# Xenobiotic Metabolism and Detoxification

> Xenobiotic metabolism — cytochrome P450, phase I and II reactions, paracetamol poisoning, CYP induction and inhibition for MBBS Biochemistry.

- Canonical URL: https://prepelephant.com/topics/mbbs/biochemistry/xenobiotic-metabolism
- Exam / course: MBBS · Subject: Biochemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Xenobiotic Metabolism and Detoxification", PrepElephant, https://prepelephant.com/topics/mbbs/biochemistry/xenobiotic-metabolism

## Direct answer

Biotransformation converts lipid-soluble xenobiotics — drugs, pesticides, pollutants — into water-soluble products the kidney and bile can excrete, mainly in the liver's smooth endoplasmic reticulum. Phase I chemistry (oxidation by the cytochrome P450 system, plus reductions and hydrolyses) installs or exposes a functional group; phase II conjugation (glucuronidation, sulphation, acetylation, methylation, glycine and glutathione conjugation) masks it with a hydrophilic handle. The same machinery sometimes activates procarcinogens and poisons, and paracetamol overdose — NAPQI formation exhausting glutathione — is the exam's textbook demonstration.

## What you must remember

- Cytochrome P450 is a haem-thiolate mono-oxygenase family of roughly 57 human genes; CYP3A4 metabolises the largest share of clinical drugs (in the order of a third or more).
- Pharmacogenetic classics: CYP2D6 poor metabolisers get no analgesia from codeine (cannot form morphine), while ultrarapid metabolisers (gene amplification) risk toxicity; CYP2C9 and VKORC1 variants guide warfarin dosing.
- Phase II reactions with their donors: glucuronidation (UDPGA, most common — bilirubin, morphine, steroids), sulphation (PAPS), acetylation (acetyl-CoA, via NAT2 fast/slow acetylator polymorphism), methylation (SAM), glycine conjugation (benzoate to hippurate) and glutathione conjugation (mercapturic acid pathway).
- Slow acetylators: isoniazid peripheral neuropathy and drug-induced lupus with hydralazine and procainamide; fast acetylators need higher isoniazid doses and are prone to hepatotoxicity from rapid acetylhydrazine formation — the classic Indian tuberculosis-therapy viva.
- Paracetamol: about 5 per cent is converted by CYP2E1 (also 1A2, 3A4) to NAPQI, detoxified by glutathione; overdose (roughly 150 mg/kg) depletes glutathione, NAPQI adducts hepatocyte proteins, and centrilobular necrosis follows — N-acetylcysteine replenishes glutathione and is the antidote, most effective within 8–10 hours.
- Enzyme induction (rifampicin, phenytoin, carbamazepine, chronic alcohol, St John's wort) causes therapeutic failure through accelerated clearance; inhibition (ketoconazole, ciprofloxacin, cimetidine, grapefruit juice inactivating intestinal CYP3A4) causes toxicity.
- Procarcinogen activation: benzo[a]pyrene in smoke and aflatoxin B1 from Aspergillus-contaminated groundnuts and maize are converted by P450 to epoxides; aflatoxin's signature is the TP53 codon 249 mutation in hepatocellular carcinoma, relevant to Indian food storage.
- Alcohol metabolism: alcohol dehydrogenase (zero-order kinetics), the induced microsomal ethanol oxidising system (CYP2E1) and catalase; excess NADH drives fatty liver, hypoglycaemia and lactic acidosis; disulfiram blocks aldehyde dehydrogenase.
- Phase III elimination: ATP-dependent transport such as P-glycoprotein (MDR1) pumping conjugates and drugs into bile and urine — the basis of multidrug resistance in cancer chemotherapy.

## Reasoning through two overdoses and one interaction

First, a young woman ingests 30 paracetamol tablets and presents after 14 hours. The Rumack–Matthew nomogram plots her four-hour-post-ingestion-equivalent level above the treatment line; whatever the delay, N-acetylcysteine is started because it repletes glutathione and directly detoxifies NAPQI. Second, a patient on stable warfarin is given co-trimoxazole for a urinary infection; three days later the INR is 7 — sulphamethoxazole inhibited CYP2C9, warfarin clearance fell, and the anticoagulant effect surged. The general rule: check every new prescription against the CYP profile of the drugs the patient already takes. Third, a man on rifampicin for tuberculosis starts oral contraceptive pills and conceives — rifampicin induced CYP3A4, accelerating oestrogen breakdown. Induction loses drugs; inhibition accumulates them; and the same P450 that detoxifies can, with aflatoxin on the plate, activate a carcinogen that mutates p53.

## Where students slip

Students assume biotransformation equals detoxification; phase I frequently creates reactive intermediates — paracetamol to NAPQI, aflatoxin to epoxide — so the honest phrase is "biotransformation to more polar, usually less active products". Second, conjugation reactions sound interchangeable but are not all detoxifying: acetylation of isoniazid itself produces the hepatotoxic acetylhydrazine. Third, they forget glucuronidation of bilirubin is the same UGT system that handles many drugs — Gilbert syndrome patients conjugate some drugs sluggishly too. And zero-order kinetics trips candidates: ethanol saturates its dehydrogenase, so a constant amount (not fraction) is metabolised per hour — the reason blood alcohol does not fall faster when you drink more.

## Frequently asked questions

### What are phase I and phase II reactions of xenobiotic metabolism?
Phase I inserts or unmasks functional groups — chiefly cytochrome P450 oxidations, plus reductions and hydrolyses; phase II conjugates the product with glucuronic acid, sulphate, acetate, methyl, glycine or glutathione, rendering it water-soluble for excretion.

### Why is N-acetylcysteine the antidote for paracetamol poisoning?
Overdose floods the CYP2E1 pathway with NAPQI, which consumes hepatic glutathione and then adducts cellular proteins; N-acetylcysteine replenishes glutathione stores and itself binds NAPQI, preventing centrilobular hepatic necrosis when given early.

### What is the clinical impact of CYP induction and inhibition?
Inducers (rifampicin, phenytoin, alcohol) accelerate clearance and cause therapeutic failure of co-administered drugs such as oral contraceptives and warfarin; inhibitors (ketoconazole, ciprofloxacin, grapefruit juice) reduce clearance and cause toxicity.

### What determines fast and slow acetylator status and its consequences?
NAT2 gene polymorphism in the acetylation pathway: slow acetylators accumulate isoniazid (peripheral neuropathy, drug-induced lupus with hydralazine), while fast acetylators clear it quickly but form more hepatotoxic metabolite.

### How does aflatoxin B1 cause hepatocellular carcinoma?
Liver P450 converts it to a reactive epoxide that binds DNA, producing a signature G-to-T mutation at codon 249 of TP53; chronic dietary exposure from fungal-contaminated groundnuts and maize synergises with hepatitis B in hepatocarcinogenesis.