Q5 LONG ANSWER 10M ★ High-yield

Classify the enzymes of biological oxidation. Describe the cytochrome P450 system and the phases of drug biotransformation.

Biochemistry · 1st Professional — Pre-Clinical

Definition

Biological oxidation = the enzymatic removal of hydrogen or electrons from a substrate in living cells, releasing energy in controlled steps.

Unlike combustion, it proceeds at body temperature, in small steps, with much of the energy conserved as ATP

Classification of Oxidoreductases

Class	Action	Examples
Oxidases	Transfer electrons to O2, forming water	Cytochrome oxidase
—	Form H2O2 instead	Xanthine oxidase, D-amino acid oxidase, monoamine oxidase
Dehydrogenases	Remove hydrogen; the acceptor is not oxygen	NAD-linked (lactate, malate DH); FAD-linked (succinate DH)
Hydroperoxidases	Use H2O2 as substrate	Catalase, peroxidases
Oxygenases	Incorporate oxygen into the substrate	See below

Oxygenases

Type	Oxygen atoms incorporated	Examples
Dioxygenase	Both into the substrate	Homogentisate oxidase, tryptophan pyrrolase
Monooxygenase (mixed function oxidase)	One into the substrate, one into water	Cytochrome P450, phenylalanine hydroxylase, dopamine β-hydroxylase

Monooxygenase reaction: RH + O2 + NADPH + H+ → ROH + H2O + NADP+

Cytochrome P450

A haem-containing superfamily, named because the reduced form binds carbon monoxide and absorbs maximally at 450 nm
Located chiefly in the smooth endoplasmic reticulum (microsomes) of liver; also mitochondria of adrenal cortex, gonads, kidney
Requires NADPH and molecular oxygen; works with NADPH-cytochrome P450 reductase
Over 50 functional isoforms in man; CYP3A4 handles about half of all drugs

Functions

Drug and xenobiotic metabolism — the largest role
Steroid hormone synthesis — side-chain cleavage, and the 11-, 17- and 21-hydroxylases
Vitamin D activation — 25-hydroxylase in liver, 1α-hydroxylase in kidney
Bile acid synthesis — 7α-hydroxylase
Fatty acid ω-oxidation; prostaglandin metabolism; cholesterol degradation

Biotransformation of Drugs

Phase	Reactions	Effect	Enzymes
Phase I	Oxidation, reduction, hydrolysis	Introduces or exposes a functional group; product may be more or less toxic	Cytochrome P450, esterases
Phase II	Conjugation — with glucuronate, sulphate, glutathione, glycine, acetate, methyl	Makes the compound water-soluble and excretable; almost always detoxifying	UDP-glucuronyl transferase, sulphotransferase, GST, NAT

CLINICAL PEARL

Phase I does not necessarily detoxify. It often creates a more reactive metabolite — paracetamol to NAPQI, benzpyrene to its epoxide. Toxicity arises when phase I outpaces phase II, which is exactly what happens in paracetamol overdose and in the induced liver of a chronic alcoholic.

Induction and Inhibition

Inducers (↑ metabolism, ↓ drug level)	Inhibitors (↓ metabolism, ↑ drug level)
Rifampicin	Cimetidine
Phenytoin, carbamazepine, phenobarbitone	Erythromycin, clarithromycin
Chronic alcohol	Ketoconazole, fluconazole
Cigarette smoke (CYP1A2)	Grapefruit juice (CYP3A4)
St John's wort	Ritonavir, ciprofloxacin
Griseofulvin, glucocorticoids	Acute alcohol, valproate

Induction takes days to weeks, since new protein must be made; inhibition is immediate — a practical difference in predicting interactions
Note that alcohol induces chronically but inhibits acutely

Non-microsomal Oxidation Systems

Enzyme	Location	Substrate	Clinical relevance
Alcohol dehydrogenase	Cytosol	Ethanol, methanol	Ethanol as antidote in methanol poisoning
Aldehyde dehydrogenase	Mitochondria	Acetaldehyde	Disulfiram inhibits it; a common East Asian variant causes flushing
Monoamine oxidase	Mitochondrial outer membrane	Catecholamines, serotonin, tyramine	MAO inhibitors and the cheese reaction
Xanthine oxidase	Cytosol	Hypoxanthine, xanthine	Allopurinol inhibits it
Catalase, peroxidases	Peroxisomes	H2O2	Antioxidant defence

Applied Aspects

Rifampicin causes oral contraceptive failure and destabilises warfarin — the classic examination example of enzyme induction
Paracetamol overdose — conjugation is saturated, P450 makes NAPQI, glutathione is exhausted and hepatocytes die. Alcoholics and the malnourished are at far greater risk, having induced P450 and low glutathione. Treated with N-acetylcysteine
Pharmacogenetics — CYP2D6 shows poor and ultrarapid metaboliser phenotypes; codeine is useless in poor metabolisers, who cannot convert it to morphine, and dangerous in ultrarapid ones
Slow acetylators (NAT2) are prone to isoniazid-induced neuropathy and hepatitis, and to hydralazine-induced lupus — a phase II polymorphism common in India
Neonates conjugate poorly — immature UDP-glucuronyl transferase explains physiological jaundice and grey baby syndrome with chloramphenicol
Grapefruit juice inhibits intestinal CYP3A4 → raised levels of statins, calcium channel blockers and ciclosporin; the effect lasts up to 24 hours
Induction is the reason antitubercular therapy destabilises so many drugs — rifampicin affects contraceptives, warfarin, antiretrovirals and steroids

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