SEMINAR
ON BIOACCUMULATION OF XENOBIOTICS
TABLE
OF CONTENT
TITLE
PAGE
CERTIFICATE
PAGE
DEDICATION
ACKNOWLEDGEMENT
ABSTRACT
TABLE
OF CONTENT
CHAPTER
ONE
1.0 INTRODUCTION
1.1 XENOBIOTICS
DESCRIPTION
1.2 ENTRY
ROUTES WITH THE BODY
CHAPTER
TWO
2.0 BIOTRANSFORMATION
OF XENOBIOTICS IN FISH
2.1 XENOBIOTIC
COMPOUND
2.2 HAZARDS
POSED BY XENOBIOTIC COMPOUND
2.3 MECHANICS
INVOLVED IN BIODEGREDATION OF XENOBIOTICS
2.4 BIODEGRADATION
CHAPTER
THREE
3.0 SIGNIFICANCE
OF XENOBIOTICS METABOLISM FOR BIOACCUMULATION KINETICS OF ORGANIC CHEMICALS
3.1 BIOMAGNIFICATIONS
3.2 BIOMAGNIFICATION
AND FOOD-WEB ACCUMULATION
3.3 BIOMAGNIFICATION
OF INORGANIC CHEMICAL
3.4 BIOMAGNIFICATION
OF CHLORINATED HYDROCARBONS
CHAPTER
FOUR
4.0 CONCLUSION
4.1 RECOMMENDATION
REFERENCES
CHAPTER ONE
INTRODUCTION
Bioaccumulation
refers to the accumulation of substances, such as pesticides,
or other organic chemicals in an organism. Bioaccumulation occurs when an
organism absorbs a toxic substance
at a rate greater than that at which the substance is lost. Thus, the longer the
biological half-life of the
substance the greater the risk of chronic poisoning, even if environmental
levels of the toxin are not very high. Bioaccumulation, for example in fish,
can be predicted by models. Hypotheses for molecular size cutoff criteria for
use as bioaccumulation potential indicators are not supported by data.
Biotransformation can strongly modify bioaccumulation of chemicals in an
organism.
Xenobiotic
A
xenobiotic is a foreign chemical substance found
within an organism that is not
normally naturally produced by or expected to be present within that organism.
It can also cover substances
which are present in much higher concentrations
than are usual. Specifically, drugs such as antibiotics are xenobiotics
in humans because the human body does not produce them itself, nor are they
part of a normal diet.
Bioconcentration
is a related but more specific term, referring to uptake and accumulation of a
substance from water alone. By contrast, bioaccumulation refers to uptake from
all sources combined (e.g. water, food, air, etc.)
Bioaccumulation
is the accumulation of contaminants by species in concentrations that are
orders of magnitude higher than in the surrounding environment. .Biodegradation,
Biotransformation, and Co-metabolism
More
than ten million organic compounds are generated by biosynthetic pathways in
animals, plants, and microorganisms, by other natural processes, and by
industrial synthesis. Whilst the organic structures found in nature are created
by many organisms and processes, microorganisms (bacteria and fungi) perform
most of the biodegradation of both natural products and industrial chemicals.
Collectively, microorganisms play a key role in the biogeochemical cycles of
the Earth.
The
substances transformed or degraded by microorganisms are used as a source of
energy, carbon, nitrogen, or other nutrient, or as final electron acceptor of a
respiratory process [see also - Cell thermodynamics and energy metabolism].
'Biodegradation' involves the breakdown of organic compounds, usually by
microorganisms, into biomass and less complex compounds, and ultimately to
water, carbon dioxide, and the oxides or mineral salts of other elements
present. The complete breakdown of an organic compound into inorganic
components is termed 'mineralization', but
'(ultimate/complete)
biodegradation' and '(complete) mineralization' are often used interchangeably,
although 'biodegradation' involves the formation of biomass as well as
inorganic compounds. Of course, biomass finally will also undergo
mineralization.
Degradation
of an organic compound to a less complex organic compound is referred to as
'incomplete (partial) biodegradation'.
'Biotransformation'
is the metabolic modification of the molecular structure of a compound,
resulting in the loss or alteration of some characteristic properties of the
original compound, with no (or only minor) loss of molecular complexity.
Biotransformation
may effect the solubility, mobility in the environment, or toxicity of the
organic compound.
A
microbial population growing on one compound may fortuitously transform a
contaminating chemical that cannot be used as carbon and energy source, a
process referred to as 'co-metabolism'. The phenomenon has also been called
'co-oxidation' and 'gratuitous' or 'fortuitous' metabolism. Usually, the
primary substrate induces production of (an) enzyme(s) that fortuitously
alter(s) the molecular structure of another compound.
The
organisms do not benefit from the co-metabolic process. Co-metabolic
transformation may result in a minor modification of the molecule, or it may
lead to incomplete or even complete degradation. The products of
partial biodegradation, or biotransformation, or co-metabolic conversion of a
xenobiotic may be less harmful as the original compound, or they may be as
hazardous or even more hazardous as the original compound. For example,
tetrachloroethene and trichloroethene can be microbially reduced to vinyl
chloride, a known carcinogen, in anoxic habitats. In natural environments, the
products of bioconversion processes may be further transformed or degraded by
other microorganisms, maybe eventually leading to complete degradation by the
microbial consortium. Co-metabolic processes, and biodegradation by microbial
consortia are thought to be of enormous ecological importance. However,
persistent xenobiotics and metabolic dead-end products will accumulate in the
environment, become part of the soil humus, or enter the food chain leading to
biomagnification.
What
are Xenobiotics?
Xenobiotics
(greek xenos= strange, foreign, foreigner) are chemically synthesized compounds
that do not occur in nature and thus are 'foreign to the biosphere'. They have
'unnatural' structural features to which microorganisms have not been exposed
to during evolution. Xenobiotics may resist biodegradation, or they undergo
incomplete biodegradation or just
biotransformation. The definition of xenobiotics as compounds 'foreign to life'
exhibiting 'unnatural' structural features does not necessarily imply that
xenobiotics are toxic compounds, but many xenobiotics indeed are harmful to
living organisms.
Whereas
xenobiotics may persist in the environment for months and years, most biogenic
compounds are biodegraded rapidly. Exceptions are lignin, the structural polymer
of woody plants, and, above all, the melanin polymers which are constituents of
the cell wall of the spores of a numberof fungi. Recalcitrance (i.e., the
structure-immanent stability) of a xenobiotic molecule is mainly due to
'unphysiological' chemical bonds and/or substituents, which block the attack by
microbial catabolic enzymes (see Table 1 and Figure 2). Type, number and
position of bonds and substituents affect the xenobiotic character. However, it
is not always easy to determine which structural moieties indeed are xenobiotic
in the sense of 'foreign to life'. Some natural compounds show principally the
same unusual structural features as xenobiotics, such as halogen substituents
or nitro groups found in some antibiotics, or they contain stable chemical
bonds like the ether and carbon-carbon bonds stabilizing lignin.
Moreover,
microorganisms throughout geological time have also been exposed to a variety
of chemicals produced by abiotic natural processes:
"Many
of these compounds bear little relationship to the biological products from
which they were originally derived. For example, soils and young sediments
contain thousands of substituted polycyclic aromatic hydrocarbons. These
molecules, formed by the thermal alteration of cellular material, have been in
contact with living organisms throughout evolutionary periods of time.
Consequently, one would predict the existence of microorganisms that will
degrade them, and organisms that metabolize aromatic hydrocarbons ranging in
size from benzene to benzo[a]pyrene have been described."
(D.
T. Gibson, 1980).
•High
molecular mass
•Low
solubility in water
•Condensed
benzene and pyridine rings, especially: polycyclic structures
•Three-fold
substituted N atoms
•Quarternary
C atoms
•Unphysiological
bonds and substituents R-X (especially, polysubstitution):
Typical
features of recalcitrant organic compounds. Type, number, and position of
'unphysiological' substituents influence recalcitrance. It should be noted that
organic chemicals of anthropogenic origin are not necessarily recalcitrant.
There are a number of industrial products that are degraded by microorganisms.
These compounds obviously are readily recognized by microbial catabolic
enzymes. Besides, research in biodegradation has demonstrated that a number of
xenobiotic compounds such as polychlorinated biphenyls (PCBs) and
nitroaromatics which once were thought to be recalcitrantare
subject to microbial attack (see the following sections).
Bioaccumulation
is the sum of two processes: bioconcentration and biomagnification.
Bioconcentration is the direct uptake of a substance by a living organism from
the medium (e.g., water) via skin, gills, or lungs, whereas biomagnification
results from dietary uptake. Many synthetic contaminants are more soluble in
fat than in water. Polychlorinated biphenyls (PCBs), for example, which can be
present in lake or river water, tend to either adsorb to particles or to
diffuse into cells of organisms. Thus, PCBs bioconcentrate in low trophic
levels, for example, in phytoplankton by a factor of around 250. Fish that
actively filter large amounts of water through their gills are subject to a
much higher bioconcentration. Additionally, biomagnification takes place in
predatory organisms. The PCB burden of the prey is transferred to the predator.
Fish like smelt that consume large quantities of mysids and zooplankton magnify
the PCB concentration. This leads to bioaccumulation factors as high as 2.8
million in predatory fish species such as lake trout and striped bass. Mammals—including
humans that eat the fish, reptiles, and birds—further accumulate PCBs.
Finally,
in the leading predators among marine life—the seal and polar bear—PCBs and
other persistent organic pollutants (POPs) reach concentrations that cause
obvious impairments of the immune and reproductive system. A significant
proportion of these accumulated contaminants is transferred to the offspring by
the mother's milk, resulting in, for example, abnormal sexual development,
behavioral dysfunctions, and cancer. Prerequisites for a substance's strong
bioaccumulation are its affinity for fat and low biodegradability, or
persistence in the environment. Bioaccumulating contaminants thus far
identified are the first-generation organochlorine pesticides (e.g., DDT, chlordane,
and toxaphene), PCBs, dioxins, brominated flame retardants, but also some
organo-metal compounds, for example, methyl mercury and tributyltin (TBT).
Because of their strong bioaccumulation and toxicity, some of these substances
were banned in North America
and Western Europe after 1970. The bioconcentration factor (BCF) often serves
as a trigger for the hazard classification of chemicals. In the European Union
a BCF greater than one hundred leads to a substance's classification as
"dangerous to the environment." The U.S. Environmental Protection
Agency (EPA) uses a BCF of greater than 1,000 for environmentally harmful
substances. In Canada chemicals with a BCF
greater than 5,000 are recommended for "virtual elimination."
1.1
Xenobiotics descriptions
Natural
compounds can also become xenobiotics if they are taken up by another organism,
such as the uptake of natural human hormones by fish found downstream of sewage
treatment plant outfalls, or the chemical defenses produced by some organisms
as protection against predators.[1]
The
term xenobiotics, however, is very often used in the context of pollutants such as dioxins and polychlorinated
biphenyls and their effect on the biota, because xenobiotics
are understood as substances foreign to an entire biological system, i.e.
artificial substances, which did not exist in nature before their synthesis by
humans. The term xenobiotic is derived from the Greek words ξÎνος (xenos) =
foreigner, stranger and βίος (bios, vios) = life, plus the Greek suffix for
adjectives -τικός, -ή, -ό (tic).
Xenobiotic
metabolism
The
body removes xenobiotics by xenobiotic
metabolism. This consists of the deactivation and the
excretion of xenobiotics, and happens mostly in the liver. Excretion routes are
urine, feces, breath, and sweat. Hepatic enzymes are responsible for the
metabolism of xenobiotics by first activating them (oxidation, reduction,
hydrolysis and/or hydration of the xenobiotic), and then conjugating the active
secondary metabolite with glucuronic acid,
sulphuric
acid,
or glutathione, followed by
excretion in bile or urine. An example of a group of enzymes involved in
xenobiotic metabolism is hepatic microsomal cytochrome P450.
These enzymes that metabolize xenobiotics are very important for the
pharmaceutical industry, because they are responsible for the breakdown of
medications.
Organisms
can also evolve
to tolerate xenobiotics. An example is the co-evolution
of the production of tetrodotoxin
in the rough-skinned
newt
and the evolution of tetrodotoxin resistance in its predator, the common
garter snake. In this predator-prey pair, an evolutionary
arms race has produced high levels of toxin in the newt and
correspondingly high levels of resistance in the snake. This evolutionary
response is based on the snake evolving modified forms of the ion channels
that the toxin acts upon, so becoming resistant to its effects.
Xenobiotics in the
environment
Xenobiotic
substances are becoming an increasingly large problem in Sewage Treatment
systems, since they are relatively new substances and are very difficult to
categorize. Antibiotics,
for example, were derived from fungi originally, and so mimic naturally
occurring substances. This, along with the natural monopoly
nature of municipal Waste Water Treatment Plants makes it nearly impossible to
remove this new pollutant load.
Some
xenobiotics are resistant to degradation. For example, they may be synthetic organochlorides
such as plastics and pesticides, or naturally occurring organic chemicals such
as polyaromatic
hydrocarbons (PAHs) and some fractions of crude oil
and coal. However, it is believed that microorganisms are capable of degrading
almost all the different complex and resistant xenobiotics found on the earth.[4] Many
xenobiotics produce a variety of biological effects, which is used when they
are characterized using bioassay.
Inter-species organ
transplantation
The
term xenobiotic is also used to refer to organs transplanted
from one species to another. For
example, some researchers hope that hearts and other organs could
be transplanted from pigs to humans. Many people
die every year whose lives could have been saved if a critical organ had been
available for transplant. Kidneys are currently
the most commonly transplanted organ. Xenobiotic organs would need to be
developed in such a way that they would not be rejected by the immune system.
1.2
Xenobiotic cell entry Route with the Body (Drug Biotransformational Systems –
Origins and Aims) (Human Drug Metabolism)
Role
of the liver
Drugs,
toxins and all other chemicals can enter the body through a variety of routes.
The major route is through the digestive system, but chemicals can by-pass the
gut via the lungs and skin. Although the gut metabolizes many drugs, the liver
is the main biotransforming organ and the CYPs and other metabolizing enzymes
reside in the hepatocytes. These cells must perform two essential tasks at the
same time. They must metabolize all substances absorbed by the gut whilst also
processing all agents already present (from whatever source) in the peripheral
circulation. This would not be possible through the conventional way that
organs are usually supplied with blood from a single arterial route carrying
oxygen and nutrients, leading to a capillary bed that becomes a venous outflow
back to the heart and lungs. The circulation of the liver and the gut have
evolved anatomically to solve this problem by receiving a conventional arterial
supply and a venous supply from the gut simultaneously (Figure 2.3); all the
blood eventually leaves the organ through the hepatic vein towards the inferior
vena cava.
The
hepatic arterial blood originates from the aorta and the venous arrangement is
known as the hepatic portal system, which subsequently miniaturizes inside the
liver into sinusoids, which are tiny capillary blood-filled spaces. This
capillary network effectively routes everything absorbed from the gut direct to
the hepatocytes, which are bathed at the same time in oxygenated arterial
blood. Metabolic products can leave the hepatocytes through the hepatic vein or
by a separate system of canalicali, which ultimately form the bile duct, which
leads to the gut. So, essentially, there are two blood routes into the
hepatocytes and one out, which ensures that no matter how a xenobiotic enters
the body, it will be presented to the hepatocytes for biotransformation.
Drug
and xenobiotic uptake: transporter systems
Although an agent might be presented
to the vicinity of a hepatocyte, there is no
guarantee it will enter the cell. This depends on the lipophilicity, size,
charge and other physiochemi-cal properties of the agent. If an agent is too
lipophilic, as described in section 2.3, it may enter a cell and become trapped
in the membrane. Alternatively, if a drug is very water soluble, it would not
be capable of crossing the lipid membrane bilayer of the cell. Until the last
decade or so, it was often assumed that drug absorption would usually be simply
through passive diffusion from high to low concentration. It is now apparent
that many drugs and toxins which are charged or amphipathic diffuse rather
poorly across lipid membranes and their successful cellular and systemic
absorption is in a large part due to their exploitation of the complex membrane
transport systems which are found not only in the gut, but also on the
sinusoidal (sometimes called the basolateral) membranes of hepatocytes, which
are bathed in blood from the portal circulation direct from the gut, as well as
arterial blood. These membrane transporters regulate cellular entry of amino
acids, sugars, steroids, lipids and hormones which are vital for homeostasis.
We know this because if the hepatocyte transporters are inhibited, the
bioavailability of several drugs increases because they escape hepatic
clearance by the CYPs and other systems. Transporter proteins are found in all
tissues and can be broadly categorized into two ‘superfamilies’; those that
assist the entry of drugs, toxins and nutrients into cells (uptake, or influx
transporters) and those that actively pump them out using ATP in the process,
usually against concentration gradients.
Hepatic
and gut uptake (influx) transporter systems
These transporters,
usually known as the solute carriers (SLCs), are found in the liver, gut,
brain, kidney and the placenta. These systems operate without using ATP and
transport everything from small peptides to anions like bilirubin-related
metabolites. The main hepatic uptake transporters are known as organic anion
transporting peptides, or OATPs. These transporters originate from a gene known
as SLCO1B1 which is found on chromosome 12. OATPs are sodium independent and
they effectively operate a process of facilitated diffusion, known as
electroneutral exchange. For every amphipathic molecule they pump in, they
expel a neutralizing anion, like glutathione (GSH), bicarbonate or even a drug
metabolite. The system is rather like a revolving door and many drugs enter gut
epithelial cells and hepatocytes this way, particularly the more hydrophilic
statins. The best documented OATPs are OATP1A2, OATP1B1 and OATP1B3. These
transporters are vital to the uptake of several classes of drugs and OATP1B1
can be inhibited by gemfibrozil, rifampicin, cyclosporine and by the antiiHIV
protease inhibitors such as ritonavir.
Regarding other hepatic transporters,
NTCP (sodium taurocholate cotransporting polypeptide) transports bile salts,
but also can handle rosuvastatin and NTCP has also been used to selectively
target liver tumours by linking cytotoxic agents to bile salts. There are
several other uptake transporters which are of most relevance in tissues other
than the liver, such as the kidneys and the gut. The OATs pump small anions
mainly in the kidney, but OAT2 and OAT5 are hepatic. OATs can be inhibited by
the cephalosporin antibiotics, which may be linked with their renal toxicity.
Aims
of biotransformation
Once drugs or toxins enter the
hepatocytes, they are usually vulnerable to some
form of biotransformation. Although you can see some of the many functions of
CYPs and other biotransformational enzymes (Figure 2.2), it is essential to be
clear on what they have to achieve with a given molecule. Looking at many
endogenous substances like steroids or xenobiotic agents, such as drugs, all
these compounds are mainly lipophilic. Drugs often parallel endogenous
molecules in their oil solubility, although many are considerably more
lipophilic than these molecules. Generally, drugs, and xenobiotic compounds,
have to be fairly oil soluble or they would not be absorbed from the GI tract.
Once absorbed these molecules could change both the structure and function of
living systems and their oil solubility makes these molecules rather ‘ elusive
, , in the sense that they can enter and leave cells according to their
concentration and are temporarily beyond the control of the living system. This
problem is compounded by the difficulty encountered by living systems in the
removal of lipophilic molecules. As previously mentioned,even after the kidney
removes them from blood by filtering them, the lipophilicity of drugs, toxins
and endogenous steroids means that as soon as they enter the collecting
tubules, they can immediately return to the tissue of the tubules, as this is
more oil-rich than the aqueous urine. So the majority of lipophilic molecules
can be filtered dozens of times and only low levels are actually excreted. In
addition, very high lipophilicity molecules like some insecticides and fire
retardants might never leave adipose tissue at all (unless moved by dieting or
breast feeding, which mobilizes fats). Potentially these molecules could stay
in our bodies for years. This means that for lipophilic agents:
•
the more lipophilic they are, the more these agents are trapped in membranes,
affecting fluidity and causing disruption at high levels;
•
if they are hormones, they can exert an irreversible effect on tissues that is
outside normal physiological control;
•
if they are toxic, they can potentially damage endogenous structures;
•
if they are drugs, they are also free to cause any pharmacological effect for
a considerable period of time.
The aims of a biotransformational
system include assembly of endogenous molecules, as
well as clearance of these and related chemicals from the organism. These aims
relate to control for endogenous steroid hormones (assembly and elimination),
as well as protection- in the case of highly lipophilic threats, like drugs,
toxins and hormone ‘mimics ’ (endocrine disruptors). Metabolizing systems have
developed mechanisms to control balances between hormone synthesis and
clearance so the organism can finely tune the effects of potent hormones such
as sex-steroids. These systems also actually detect the presence of drugs and
act to eliminate them.
Task
of biotransformation
Essentially,
the primary function of biotransforming enzymes such as CYPs is to ‘move’ a
drug, toxin or hormone from the left-hand side of Figure 2.1 to the right-hand
side. This means making very oil-soluble molecules highly water-soluble. This
sounds impossible at first and anyone who has tried to wash their dishes
without using washing up liquid will testify to this problem. However, if the
lipophilic agents can be structurally altered, so changing their physicochemical
properties, they can be made to dissolve in water. Once they are water-soluble,
they can easily be cleared by the kidneys into urine and they will finally be
eliminated.
Phase’s
I-III of biotransformation
Most lipophilic agents that invade
living systems, such as aromatic hydrocarbons,
hormones, drugs and various toxins, vary in their chemical stability, but many
are relatively stable in physiological environments for quite long periods of
time. This is particularly true of polycyclic aromatics. This means that a
considerable amount of energy must be put into any process that alters their
structures. This energy expenditure will be carried out pragmatically. This
means that some molecules may require several changes to attain water
solubility, such as polycyclics, whilst others such as lorazepam and AZT, only
one. The stages of biotransformation are often described as ’ Phases ’ I, II
and III. Phase I metabolism mainly describes oxidative CYP reactions, but
non-CYP oxidations such as reductions and hydrolyses are also sometimes
included in the broad term ‘Phase I’. This has been highlighted as rather
arbitrary and inconsistent and it is recommended that it is more accurate to
refer to a particular process specifically, rather than using the loose term
‘Phase I’.
The term , Phase II
, describes generally conjugative processes, where water, soluble endogenous
sugars, salts or amino acids are attached to xenobiotics or endogenous
chemicals. The very term ‘Phase II’ suggests that ‘Phase I’ processes must
necessarily occur prior to conjugative reactions with a molecule. Although this
does often happen, conjugation also occurs directly without prior ‘preparation’
by oxidative processes. The products of ‘Phase II’ tend also to be strongly
associated with detoxification and high water solubility. This is not always
the case either and it is important to realize that some conjugative ‘Phase II’
processes can form either toxic species, or metabolites even less water-soluble
than the parent drug. The more recent term Thase III , describes the system of
efflux pumps that excludes water-soluble products of metabolism from the cell
to the interstitial fluid, blood and finally the kidneys. The efflux pumps can
also exclude drugs as soon as they are absorbed from the gut, as well as
metabolites. Although the Phase I-III terminology remains popular and thus is
sometimes used in this topic, it is important to recognize the limitations of
these terms in the description of many processes of biotransformation.
Biotransformation has a secondary
effect, in that there is so much structural
change in these molecules that pharmacological action is often removed or
greatly diminished. Even if the metabolite retained some potential
pharmacodynamic effects, its increased polarity compared with the parent drug
means that the Phase III systems are likely to remove it relatively quickly, so
diminishing any effects it might have exerted on the target tissue.
The use of therapeutic drugs is a
constant battle to pharmacologically influence a
system that is actively undermining the drugs’ effects by removing them as fast
as possible. The processes of oxidative and conjugative metabolism, in concert
with efflux pump systems, act to clear a variety of chemicals from the body
into the urine or faeces, in the most rapid and efficient manner. The systems
that manage these processes also sense and detect increases in certain
lipophilic substances and this boosts the metabolic capability to respond to
the increased load. The next topic will outline how mainly CYP-mediated
oxidative systems achieve their aim of converting stable lipophilic agents to
water-soluble products.
CHAPTER
TWO
2.0
Biotransformation of xenobiotics in fish.
Biotransformation
of xenobiotics in fish occurs by many of the same reactions as in mammals.
These reactions have been shown to affect the bioaccumulation, persistence,
residue dynamics, and toxicity of select chemicals in fish. P-450-dependent
monooxygenase activity of fish can be induced by polycyclic aromatic hydrocarbons,
but phenobarbital-type agents induce poorly, if at all. Fish monooxygenase
activity exhibits ideal temperature compensation and sex-related variation.
Induction of monooxygenase activity by polycyclic aromatic hydrocarbons can
result in qualitative as well as quantitative changes in the metabolic profile
of a chemical. Induction can also alter toxicity. In addition, multiple P-450
isozymes have been described for several fish species. The biotransformation
products of certain chemicals have been related to specific P-450 isozymes, and
the formation of these products can be influenced by induction. Exposure of
fish to low levels of certain environmental contaminants has resulted in
induction of specific monooxygenase activities and monitoring of such activities
has been suggested as a means of identifying areas of pollutant exposure in the
wild.
2.1
Xenobiotic compounds.
These compounds are not commonly produced by
nature. Some microbes have been seen to be capable of breaking down of
xenobiotics to some extent. But most of the xenobiotic compounds are non
degradable in nature. Such compounds are known to be recalcitrant in nature.
The properties of xenobiotic compounds attributing to its recalcitrant properties are:
(i) Non recognizable as substrate by microbes to act upon and degrade it.
(ii) It does not contain permease which is needed for transport into microbial cell.
(iii) Large molecular nature makes it difficult to enter microbial cell.
(iv) They are highly stable and insolubility to water adds to this property.
(v)Mostly toxic in nature.
The recalcitrant xenobiotic compounds can be divided into different groups depending on their chemical composition
Halocarbons: They consist of halogen group in their structure. Mainly used in solvents, pesticides, propellants etc. They are highly volatile and escape into nature leading to destruction of ozone layer of atmosphere. The compounds present in insecticides, pesticides etc,. leach into soil where they accumulate and result in biomagnification.
Polychlorinated biphenyls (PCBs): They consist of a halogen group and benzene ring. They are mainly used in plasticisers, insulator coolants in transformers etc. They are chemically and biologically inert adding on to its recalcitrant nature.
Synthetic polymers: These are mainly used to form plastics like polyester, polyvinyl chloride etc. They are insoluble in water and of high molecular weight explaining the recalcitrant property.
Alkylbenzyl Sulphonates: They consist of a sulphonate group which resists break down by microbes. They are mostly found in detergents.
Oil mixtures: When oil spills occur covering a huge area the break down by action of microbes becomes non effective. They become recalcitrant as they are insoluble in water and some components of certain oils are toxic in higher concentrations.
The recalcitrant property of xenobiotic compound is directly linked to its complexity so that the higher the complexity the stronger recalcitrant property.
2.3 Hazards posed by xenobiotic compounds
The hazards posed by xenobiotics are huge. These compounds are highly toxic in nature and can affect survival of lower as well as higher eukaryotes. It also poses health hazards to humans like various skin problems, reproductively and even known as a trigger for causing cancer. These compounds are persistent and remain in the environment for many years leading to bioaccumulation or biomagnification. They also find a way into the food chains and the concentrations of such compounds was found to be high even in organisms that do not come in contact with xenobiotics directly.
2.4 Mechanisms involved in biodegradation of xenobiotics
Xenobiotic compounds, owing to its recalcitrant nature, is hard to break down and degrade. The complexity of its chemical composition adds to this. For breaking down such compounds the enzymes act on certain groups present in the compound. For eg: in the halocarbons the halogen group is targeted. Enzymes like oxygenases play a major role. The bonds like ester-, amide-, or ether bonds present in the compounds are first attacked leading to breaking down of compounds. In some cases the aliphatic chains and in aromatic compounds the aromatic components may be targeted. The site and mode of attack depends on the action of enzyme, its concentration and the favourable conditions. Often it is seen that the xenobiotics do not act as a source of energy to microbes and as a result they are not degraded. The presence of a suitable substrate induces its breakdown. This substrate is known as co – metabolite and the process of degradation are known as co metabolism. In another process, the xenobiotics serve as substrates and are acted upon to release energy. This is called gratuitous metabolism.
2.5Biodegradation:
Certain microbes on continuous exposure to xenobiotics develop the ability to degrade the same as a result of mutations. Mutations resulted in modification of gene of microbes so that the active site of enzymes is modified to show increased affinity to xenobiotics. Certain mutations also resulted in developing new enzymatic pathway for xenobiotic degradation. Use of mixed population of microbes is usually recommended as it has been seen to yield faster results as the two different microbes attack different parts through different mechanisms resulting in effective break down. It also creates a condition of co metabolism. The modification of certain genes of microbes to break down xenobiotics is also recommended and is seen to produce high level of accuracy.
The properties of xenobiotic compounds attributing to its recalcitrant properties are:
(i) Non recognizable as substrate by microbes to act upon and degrade it.
(ii) It does not contain permease which is needed for transport into microbial cell.
(iii) Large molecular nature makes it difficult to enter microbial cell.
(iv) They are highly stable and insolubility to water adds to this property.
(v)Mostly toxic in nature.
The recalcitrant xenobiotic compounds can be divided into different groups depending on their chemical composition
Halocarbons: They consist of halogen group in their structure. Mainly used in solvents, pesticides, propellants etc. They are highly volatile and escape into nature leading to destruction of ozone layer of atmosphere. The compounds present in insecticides, pesticides etc,. leach into soil where they accumulate and result in biomagnification.
Polychlorinated biphenyls (PCBs): They consist of a halogen group and benzene ring. They are mainly used in plasticisers, insulator coolants in transformers etc. They are chemically and biologically inert adding on to its recalcitrant nature.
Synthetic polymers: These are mainly used to form plastics like polyester, polyvinyl chloride etc. They are insoluble in water and of high molecular weight explaining the recalcitrant property.
Alkylbenzyl Sulphonates: They consist of a sulphonate group which resists break down by microbes. They are mostly found in detergents.
Oil mixtures: When oil spills occur covering a huge area the break down by action of microbes becomes non effective. They become recalcitrant as they are insoluble in water and some components of certain oils are toxic in higher concentrations.
The recalcitrant property of xenobiotic compound is directly linked to its complexity so that the higher the complexity the stronger recalcitrant property.
2.3 Hazards posed by xenobiotic compounds
The hazards posed by xenobiotics are huge. These compounds are highly toxic in nature and can affect survival of lower as well as higher eukaryotes. It also poses health hazards to humans like various skin problems, reproductively and even known as a trigger for causing cancer. These compounds are persistent and remain in the environment for many years leading to bioaccumulation or biomagnification. They also find a way into the food chains and the concentrations of such compounds was found to be high even in organisms that do not come in contact with xenobiotics directly.
2.4 Mechanisms involved in biodegradation of xenobiotics
Xenobiotic compounds, owing to its recalcitrant nature, is hard to break down and degrade. The complexity of its chemical composition adds to this. For breaking down such compounds the enzymes act on certain groups present in the compound. For eg: in the halocarbons the halogen group is targeted. Enzymes like oxygenases play a major role. The bonds like ester-, amide-, or ether bonds present in the compounds are first attacked leading to breaking down of compounds. In some cases the aliphatic chains and in aromatic compounds the aromatic components may be targeted. The site and mode of attack depends on the action of enzyme, its concentration and the favourable conditions. Often it is seen that the xenobiotics do not act as a source of energy to microbes and as a result they are not degraded. The presence of a suitable substrate induces its breakdown. This substrate is known as co – metabolite and the process of degradation are known as co metabolism. In another process, the xenobiotics serve as substrates and are acted upon to release energy. This is called gratuitous metabolism.
2.5Biodegradation:
Certain microbes on continuous exposure to xenobiotics develop the ability to degrade the same as a result of mutations. Mutations resulted in modification of gene of microbes so that the active site of enzymes is modified to show increased affinity to xenobiotics. Certain mutations also resulted in developing new enzymatic pathway for xenobiotic degradation. Use of mixed population of microbes is usually recommended as it has been seen to yield faster results as the two different microbes attack different parts through different mechanisms resulting in effective break down. It also creates a condition of co metabolism. The modification of certain genes of microbes to break down xenobiotics is also recommended and is seen to produce high level of accuracy.
CHAPTER
THREE
3.0
Significance of Xenobiotic Metabolism for Bioaccumulation Kinetics of Organic
Chemicals in Gammarus pulex
Bioaccumulation
and biotransformation are key toxicokinetic processes that modify toxicity of
chemicals and sensitivity of organisms. Bioaccumulation kinetics vary greatly
among organisms and chemicals; thus, we investigated the influence of
biotransformation kinetics on bioaccumulation in a model aquatic invertebrate
using fifteen 14C-labeled organic xenobiotics from diverse chemical
classes and physicochemical properties (1,2,3-trichlorobenzene, imidacloprid,
4,6-dinitro-o-cresol, ethylacrylate, malathion, chlorpyrifos, aldicarb,
carbofuran, carbaryl, 2,4-dichlorophenol, 2,4,5-trichlorophenol,
pentachlorophenol, 4-nitrobenzyl-chloride, 2,4-dichloroaniline, and sea-nine
(4,5-dichloro-2-octyl-3-isothiazolone)). We detected and identified metabolites
using HPLC with UV and radio-detection as well as high resolution mass
spectrometry (LTQ-Orbitrap). Kinetics of uptake, biotransformation, and
elimination of parent compounds and metabolites were modeled with a first-order
one-compartment model. Bioaccumulation factors were calculated for parent
compounds and metabolite enrichment factors for metabolites. Out of 19 detected
metabolites, we identified seven by standards or accurate mass measurements and
two via pathway analysis and analogies to other compounds.
1,2,3-Trichlorobenzene, imidacloprid, and 4,6-dinitro-o-cresol were not
biotransformed. Dietary uptake contributed little to overall uptake.
Differentiation between parent and metabolites increased accuracy of
bioaccumulation parameters compared to total 14C measurements.
Biotransformation dominated toxicokinetics and strongly affected internal
concentrations of parent compounds and metabolites. Many metabolites reached
higher internal concentrations than their parents, characterized by large
metabolite enrichment factors.
An
example of poisoning in the workplace can be seen from the phrase "as mad as a hatter". The process for
stiffening the felt used in making hats involved mercury, which forms organic
species such as methylmercury, which is lipid soluble, and tends to
accumulate in the brain resulting in mercury poisoning.
Other lipid (fat) soluble
poisons include tetraethyllead compounds (the lead
in leaded petrol),
and DDT.
These compounds are stored in the body's fat, and when the fatty tissues
are used for energy, the compounds are released and cause acute poisoning.
Strontium-90,
part of the fallout from atomic bombs,
is chemically similar enough to calcium that it is utilized in osteogenesis,
where its radiation can cause damage for a long time.
Naturally
produced toxins can also bioaccumulate. The marine algal blooms
known as "red tides" can result in local filter
feeding organisms such as mussels
and oysters
becoming toxic; coral fish can be responsible for the poisoning known as ciguatera
when they accumulate a toxin called ciguatoxin
from reef algae.
Some
animal species exhibit bioaccumulation as a mode of defense; by consuming toxic
plants or animal prey, a species may accumulate the toxin which then presents a
deterrent to a potential predator. One example is the tobacco hornworm,
which concentrates nicotine
to a toxic level in its body as it consumes tobacco
plants. Poisoning of small consumers can be passed along the food chain to
affect the consumers later on. Other compounds that are not normally considered
toxic can be accumulated to toxic levels in organisms. The classic example is
of Vitamin A,
which becomes concentrated in carnivore
livers
of e.g. polar bears: as a pure carnivore that feeds on
other carnivores (seals), they accumulate extremely large amounts of Vitamin A
in their livers. It was known by the native peoples of the Arctic that the
livers of carnivores should not be eaten, but Arctic explorers
have suffered Hypervitaminosis A from eating the bear
livers (and there has been at least one example of similar poisoning of Antarctic explorers
eating husky
dog
livers). One notable example of this is the expedition of Sir Douglas Mawson,
where his exploration companion died from eating the liver of one of their
dogs.
Coastal fish
(such as the smooth toadfish) and seabirds
(such as the Atlantic Puffin) are often monitored
for heavy metal bioaccumulation.
In
some eutrophic aquatic systems, biodilution
can occur. This trend is a decrease in a contaminant with an increase in
trophic level and is due to higher concentrations of algae and bacteria to
"dilute" the concentration of the pollutant.
3.2
Biomagnifications (or bioaccumulation) refers to the ability of living
organisms to accumulate certain chemicals to a concentration larger than that
occurring in their inorganic, non-living environment, or in the case of
animals, in the food that they eat. Organisms accumulate any chemical needed
for their nutrition. The major focus of biomagnification, however, is the
accumulation of certain non-essential chemicals, especially certain chlorinated
hydrocarbons that are persistent in the environment. These compounds are
insoluble in water, but highly soluble in fats. Because almost all fats within
ecosystems occur in the living bodies of organisms, chlorinated hydrocarbons
such as 4,4’-(2, 2, 2-trichloroethane-1, 1-diyl)-bis(chlorobenzene) (DDT) and
polychlorinated biphenyls (PCBs) tend to selectively accumulate in organisms.
This can lead to ecotoxicological problems, especially for top predators at the
summit of ecological food webs, who ingest the toxic prey.
3.3Biomagnification
and food-web accumulation
Organisms
are exposed to a myriad of chemicals in their environment. Some of these
chemicals occur in trace concentrations in the environment, and yet they may be
selectively accumulated by organisms to much larger concentrations that can
cause toxicity. This tendency represents biomagnification.
Some
of the biomagnified chemicals are elements such as selenium, mercury, nickel,
or organic derivatives such as methylmercury. Others are in the class of
chemicals known as chlorinated hydrocarbons (or organo-chlorines). These are
extremely insoluble in water, but are freely soluble in organic solvents,
including animal fats and plant oils (these are collectively known as lipids).
Many of the chlorinated hydrocarbons are also very persistent in the
environment, because they are not easily broken down to simpler chemicals through
the metabolism of microorganisms, or by ultraviolet radiation or other
inorganic processes. Common examples of bioaccumulating chlorinated
hydrocarbons are the insecticides DDT and dieldrin, and a class of industrial
chemicals abbreviated as PCBs.
Food-web
accumulation is a special case of biomagnification, in which certain chemicals
occur in their largest ecological concentration in predators at the top of the
food web. An ecological food web is a complex of species that are linked
through their trophic interactions, that is, their feeding relationships. In
terms of energy flow, food webs are supported by inputs of solar energy, which
is fixed by green plants through photosynthesis. Some of this fixed energy is
used by the plants in their own respiration, and the rest, as plant biomass, is
available to be passed along to animals, which are incapable of metabolizing
any other type of energy. Within the food web, animals that eat plants are
known as herbivores. These are eaten by carnivores, which in turn may be eaten
by other carnivores. Top predators (examples include wolves, bears, and seals)
occur at the summit of the food web. In general, food webs have a pyramidal
structure, with plant productivity being much greater than that of herbivores,
and these being more productive than their predators. Top predators are usually
quite uncommon. Within food webs, biomagnifying chemicals such as DDT,
dieldrin, and PCBs have their largest concentrations, and cause the greatest
damage, in top predators.
3.4
Biomagnification of inorganic chemicals
All
of the naturally occurring elements occur in the environment. Some occur at
very low concentration, while others are more abundant. This contamination is
always detectable, as long as the analytical chemistry method of detection is
sensitive enough to detect even trace amounts of the target chemical. About 25
of the elements are required by plants and/or animals, including the
micronutrients copper, iron, molybdenum, zinc, and rarely, aluminum, nickel,
and selenium. However, under certain ecological conditions these micronutrients
can biomagnify to very large concentrations, and even cause toxicity to
organisms.
One
example is serpentine soil and the vegetation that grows in it. Serpentine
minerals contain relatively large concentrations of nickel, cobalt, chromium,
and iron. Soils derived from this mineral can be toxic to plants. However, some
plants grown on serpentine soils are physiologically tolerant of these metals,
and can bioaccumulate them to very large concentrations. For example, the
normal concentration of nickel in plants is about 1-5 ppm (parts per million, a
concentration equivalent to mg/kg). However, on sites with serpentine soils
much larger concentrations of nickel occur in plant foliage and other tissues.
Nickel concentrations as large as 16% occur in tissues of a plant in the
mustard family, Streptanthus
polygaloides, in California, and 11-25%
nickel occurs in the blue-colored latex of Sebertia acuminata on the island of New
Caledonia in the Pacific Ocean.
It is common for plants growing on serpentine soils to have nickel
concentrations of thousands of parts per million, which is usually considerably
larger than the concentration in soil.
Another
case of biomagnification occurs on some sites in semiarid regions in which the
soil is contaminated by selenium, which may then be hyperaccumulated (i.e.,
extremely accumulated) by specialized species of plants. These plants are
poisonous to grazing livestock and other large animals, causing a toxic
reaction called “blind staggers.” The most important selenium-accumulating
plants in North America
are milk vetches in the genus Astragalus,
in the legume family. There are 500 species of Astragalus in North America, of which 25 are
accumulators of selenium. The foliage of these plants can contain thousands of
parts per million (ppm; equivalent to 1 milligram per liter) of selenium, to a
maximum of about 15,000 ppm, much larger than the concentration in soil.
Sometimes, accumulator and non-accumulator Astragalus species grow together, as in the
case of a place in Nebraska with 5 ppm selenium in
soil, and 5,560 ppm in Astragalus
bisulcatus, but only 25 ppm in A. missouriensis.
Mercury
can also be biomagnified from trace concentrations in the environment. In this
case, trace concentrations of mercury in water can result in large
contaminations of fish and other predators. For example, fish species known to
bioaccumulate mercury in offshore waters of North America include Atlantic
swordfish, Pacific blue marlin, tunas, and halibut, among others. These fish
can accumulate mercury from trace concentrations in seawater (less than 0.1
ppm) to concentrations in flesh that commonly exceed 0.5 ppm of the fresh
weight of the fish, the maximum acceptable concentration in fish for human
consumption. The contamination of oceanic fish by mercury is probably natural,
and is not only a modern phenomenon. Studies have found no difference in
mercury contaminations of modern tuna and museum specimens collected before
1909, or concentrations in feathers of pre-1930 and post-1980 seabirds
collected from islands in the northeast Atlantic Ocean.
In this phenomenon of mercury biomagnification, there is a tendency for larger,
older fish to have relatively large concentrations. In a study of Atlantic
swordfish, for example, the average mercury concentration of animals smaller
than 51 lb (23 kg) was 0.55 ppm, compared with 0.86 ppm for those 51-99 lb
(23-45 kg) in weight, and 1.1 ppm for those heavier than 45 kg. Large
concentrations of mercury also occur in fish-eating marine mammals and birds
that are predators at or near the top of the marine food web.
3.5
Biomagnificaiton of chlorinated hydrocarbons
Chlorinated
hydrocarbons such as some insecticides (examples include DDT, dieldrin, and
methoxychlor), PCBs, and dioxin have a low solubility in water. In other words,
they tend not to dissolve in water to forma solution. As a result, these
chemicals cannot be diluted into the larger volume of water. However,
chlorinated hydrocarbons are highly soluble in lipids. Because most lipids
within ecosystems occur in biological tissues, the chlorinated hydrocarbons
have a strong affinity for living organisms, and they tend to biomagnify by
many orders of magnitude from vanishingly small aqueous concentrations.
Furthermore, because chlorinated hydrocarbons are persistent in the
environment, they accumulate progressively as organisms grow older, and they
accumulate into especially large concentrations in top predators, as described
previously. In some cases, older individuals of top-predator animals such as
raptorial birds and fish-eating marine mammals have been found to have
thousands of ppm of DDT and PCBs in their fatty tissues. The toxicity caused by
these animals accumulated exposures to DDT, PCBs, and other chlorinated
hydrocarbons is a well-recognized environmental problem.
The
biomagnification and food-web accumulation characteristics of DDT are
especially well known. Typically, DDT has extremely small concentrations in air
and water, and, to a lesser degree in soil. However, concentrations are much
larger in organisms, especially in animals at or near the top of their food
web, such as humans and predatory birds. The food-web biomagnification of DDT
can be illustrated by the case of Lake Kariba, Zimbabwe. Although banned in
most industrialized countries since the early 1970s, DDT is still used in many
tropical countries for agriculture purposes and to control insect vectors of human
diseases. The use of DDT in agriculture was banned in Zimbabwe in 1982, but DDT
continues to be used to control mosquitoes and tsetse flies, insects that
spread malaria and diseases of livestock. The concentration of DDT in the water
of Lake Kariba was less than 0.002 ppb, but concentrations in sediment were 0.4
ppm (because sediment contains a relatively large concentration of organic
matter, it contains much more DDT than the overlying water). Planktonic algae
contained 2.5 ppm. A filter-feeding mussel had 10 ppm (values for animal
tissues are for DDT in fat), while two species of plant-eating fish contained 2
ppm, and a bottom-feeding fish contained 6 ppm. A predatory fish and a
fish-eating bird, the great cormorant, contained 5-10 ppm. The Nile crocodile is the top predator in Lake
Kariba (other than humans), and it had 34 ppm. Therefore, the data for Lake
Kariba illustrates a substantial biomagnification of DDT from water, and to a
lesser degree from sediment, as well as a marked food-web accumulation from
herbivores to top carnivores.
The
widespread occurrence of food-web biomagnification of DDT and other chlorinated
hydrocarbons caused chronic, ecotoxicological damage to birds and mammals of many
species, even in habitats remote from sprayed sites. In some species, effects
on predatory birds were severe enough to cause large declines in abundance
beginning in the early 1950s, and resulting in local or regional losses of
populations. Prominent examples of North American birds that suffered
population decreases because of exposure to chlorinated hydrocarbons include
bald eagle, golden eagle, peregrine falcon, osprey, brown pelican, and
double-crested cormorant, among others. However, since the banning of the use
of DDT in North America in the early 1970s, these birds have increased in
abundance. In the case of the peregrine falcon, this increase was enhanced by a
captive-breeding and
KEY
TERMS
Biomagnification—
Tendency of organisms to accumulate certain chemicals to a concentration larger
than that occurring in their inorganic, nonliving environment, such as soil or
water, or in the case of animals, larger than in their food.
Ecotoxicology—
The study of the effects of toxic chemicals on organisms and ecosystems.
Ecotoxicology considers both direct effects of toxic substances and also the
indirect effects caused, for example, by changes in habitat structure or the
abundance of food.
Food-web
accumulation— Tendency of certain chemicals to occur
in their largest concentration in predators at the top of the ecological food
web. As such, chemicals such as DDT, PCBs, and mercury in the aquatic
environment have their largest concentrations in predators, in comparison with
the non-living environment, or with plants and herbivores.
Hyperaccumulation—
A syndrome in which a chemical is bioaccumulated to an extraordinary degree.
release
program over much of its former range in eastern North America.
In
some African countries where malaria is a problem, the use of DDT to control
mosquitoes (which can transfer the malaria-causing microorganism from
person-to-person as they obtain their blood meal) has been advocated. If
implimented, DDT spraying programs would have to be controlled, so as not to
contaminate ground- and surface-water supplies.
CHATER
FOUR
CONCLUTION
Xenobiotic
compounds are chemicals which are foreign to the biosphere. Depending on their
fate in air, water, soil, or sediment, xenobiotic pollutants may become
available to microorganisms in different environmental compartments. Actually,
the dominant means of transformation and degradation of xenobiotic compounds on
Earth resides in microorganisms. In natural habitats, the physicochemical
properties of the environment may affect and even control biodegradation
performance. Sorption to soil and sediment as well as micropore entrapment are
major causes for the persistence of many xenobiotics. 'Polycyclic aromatic
hydrocarbons, halogenated aliphatic as well as aromatic hydrocarbons,
nitroaromatic compounds, azo compounds, s-triazines, organic sulfonic acids,
and synthetic polymers are important classes of pollutants with xenobiotic
structural features. This article is focused on the mechanisms and pathways of
microbial degradation of these compounds. Fungi, and aerobic as well as
anaerobic bacteria are involved in the degradation of xenobiotics. Sometimes
these microbial transformation processes are fortuitous, a phenomenon that is
not uncommon in microbiology. On the other hand, microorganisms may use
xenobiotic compounds as a source of energy, carbon, nitrogen, or sulfur.
Degradation of many xenobiotic chemicals requires microbial communities. Some
xenobiotics, however, appear to resist microbial attack.
RECOMMENDATION
I
recommend that the collective knowledge in the field of microbial degradation
may enable scientists to establish rules to predict the biodegradability and
the biodegradation pathways of xenobiotic compounds.
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