Question

1. Explain or describe Albumin Binding.

2. Explain how oxidation influence Glutathione conjugates, Sulfation, Methylation, and Glucuronidation.

3. List Toxic Xenobiotics Classed by Reactive Metabolites/Intermediates.

4. Explain or describe Phase I Oxidations.

5. Describe a notable feature of flavin monoxygenases.

6. Cytochrome P450 family members can differ in various aspects. The developed nomenclature is based on amino acid homology. List four sources for this nomenclature:

7. Explain or describe Phase I Hydrolysis.

8. Explain or describe glucuronidation.

9. Explain or describe the reaction acetylation.

10. List and explain the five major “complexities” associated with xenobiotic-metabolizing enzyme induction.

Answer 1

8) Glucuronidation is an important reaction of xenobiotic biotransformation in almost all mammalian species, and the process requires a cofactor called uridine diphosphate-glucuronic acid. The reaction is catalised by by UGTs (also known as glucuronyltransferases), which are seen in many tissues. The site of glucuronidation is generally an electron-rich nucleophilic (positive charge loving) heteroatoms like oxygen, nitrogen, or sulfur etc. Human UGTs are a family of enzymes that detoxify hundreds of compounds through their conjugation to glucuronic acid, making them harmless, more water-soluble, and excretable. Genetic inheritance, age, and environmental factors largely influence the different profiles of the inducible hepatic UGTs. Variation in the complement of these UGTs may result in huge differences in the safe elimination of toxic metabolites.

9) Acetylation is a reaction that occurs with amino, hydroxyl, or sulfhydryl groups. The acetyl group is transferred from acetyl-coenzyme A, and the reaction is catalyzed by the enzyme acetyltransferases. In a few cases, the conjugates are further metabolized to toxic compounds. Although acetylation is not a major xenobiotic-conjugation pathways, for number of molecules it is the only reaction utilized for their metabolism. It is a metabolic pathway for amine (almost exclusively aromatic), sulphonamido, and hydrazino functional groups. These functional groups may be present on the parent xenobiotic, or may be generated by phase I metabolism. Drugs that undergo this reaction are p-aminobenzoic acid, aminoglutethimide, p-aminosalicylic acid, dapsone, hydralazine, isoniazid, procainamide, sulfamethazine, sulfamethoxazole, and sulfanilamide etc. Drugs with phase I metabolites that are acetylated include caffeine, clonazepam, nitrazepam, and sulfasalazine. It is considered a phase II reaction that masks reactive centers rather than increasing clearance because the final conjugate is less water-soluble than the parent compound or the phase I metabolite. For bicyclic aromatic amines and heterocyclic amines , acetylation act as a way to bioactivation, and the acetylation “status” or phenotype of the individual may indicate the risk of cancer.An example of acetylation reaction of Isoniazid

7) Phase I metabolism (also termed as non synthetic reaction) includes binding of oxygen, hydrogen, water, or amino acids to the lipid loving (liophilic) drug molecule to expose or introduce a hydroxyl (-OH), amino (-NH₂), sulfhydryl (-SH), or carboxyl (-COOH) polar functional group, thereby increasing the drug's water solubility. These reactions include hydrolysis, reduction, and oxidation.

Hydrolysis

It involves the splitting of the parent drug molecule in prescence of water. A bond in the compound is broken, giving two compounds. At the same time the water molecule splits in two, with a hydrogen transferring to one of the compounds produced and a hydroxide to the other compound.The enzymes of hydrolysis reactions are esterases, peptidases, and amidases. The general equation of hydrolysis reaction is as follows,

R-COO-R' + H₂O ⇒ R-COOH + R'-OH

R-CO-NH-R' + H₂O ⇒ R-COOH + R'-NH₂

Example: conversion of cocaine to benzoylecognine and ecognine methyl ester .

Reduction

It is defined as the reactions resulting in the addition of hydrogen and/or the removal of oxygen. Enzymes involved in reduction reactions are called reductases. The different types of reduction reactions include de-hydroxylation, hydrogenation, decarboxylation, amination, methylation. The general equations of the reactions are shown below,

R-OH ⇒ R-H (de-hydroxylation)

R-C=O ⇒ R-C-OH (hydrogenation)

R-COOH ⇒ R-C=O (decarboxylation)

R-NO₂⇒ R-NH₂ (amination)

R-C-H ⇒ R-CH₃ (methylation)

Example : The inactivation of warfarin by the transformation of a ketone group to a hydroxyl group..

Oxidation

It is defined as the reactions resulting in the addition of oxygen and/or the removal of hydrogen. The enzymes caalysing oxidation include mixed-function oxidases, monoxygenases, and cytochrome P450 enzymes. The different types of oxidation reaction include Conversion of a hydrogen to a hydroxyl group, Conversion of a hydroxyl group to a carbonyl group, Conversion of a carbonyl group to a carboxyl group,Conversion of an amino group to a carbonyl group, Conversion of a methyl group to a hydrogen.

R-H ⇒ R-OH (hydroxylation)

R-C-OH ⇒ R-C=O (dehydrogenation)

R-C=O ⇒ R-COOH (carboxylation)

R-C-NH₂⇒R-C=O (deamination)

R-CH₃⇒ R-H (demethylation)

Example: Hydroxylation of amphetamine to 4-hydroxyamphetamine and norephedrine.

               Hydroxylation of delta-9-THC to 11-OH-delta-9-THC.

The enzymes of oxidation include mixed-function oxidases, monoxygenases, and cytochrome P450 enzymes.