A xenobiotic (toxicant) may be absorbed into an animal via different routes. After absorption it is distributed to different parts of the body and, finally, is available for excretion. Many xenobiotics are known to undergo biotransformation while in the organs and tissues. Biotransformation is also termed metabolic transformation.
Definition of Biotransformation:
Biotransformation may be defined as “the biologically catalysed conversions of chemicals, other than the normal body constituents into other chemicals”.
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Or,
Biotransformation is a process that, in general, converts the parent xenobiotic compounds or toxicants into their metabolites and then form conjugates and facilitate their release from the body.
Or,
The biochemical modification of the xenobiotic (toxicant) molecules through the living cell is termed biotransformation or metabolic transformation.
Or,
The act of reduction in the potentiality of the toxicant by the internal system of animal may be termed biotransformation.
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During biotransformation, generally the parent toxic compounds undergo detoxification i.e., the formed metabolites and, finally, conjugates prove to be lesser toxic than the original or parent toxicants. But in certain cases the metabolites have been proved more toxic than the parent xenobiotics (toxicants) and such reaction is termed bioactivation.
Biotransformation Sites:
The organ/tissue wherein the biotransformation takes place is called biotransformation site. The biotransformation of xenobiotics are often catalyzed by the enzymes which chiefly occur in the liver of vertebrates. In the vertebrates these enzymes also occur in the skin, kidney, lungs, intestine, placenta, gonads, embryonic liver, aorta, lymphocytes, blood platelets, adrenal cortex and medulla but not in nervous system, though their activity have been anticipated. In insects, such enzymes have been reported in mid-gut, fat body and Malpighian tubules.
The most important biotransformation sites in the vertebrates including human are- liver, kidney, intestine, skin and lungs where the biotransformation takes place in the following order:
Liver > Kidney > Intestine > Lungs > Skin
Principal Objectives of Biotransformation:
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The principal objectives of the biotransformation processes are to detoxify the toxicants and their elimination from the body.
It includes:
1. Conversion of water-insoluble substances to water-soluble form,
2. Emulsification of substances with bile, so as to facilitate elimination in colloidal form with feces,
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3. Incorporation of toxic molecules within inactive proteins, for example, metal molecules in metallothionien protein and formation of metalloprotein for elimination.
Nature of Biotransformation Enzymes:
Actually the enzymes involved in biotransformation of xenobiotic compounds (toxicants) have relatively low degree of substrate specificity in comparison to those involved in the metabolism of constitutive chemicals.
The responsible enzymes in biotransformation of xenobiotics are monooxygenases or cytochrome P-450 species.
The reaction catalyzed by a mono-oxygenase is:
RH + O2 + NADP + H+ → R-OH + HOH + NADP+
RH represents a very wide variety of xenobiotics including endogenous compounds such as steroids.
The reaction catalyzed by cytochrome P-450 may also be represented as:
Reduced cyt. P-450 → Oxidized cyt. P-450.
RH + O2 → R-OH + HOH
The major mono-oxygenases in the ER are cytochrome P-450 species. Cytochrome P-450 is notable because of the fact that approximately 50% of the drugs ingested are metabolized by species of cytochrome P-450. The same enzyme also acts on various carcinogens and pollutants.
Salient Features of Cytochrome P-450 Species:
1. These are hemoproteins.
2. These are present in high concentration in the membranes of the endoplasmic reticulum (ER) or liver.
3. These are also present in the mitochondria as well as in the ER of the adrenal gland.
4. Six closely related species of cytochrome P-450 present in the liver ER that act on a wide variety of drugs, carcinogens, and other xenobiotics. In recent years, the genes for these species of cytochrome P-450 have been isolated and studied.
5. The enzyme that uses NADPH in the reaction mechanism of cytochrome P-450 is called the NADPH-Cytochrome P-450 reductase.
6. The phosphatidyl choline, the major lipid found in the membranes of ER, is the principal component of the cytochrome P-450 system.
7. Most species of cytochrome P-450 are inducible e.g., the administration of phenobarbital (PB) causes a hypertrophy of the smooth ER and a 3 to 4 fold increase of the amount of cytochrome P-450 within 4-5 days. The mechanism of induction involves increased transcription of mRNA for cytochrome P-450.
8. Another species called cytochrome P-448 is specific for the metabolism polycyclic aromatic hydrocarbons (PAHs) and related molecules; hence it is said to be aromatic hydrocarbon hydroxylase (AHH). This enzyme is very important in the metabolism of PAHs and in carcinogenesis produced by these agents. Researches reveal that smokers have higher levels of this enzyme in some of their cells and tissues than do non-smokers. It is noticed that the activity of this enzyme may be increased in the placentae of women who smoke, thus altering the quantities of metabolites of PAHs to which the fetus is exposed.
The biological significance for the presence of these enzymes may be to evolve a protective device against xenobiotic compounds.
Mechanism of Biotransformation:
R. T. Williams, in 1959, firstly studied the mechanism of biotransformation of xenobiotics and divided the entire steps taking part in enzymatic biotransformation into two phases:
1. Phase I reactions or non-synthetic reactions involving oxidation, reduction and hydrolysis.
2. Phase II reactions or synthetic reactions involving the formation of a conjugate, i.e., metabolites of parent toxicant + endogenous polar or ionic moiety.
To sum up, biotransformation is such a process in which the parent xenobiotic compound is converted into metabolites and then conjugates are formed. For example, benzene undergoes oxidation, a phase I reaction, to form phenol, which conjugates with sulphate, a phase II reaction. However, some chemicals form conjugates without proceeding in phase I reaction.
For example phenol may conjugate with sulphate without proceeding in phase 1 reactions. The metabolites and conjugates are usually more water-soluble and polar, hence more readily excretable. Biotransformation can, therefore, be considered in general as a mechanism of detoxification by the host animal.
The type and rate of biotransformation of any xenobiotic compound differs from one species of animal to another and even from one strain to another, which often accounts for the difference of toxicity in animals. Apart from age and sex of the animal, exposures to other chemicals may also alter the biotransformation. Knowledge of such factors is important in designing the toxicological studies and in also the interpretation of possible health hazards to humans.
Complex Nature of Biotransformation:
Certain toxicants in their mode of action during biotransformation show a great degree of complexity because they generally undergo various types of biotransformation, forming a variety of metabolites and conjugates. Some of the metabolites and conjugates of bromobenzene and carbaryl are shown in Figs. 21.1 and 21.2, respectively.
WHO (1971) reported that organophosphorus insecticides, viz. fenitrothion, omethoate, etc., can be metabolised through dealkylation, oxidation, hydrolysis or desulphuration, yielding ten or more different metabolites. Parathion, which is an organophosphorus pesticide, is bio-activated in the liver to paraoxon, which is much more potent cholinesterase inhibitor.
Factors Affecting Biotransformation:
The liver is the main organ wherein biotransformation of xenobiotics takes, place. However, diseases such as acute and chronic hepatitis, cirrhosis of the liver, and hepatic necrosis often decrease the biotransformation.
The biotransformation capability of infants of premature births is extremely low in comparison to the adults.
Certain other factors that affect the efficiency of liver to bio-transform the toxicant/drug are- nutritional status, sex, age, procedure of administration of the drug and duration of drug administered.
Starvation of organisms results in decrease in the levels of biotransformation enzymes. Therefore, starved animals are often more sensitive to toxicants than the normally fed individuals.
The biotransformation of toxicants is catalyzed by the MMFO. A deficiency of essential fatty acids generally depresses MMFO activities. This is also true with protein deficiency. Deficiency of certain vitamins like vits. A, C and E depresses the MMFO. However, thiamine deficiency has the opposite effect.
Some foods contain appreciable amount of chemicals that are actually strong inducers of the MMFO, e.g., safrole, xanthines, and indoles. In addition, potent inducers such as DDT and PCB are present as contaminants in many foods.
The rate of biotransformation of xenobiotics varies according to the sex of the organism. For example, adult male rats bio-transform xenobiotics at high rates than those of adult females.
Environmental factors like temperature and ionizing radiations also affect the biotransformation process. For instance, exposure of rodents to ionizing radiations reduce the rate of biotransformation of xenobiotics. It is because ionizing radiations have been reported to reduce the hydroxylation of steroids, desulphuration activity and glucuronide formation.
Bioactivation:
The conversion of certain chemically stable compounds to highly chemically reactive metabolites is termed bioactivation. In other words, bioactivation is the biotransformation in which the formed metabolite proves to be highly toxic than the parent compound, i.e., toxicant or drug. These reactive compounds become covalently bound to tissue macromolecules and cause injury. In addition, other types of metabolites produce deleterious effects via other mechanism also.
The biotransformation of bromobenzene to its epoxide and subsequent reactions serve as an interesting example of bioactivation and its consequences. Although bromobenzene epoxide may become covalently bound to tissue macromolecules and cause injury, the alternative routes of metabolism may prevent or reduce the injury.
Likewise parathion, an organophosphorous pesticide, is bioactivated in the liver to paraoxon, which is much more potent cholinesterase inhibitor.
Important site of bioactivation of many toxicants is liver; hence it is a common target organ. Liver is regarded as the largest metabolic gland. However, if the metabolites are sufficiently stable, they may affect other organs after being transported there (e.g., bromobenzene on the kidney). In rare cases, other sites may be lungs, stomach, adrenal, retina and bone marrow.