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Everything you need to know about xenobiotics. Some of the most frequently asked questions are as follows:-
Q.1. What is a xenobiotic?
Ans: The scientists recently have coined a new term for foreign chemicals found in the body, which are called xenobiotics. The term comes from the words xenos, meaning foreign, and bios, meaning life. Xenobiotics are found when the body absorbs chemicals that are not nutrients, or when normally occurring substances (nutrients orendogenously produced biochemical) become denatured and get modified from original molecular structure or are in excess. In other words, xenobiotics are those chemicals, which are normally not produced or expected to be present in an organism, but still they are found in it.
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It also covers substances, which are present in much higher concentrations than are usual. It will be worthwhile to add that a xenobiotic is a foreign compound to which the body is exposed. The compounds referred to as xenobiotics include, medicinal agents (drugs), industrial chemicals, and environmental chemicals.
Q.2. Discuss implications of xenobiotics.
Ans: Natural compounds can also become xenobiotics if they are taken up by another organism (e.g., uptake of natural hormones excreted from humans by fish downstream of sewage treatment plant outfalls). 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 animals to humans. Some authors suggest that we are burdened by as many as 100,000 man-made xenobiotics— and that doesn’t take into account the natural ones.
Q.3. What are the major sources of xenobiotics?
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Ans: The major sources of xenobiotics are food additives such as colourings, flavours, preservatives, etc. The other sources are fungicides, pesticides, herbicides, antibiotic or hormone residues, various other drugs and environmental pollutants such as smoke produced by automobiles, brick-kilns, industries, crackers and fireworks used on Diwali and other festive occasions.
Q.4. Mention the diseases that are associated with xenobiotics.
Ans: A considerable number of clinical studies have revealed that xenobiotic chemicals can cause precise and direct toxic effect on human body.
Diseases directly associated with xenobiotics are as under:
(i) Physical:
Cardiac arrhythmia, eczema, edema, epilepsy, fatigue, headache, hypertension, multiple sclerosis, tinnitus, rheumatism, pain, psoriasis, vasculitis.
(ii) Psychological:
Autism, aggressive behavior, anxiety, fatigue, insomnia, and organic mental disorders.
Diseases that may be triggered or worsened by food allergies are:
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(i) Physical:
AIDS, alcoholism, bronchial asthma, constipation, Crohn’s disease, dermatitis, diarrhoea, gallbladder disease, glaucoma, hypoglycaemia, irritable bowel syndrome, lupus, obesity, osteoarthritis, Raynaud’s syndrome, rheumatoid arthritis, ulcerative colitis, ulcers (duodenal and gastric)and uticaria.
Q.5. How can we get rid of xenobiotics?
Ans: The body gets rid of xenobiotics by xenobiotic metabolism. This consists of the deactivation and the secretion of xenobiotics, and happens mostly at the liver. Secretion routes are urine, faeces, breath and sweat. An example of a group of enzymes involved in xenobiotic metabolism is the hepatic microsomal cytochrome P450s. Because of the significant overload of xenobiotics in our modern environment, we should undertake a light detoxification programme. However, learning the sources of xenobiotics and how to avoid them is even more important.
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Here are some useful suggestions:
(i) Avoid all artificial colours, flavours, preservatives and sweeteners.
(ii) Eat low on the food chain; the closer a food is to its natural state, the less the toxin risk.
(iii) Eat organically grown food as far as possible. Therefore, organic farming should be encouraged.
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(iv) Carefully select body care products, avoiding petroleum-based cosmetics and body care products.
(v) Avoid using chemical-based household cleaners, laundry products and air fresheners. Buy toxin-free, biodegradable products made with natural ingredients.
(vi) Drink spring or filtered water.
(vii) Fill your home and work environments with houseplants that absorb toxic gases from the air. Spider plants, Boston ferns, English ivy and Dracaena are easy to grow and provide natural air pollution control.
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Q.6. What is cause of xenobiotic organic compound proving recalcitrant?
The reason is, the unusual substitution of with chlorine, halogen and heavy metals, unusual bonds or bond resuences at tertiary and quarternary carbon atoms, high condensation of aromatic rings and excessive molecular size as in case of polythene and other plastics.
Q.7. Define biodegradation.
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This is the process of chemical breakdown or degradation of a substance to smaller units, mediated by microorganisms or their enzymes.
Q.8. What is mineralization? Why was this term coined in place of biodegradation?
The mineralization is the term used for microbial breakdown of organic materials into inorganic materials mainly due to microorganism. The term mineralization was required because sometimes there were reports that DDT an insecticide is degradable as that is converted in parts to DDD, DDE and other closely related compounds but others would argue with the plea that DDT is persistent because the basic carbon skeleton remains unchanged.
Q.9. Which are the main categories of chemical compounds that come under genobiotic pollutants?
The broad categories of genobiotic pollutants are:
(i) Recalcitrant hydrocarbons.
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(ii) Haloalkyl propellants and solvents.
(iii) Recalcitrant nitroaromatic compounds.
(iv) Polychlorinated biphenyls (PCBs) and dioxins.
(v) Synthetic polymers.
(vi) Alkyl benzyl sulphonates.
(vii) Petroleum hydrocarbons
(viii) Acid mine drainage.
Q.10. What are recalcitrant halocarbons?
The carbon – halogen bond in them is very stable. Cleavage of this bond is not exothermic and requires a substantial energy input as it is an endothermic reaction. There seems to be little prospect for microorganisms ever to evolve a capability for utilization of extensively halogenated carbon compounds as grow material none the less some cometabolism and photodegradation may occur in them.
Prominent groups in them are C, – C2 haloalkyl propellants, solvents and refrigerants, haloaromatics like chlorobenzenes, chlorophenols and chlorobenzoates, polychlorinated or polybrominated biphenyls and triphenyls, chlorodibenzodioxins and chlorodibenzofurans. Besides them organochlorine insecticides have also been found to be recalcitrants.
Q.11. Give an introduction to halo benzenes, halo phenols and halo benzoates.
Chloro benzenes comprise major industrial solvents. Their aerobic bidegrability declines with the number of halo-substituents but the same halogenated aromatics are dechlorinated anaerobically. On the other hand Pseudomonas and Alcaligenes use the dioxygenase to produce chlorocatechols from aerobic biodegradation of trichloro and tetrachlorobenzenes. The highly chlorinated chlorophenols as pentachlorophenol (PCP) are used as preservatives for wood and canvas. Some chlorophenols are used to make pesticides.
A hope has been generated that PCP is converted aerobically by a monooxygenase to tetrachlorohydro quinone through the oxidative elimination of the chlorine para to phenolic hydroxyl, through a species of Flavobacterium and Mycobacterium chlorophenoticum. Dechlorination step by step, to 2, 5 dichlorohydroquinone is followed by ring opening, further, through anaerobic conditions PCP is reductively dechlorinated step by step to phenol, and by further anaerobic treatment of phenol to methane and carbon dioxide.
Q.12 . How can halo benzoates be degraded?
Chlorophenols and chlorobenzoates can be degraded anaerobically by complex methanogenic or sulphidogenic microbial consortia in the presence of additional carbon sources, however, dechlorinations are quite substrate specific. Desulfomonile tiedjei which is strictly anaerobic rod-shaped sulfidogen removes chlorosubstituents from the Meta position of chlorobenzoates. It also dehalogenates highly chlorinated phenols and PCE.
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Q.13. Can polychlorinated biphenyls are degraded (PCBs)?
The PCBs are mixtures of biphenyls with 1 to 10 chlorine atoms per molecule. Their structures is almost like DDT and are also known to accumulate in higher trophic levels. Although PCBs are relatively resistant to biodegradation yet some microorganisms have be isolated which can transform them. PCBs can be biodegraded aerobically using white rot fungus
Phanerochaete, by Acinetobacter and by Alkaligenes and anaerobically by reductive dehalogenation. The degradation of PCBs is by cometabolism and can be increased by adding less chlorinated analogs like dichlorophenyl.
Q.14. Can synthetic polymers are degraded?
Synthetic polymers are xenobiotics without photodamage in them only the accessible starch granules are degraded to some porosity and brittleness no degradation beyond that has been possible so far. However, efforts are in progress by producing truly biodegradable poly-P-hydroxyalkanoates produced by Alkaligenes eutrophus, Bacillus cereus and some pseudomonads. The intracellular storage products bear thermoplastic properties, therefore, can be moulded. But the price of such a product will be 5 to 7 times more than the petrochemical based polyethylene.
Q.15. What are alkyl benzyl sulphonates (ABS)?
They are main components of anionic detergents. They are surface active with polar sulphate and nonpolar alkyl end in their molecule. Due to their characteristic of emulsification of fatty substances and thereby cleaning occurs while these molecules make a monolayer around lipophilic droplets or particles.
Their molecules orient with their nonpolar end towards the lipophilic substance and the sulphonate end towards water. Nonlinear alkylbenzyl sulphonates are recalcitrants and resistant to biodegradation and cause foaming in the rivers in plenty.
Though ABS are easier to manufacture and bear superior detergent properties but the methyl branching of alkyl chain in them interferes with biodegradation of ABS. It is specifically because tertiary carbon atom blocks the normal p oxidation sequence in the molecule. That is why detergent industry turned from ABS to LAS (linear alkylbenzyl sulphonates), which are biodegradable.
Q.16. Are petroleum hydrocarbons pollutants?
Most xenobiotic pollutants are substituted or modified hydrocarbons. Their short chains are toxic to organisms but they evaporate quickly from the oil slicks. However, very long chain alkanes are resistant to biodegradation. Increasing chain length makes alkanes to exceed molecular weight of 500 and consequently alkanes stop to serve as carbon source.
Moreover, branching often reduces the rate of biodegradation because tertiary and quaternary carbon atoms interfere with degradation or at all do not degrade. Aromatic compounds particularly of condensed polymer type are degraded at a much slow rate in comparison to alkanes.
The alicyclic compound frequently do not serve as the sole carbon source for microbial growth until and unless they are provided with long aliphatic side chain but they cannot be degraded through cometabolism using 2 or more strains that exhibit cooperative mechanisms because of their complementary metabolic ability.
Q.17. What are hydrocarbons?
The hydrocarbons are the substances formed of hydrogen and carbon or the compounds containing carbon and hydrogen only, e.g. benzene, paraffin and coal gas. They occur in petroleum. Therefore, hydrocarbons have also been called petroleum hydrocarbons.
