Prof Ram Kumar

Department of Environmental Science

Xenobiotic compounds & Biodegradation

• The term xenobiotics, is generally used in the context of pollutants e.g.

dioxins and Polychlorinated bipheniles and their effects on the life forms.

• 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 (xenos) = foreigner,

stranger and bios, vios) = life.

Recall our previous lecture on Xenobiotic compunds

• The term xenobiotics, is generally used in the context of pollutants e.g.

dioxins and Polychlorinated bipheniles and their effects on the life forms.

• 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 (xenos) = foreigner,

stranger and bios, vios) = life.

Recall our previous lecture on Xenobiotic compounds:

Definition, classification and Environmental concerns

Xenobiotics:

• A xenobiotic is a foreign compound (substance) enters in the body

of an organims

• They are not normally naturally produced by or expected to be

present within that organism.

• It may include substances, That are present in much higher

concentrations than the normal;

( e.g. Drugs (antibiotics) are xenobiotics in humans because the

human body does not produce them itself, nor are they part of a

normal food.

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.

A variety of Systems in Toxicology

Drug and polluants toxicity: differences and similarities

Global systems

The Organism as a system

Cellular and molecular systems

Clinical response

Preclinical response

contaminants Internal

contamination biomarkers

New technologies

Internal dose

External contact

exposure

Environmental Toxicology: a global system

sources

Can we predict toxicity?

Drug Toxicity: the organism as a system

Target tissues

Toxicity

Impact de la toxicité des médicaments Drug Toxicity: a health and economical issue

Can we predict toxicity?

High throughput technologies: the « omics »

Lessons from molecular and cellular biology

Analytical Methods

Systems biology

In silico prediction

Paradise on earth: low cost, high efficiency

Predictive and Mechanistic Toxicology

Can New Technologies help?

Xenobiotics are low molecular weight foreign Substances:

Drugs Pollutants Nutrients

Similar responses at the cellular level Exposure to xenobiotics is accompanied by a stress

The Xenobiotics Stress System

What is a stress??

Stress: the word Physics: response of a metal Physiology: a defined set of responses to extreme situations (Selye) Cell biology: response of a cell to aggression Psychology-social sciences: response of an individual or of a group

Stress is an adaptive response to a significant shift in cellular conditions This response has a cost

Xenobiotics in the environment

There are numerous Xenobiotic substances in sewage so this is an issue

in Sewage Treatment systems, as

• Different xenobiotic substance will present its own problems as to how

to remove them (and whether it is worth trying to). As some xenobiotics

i.e. organochlorides such as plastics and pesticides, or naturally

occurring organic chemicals such as poly-aromatic hydrocarbons

(PAHs) and some fractions of crude oil and coal.

• Generally, it is believed that microorganisms are capable of degrading

almost all the different complex and resistant xenobiotics found on the

earth.

• Many xenobiotics produce a variety of biological effects, which is used

when they are characterized through bioassay.

• Xenobiotics may become available to microorganisms depending

upon their fate in air water, soil and sediment.

• In natural habitats the physicochemical properties of environment may affect

or even control degradation of Xenobiotic compounds.

Important classes of pollutants with Xenobiotic structural features:

• Aromatic hydrocarbons, halogenated aliphatic and aromatic

hydrocarbons, nitroaromatic compunds, azocompounds, s-triazines,

organic sulphonic acids, and synthetic polymers,

• Polychlorinated biphenyls (PCBs) are a family of 209 related chemical

compounds that were manufactured and sold as complex mixtures

differing in their average chlorination level.

• The individual PCB isomers, or PCB congeners, are described according to

the position of the chlorine substitution, e.g., 2,3,4,3',4'-

pentachlorobiphenyl (the shorthand 234-34-CB will be used in this article)

• The desirable physical and chemical properties of PCBs (excellent dielectric

and flame resistance properties, chemical and thermal stability) led to their

extensive industrial use as heat transfer fluids, hydraulic fluids, solvent

extenders, plasticizers, flame retardants, organic diluents, and dielectric

fluids.

• Extensive application of these chemically and thermally stable

compounds has resulted in widespread contamination;

e.g. several hundred million pounds have been released to the

environment.

• The high octanol/water partition coefficient (Kow) of some

PCB congeners results in their accumulation in fatty tissues

and their biomagnification in the food chain.

DISTRIBUTION OF PCBs

CAPACITORS 36%

PLASTICIZERS 27%

TRANSFORMERS 19%

HYDRAULIC FLUIDS 10%

HEAT CARRIERS 6%

DIFFERENT USAGE 2%

(paint additives, additives for pesticides, polishing waxes,

carbon copy papers, printing inks and pastes, roofing felts,

motor oils etc.)

Distribution and Transformation of Organic

Compounds

Basic principles of pollutant distribution and transformations. The factors

controlling chemodynamics include Henry’s constant, sorption/distribution

coefficients, bioconcentration factor, and KOW.

Distribution and

Transformation

of Organic

Compounds

Materials properties and

environmental behaviour

Transport and Sorption of Organic

Compounds

Degradation of Organic Compounds

Transformation processes of organic matter: Abiotic and biotic processes

• Chemical and biological transformation processes control the ultimate

fate of hydrocarbons released into the environment.

• The transformation reactions differ depending on the environmental

compartment within which the compounds reside and vary with chemical

structure.

• When hydrocarbons are released to the atmosphere or surface waters,

photochemical oxidation, an abiotic process, can occur.

• In soils and groundwater and surface waters, biologically mediated degradation

of hydrocarbons is the most important transformation process.

• In the absence of light, chemical degradation reactions at Earth surface

temperature and pressure are relatively unimportant compared to biologically

mediated degradation reactions.

Degradation of Organic Compounds

Anaerobic processes

• Anoxic conditions frequently develop in subsurface environments affected by

high concentrations of dissolved hydrocarbons because of rapid aerobic

biodegradation rates and the limited supply of oxygen.

• In the absence of oxygen, the oxidized forms of other inorganic species, and

some organic species such as humic substances, are used by microorganisms

as electron acceptors.

• The most commonly available electron acceptors in subsurface environments

include both solid (such as Fe and Mn oxides) and dissolved (such as nitrate

and sulfate) species.

• In aquifers, as geochemical conditions change, a sequence of reactions occurs,

reflecting the ecological succession of progressively less efficient modes of

metabolism.

Degradation of Organic Compounds

Degradation of Pesticides in Soils

The entire biocide is bioavailable directly after application. Within a few days

there is reversible adsorption to soil particles. Biomineralisation takes place

in the dissolved phase, which leads to the mobilisation of adsorbed biocide

constituents, where the breakdown continues. Thus, there is a maximum for

the amount of bound biocide.

Degradation of Organic Compounds

Behaviour (transport, transformation) of pesticides in soil

Degradation of Organic Compounds

Landfill leachate

Degradation of Organic Compounds

Abiotic process of degradation

Abiotic processes

• Approximately 25% of the average oil spill on the open ocean evaporates. In

the gaseous state, hydrocarbons are readily photooxidized. The dissolved

fraction of petroleum also is subject to photo-oxidation.

• The largest sink for alkanes in the atmosphere is reactions with OH and NO3

radicals (formation of photochemical smog). Mono-aromatic hydrocarbons react

only with OH radicals, forming aldehydes, cresols, and in the presence of NO,

benzylnitrates.

Degradation of Organic Compounds

Benefits of Anaerobic PCB Dechlorination:

• The benefits of anaerobic PCB dechlorination involve reductions in both

the potential risk from and potential exposure to PCBs.

• These reductions in the potential risk from PCBs include reduced dioxin

like toxicity and reduced carcinogenicity.

• The preferential loss of meta and para chlorines catalyzed by anaerobic

dechlorination results in dramatic reductions in the levels of coplanar,

dioxinlike PCB congeners in the mixture.

• These reductions in concentrations correlate with reductions in

ethoxyresorufin- O-deethylase (EROD) induction potency and toxic

equivalency factors for the mixture. Most importantly, these same extensive

reductions are occurring in the environment.

Anaerobic bacteria attack more highly chlorinated PCB congeners through

reductive dechlorination.

This is microbial process that affects the preferential removal of meta and para

chlorines, resulting in a depletion of highly chlorinated PCB congeners with

corresponding increases in lower chlorinated, ortho-substituted PCB congeners.

• The altered congener distribution of residual PCB contamination observed in

several aquatic sediments was the earliest evidence of the anaerobic

dechlorination of PCBs.

• In laboratory the selective removal of meta and para chlorines was recorded.

• The widespread dechlorination of PCBs in aquatic sediments has now been

documented for several river systems.

• Many surveys demonstrate that PCB dechlorination is prevalent in aquatic

sediments.

• Extensive PCB dechlorination has been observed in sediments of the upper

Hudson River.

• Many survey indicates that microbial dechlorination is widespread

throughout these sediments in rivers.

• Extensive changes had occurred in sediments exhibiting a broad range of

PCB concentrations, even as low as 5 ppm.

This suggests that PCB-dechlorinating activity may be the result of a common

reductive pathway present in many different anaerobic microorganisms located

throughout the environment.

• Support for this hypothesis comes from recent efforts demonstrating that

several iron and cobalt heme cofactor systems are capable of reductively

dechlorinating a wide variety of chlorinated organic compounds, including

PCBs.

• Environmental dechlorination is more extensive at higher PCB

concentrations, consistent with the faster dechlorination rates observed at

higher PCB concentrations in the laboratory.

• The reduced carcinogenicity as a result of dechlorination has been

reported.

• In these studies, only the most highly chlorinated PCB mixture (Aroclor

1260, average 6.4 chlorines per biphenyl) resulted in observable cancer

potencies.

• Aroclor 1254 (average 5.1 chlorines per biphenyl) and Clophen A30 did not

demonstrate any tumorigenic effect.

• Clophen A30 is similar in composition to Aroclor 1242, with an average 3.3

chlorines per biphenyl.

• Decreasing PCB chlorination levels and microbial anaerobic PCB

dechlorination therefore reduce carcinogenic potential.

• Further reductions in risk associated with PCB-contaminated sediments

are realized via reduced PCB exposure upon dechlorination.

Further reduction is achieved by the lightly chlorinated PCB

congeners produced upon dechlorination are more readily degraded by

indigenous aerobic bacteria.

Degradation of Organic Compounds Biotic processes

In soils and groundwater, biologically mediated processes dominate. The more

water-soluble components of crude oil and petroleum produces are most

frequently reported in groundwater downgradients from spills and leaks. These

hydrocarbons are biologically reactive and their fate in the subsurface is controlled

by microbiological as well as physical and chemical processes. Certain

microorganisms are able to degrade petroleum hydrocarbons and use them as a

sole source of carbon and energy for growth.

Aerobic processes

Aerobic processes

• Oxygen is the preferred electron acceptor by microorganisms because of the

high-energy yield of these processes.

• Aerobic degradation of hydrocarbons can occur when indigenous populations of

bacteria capable of aerobic degradation of hydrocarbons are supplied with

molecular oxygen and nutrients required for cell growth.

• Studies involving complex mixtures of hydrocarbons have demonstrated that

microorganims can degrade most of the hydrocarbons present in gasoline.

Degradation of Organic Compounds

Degradation of Organic Compounds

Summation curves of oxygen consumption in soil (S) contaminated with oil,

which was mixed with compost (C) in different ratios

BIODEGRADATION OF PCBs

• As a result of their very stable properties, PCBs are synthetic compounds

that are not readily degraded.

• The degradation of these compounds entails difficult mechanisms of

chemical, biochemical or thermal destruction.

• Biodegradation, that is, the degradation of compounds by bacteria or

other microorganisms, is a slow yet possible method for destroying PCBs

in both aerobic and anaerobic environments.

• It is the only process known to degrade PCBs in soil systems or aquatic

environments.

• The specific processes involved are aerobic oxidative dechlorination or

hydrolytic dehalogenation and anaerobic reductive dechlorination.

Degradation of Organic Compounds

PCB Biodegradation: • These compounds have been shown to undergo biodegradation under a

variety of conditions in the laboratory and in the environment.

• Two distinct biological systems capable of biodegrading PCBs have been

identified: aerobic oxidative processes and anaerobic reductive

processes.

• The aerobic bacterial biodegradation of PCBs is widely known and has been

well studied .

• Several microorganisms have been isolated that can aerobically degrade

PCBs, degrading the more lightly chlorinated congeners.

• These organisms attack PCBs via the 2,3-dioxygenase pathway, converting

PCB congeners to the corresponding chlorobenzoic acids.

• These chlorobenzoic acids can then be degraded by indigenous bacteria,

resulting in the production of carbon dioxide, water, chloride, and biomass.

• Moreover, new evidence indicates that the aerobic process is occurring

naturally in undisturbed Hudson River sediments).

• Dechlorination significantly reduces the bioaccumulation potential of the

PCB mixture through conversion to congeners that do not significantly

bioaccumulate in the food chain.

• The lightly chlorinated PCB congeners resulting from dechlorination (e.g.,

2-CB and 2-2-CB)

The complete biological degradation of PCBs should finally

give CO2, chlorine and

• The complete biological degradation of PCBs involves the removal

of chlorine from the biphenyl ring followed by cleavage and

oxidation of the resulting compound.

• Persistence of PCBs in the environment increases with the

degree of chlorination of the congener. The position of chlorine

atoms on the rings also affects the rate of biodegradation

• Polychlorinated biphenyls (PCBs) are the widely distributed

pollutants,

• They are toxic and carcinogenic

• the safe and economical degradation of which is one of the

urgent problem for mankind

• PCBs are strongly resistant to biodegradation due to their

chemical stability.

• Total amount of PCBs having been released into the biosphere

is ca. 750,000 tons.

• Parts of the PCBs have been accumulated and concentrated in the

bodies of fishes, birds and so on.

• The accumulation of PCBs have been observed even in human bodies.

• The toxic symptoms due to PCBs involve headache, pain of joints,

hypertension and so on.

• The PCBs can be degraded using heat, above 1,200°C which ,

however, gives rise to the production of dioxin notorious for its severe

toxity.

• In addition, it is impossible to remove the PCBs which have already

been widely spread over the environment using this method.

• Since 1973, a number of microorganisms that could degrade PCBs

have been isolated and characterized (Ahmed and Focht, 1973;

Furukawa and Matsumura, 1976; Furukawa and Chakrabarty, 1982;

Furukawa, 1982; Bedard et al., 1986; Furukawa and Miyazaki, 1986;

Bedard et al., 1987; Kimbara et al., 1989; Fukuda, 1993).

• The major biodegradation pathway of PCBs in microorganisms has also

been established.

• Thus, four specific enzymes,

(i) biphenyl dioxygenase (BphA),

(ii) dihydrodiol dehydrogenase (BphB),

(iii) 2,3-dihydroxybiphenyl dioxygenase (BphC) and

(iv) 2-hydroxyl-6-oxo-6-phenylhexa-2,4-dienoic acid

hydrolase (BphD)

are sequentially involved in the oxidative degradation of

PCBs into chlorobenzoates and 2-hydroxypenta-2,4-

dienoate (Furukawa & Miyazaki, 1986).

謝謝

Contents and figures have

been taken from various

sources for classroom

teaching