Importancia de la formación del profesional en el ámbito de la docencia.

2.0 Objectives 3.0 Main Content

3.1 Mechanism of Detoxification 3.2 Detoxification Reactions

3.3 Excretion of Detoxified Xenobiotic 4.0 Conclusion

5.0 Summary

6.0 Tutor Marked Assignment 7.0 References/Further reading 1.0 INTRODUCTION

Humans are exposed to various foreign chemicals (called xenobiotic) from air, nutrition (food additives, preservatives), therapy for disease (drugs) etc. Hence an understanding of the ability of the body to cope with the chemical onslaught at the cellular level is an important aspect of metabolism. Detoxification is a series of biochemical reactions that convert converts toxic substances into less harmful products and then into more easily excretable forms. Detoxification is often referred to as drug metabolism. All the detoxification reactions majorly occur in the liver, however, the kidney and intestines are also involved to a lesser extent. Several unwanted and harmful substances e.g. drugs get entry into the body by absorption from the gastrointestinal tract and so need to be eliminated regularly to prevent accumulation and prolonged cumulative action.

Compounds that are detoxified are foreign compounds or xenobiotic such as drugs, food additives, insecticides, pollutants etc, compounds produced in the body which are to be eliminated e.g. bilirubin, steroids ammonia etc and compounds produced in the intestine by bacterial putrefaction and fermentation e.g. indole and skatole (from tryptophan), histamine (from histidine), tyramine (from tyrosine) etc.

2.0 OBJECTIVES

On the completion of this unit you will get to know the definition of detoxification, mechanism and reactions of detoxification, and the excretion of xenobiotic.

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3.0 MAIN CONTENT

3.1 Mechanism of Detoxification

The overall mechanism of detoxification is to increase the water solubility (polarity) of toxic products and thus facilitate their excretion from the body mostly in urine and also through bile and faeces. Toxic products are rendered water-soluble by adding polar functional groups/conjugates.

Toxic compounds / products (less polar)

Addition of polar functional groups/conjugates

Less toxic compounds

(more polar, water soluble, easily excretable forms)

Excreted through urine / bile / faces

Figure 4.1: Mechanism of detoxification 3.2 Reactions of Detoxification

Foreign substances undergo some preliminary changes which are not different from the metabolic changes occurring to physiological substances. Detoxification reactions are grouped into two phases namely Phase-I and Phase-II detoxification reactions.

Phase I

Phase I detoxification reaction involves oxidation (also known as hydroxylation), reduction or hydrolysis. Minor reactions like deamination, desulfuration, dealkylation etc. are also induced under Phase I

a. Detoxification by oxidation: In this reaction hydrocarbons are oxidized to their corresponding alcohols; alcohols to aldehydes

and aldehydes to acids. Also aromatic hydrocarbons are reduced to phenols.

Example is the oxidation of methanol to formic acid, oxidation of benzene to benzoic acid.

Cytochrome P450 (hemeproteins) are enzymes involved in oxidation reaction and are located in the liver microsomes (endoplasmic reticulum). Fifty percent (50%) of drugs are detoxified through this system.

Methanol (CH₃-OH) Formic acid (H-COOH)

Benzoyl Benzoic acid

b. Detoxification by reduction: This reaction involves the addition of hydrogen atom to hydrocarbons. Examples are given below:

Nitrobenzene Amino benzene (Aniline)

Chloral Trichloroethanol

(Chloral is a sedative)

c. Detoxification by hydrolysis: This reaction involves the hydrolysis of ester, amide, glycosidic bond, ether bond etc by the addition of water. Also Drugs like procaine and acetyl salicylic (aspirin) acid and cardiac glycosides e.g. digitalin, atropine undergo hydrolysis.

H₂O

Aspirin salicylic acid + acetic acid

H₂O

Atropine tropic acid + tropine Phase-II

Phase-II detoxification process consists of conjugation reaction. This reaction involves the coupling of the foreign substances after the processes of oxidation, reduction and hydrolysis with some polar substances in the body making the toxic compounds more water soluble and hence easily excretable. The following are some examples:

a. Glucuronic acids: Conjugation with glucuronic acid is the most common conjugation reaction. UDP (uridine diphosphate)- glucuronic acid is the glucuronyl donor, UDP-glucuronyl

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transferases are the enzymes. Aromatic acids (e.g. benzoic acid) and phenols are conjugated with glucuronic acid. The glucuronic acid is derived from uridine diphosphate glucuronic acid. The drug chloramphenicol and the bile pigments are among the important substances conjugated with glucuronic acid.

Derivatives of steroid hormones also are conjugated with glucuronic acid before excretion.

b. Active sulfate: It is used to conjugate phenolic compounds. The derivatives are called ethereal sulfates. An increase in their amount in urine excessive intestinal putrefaction or stasis.

Adrenal cortical hormones are also excreted after conjugation with sulfuric acid. Yeast and mammalian liver contain enzymes that can activate inorganic sulfate by adding it to 3-phosphoadenosine-5-phosphate. Active sulfate is adenine-3-phosphoribose-5-phosphosulfate. Incorporation of sulfate into sulfated mucopolysaccharaides and conjugation of steroid hormones and others with sulfate are brought about after preliminary activation of sulfate.

c. Glycine: This is used to conjugate aromatic acids, cholic acid and also nicotinic acid. The formation of bile acid (glycocholic acid) from cholic acid is also brought about by conjugation with glycine.

d. Cysteine: This is used in the conjugation of aromatic compounds like benzene and halogenated ring compounds like bromobenzene.

e. Acetic acid: Acetic acid is used to conjugate with aromatic amino compounds like sulfanilamide.

f. Glutamine: This is conjugated with phenylacetic acid to form phenylacetylglutamine.

g. Methyl groups: Active methionine (s-adenosyl methionine) is used for the conjugation of certain pyridine and other heterocyclic nitrogen containing compounds like nicotinamide.

h. Interaction of highly toxic cyanides: There may also be conjugation with thiosulfate to form relatively nontoxic thiocyanates. The enzyme rhodanase converts cyanide to thiocyanate.

3.3 Excretion of detoxified xenobiotic

i. The products of phase-I reactions are either excreted directly or undergo further metabolism by phase-II reactions and then excreted.

ii. In some cases, a xenobiotic may undergo only phase-II reaction without undergoing phase-I reaction.

iii. Occasionally, a xenobiotic may be excreted unchanged.