Detoxification & Xenobiotics

Detoxification & Xenobiotic Metabolism

Xenobiotics are foreign compounds (drugs, toxins, environmental chemicals) processed primarily in the liver (also kidney, GI, lung) to make them more water-soluble for excretion. The process is called biotransformation.

Phase I Reactions

Introduce or unmask a polar functional group on the xenobiotic. Usually oxidation, reduction, or hydrolysis. Products may be more or less toxic than parent compound.

  • Cytochrome P450 (CYP) Enzymes: Most important. Located in SER (smooth ER). Require O₂ and NADPH. CYP3A4 metabolizes ~50% of drugs. Family includes CYP1A2, CYP2C9, CYP2C19, CYP2D6. Reactions: Hydroxylation, N-dealkylation, O-dealkylation, sulphoxidation, desulfuration, epoxidation.
  • Monoamine Oxidase (MAO): Deaminates biogenic amines (dopamine, serotonin, NE). MAO-A inhibitors treat depression; MAO-B inhibitors treat Parkinson's (selegiline).
  • Flavin-Containing Monooxygenases (FMO): Oxidize nitrogen and sulfur atoms in drugs.
  • Esterases and Amidases: Hydrolyze ester and amide bonds (prodrug activation — aspirin, codeine → morphine).

Phase II Reactions (Conjugation)

Add a large polar endogenous molecule to the Phase I product → greatly increased water solubility → excreted in urine or bile.

  • Glucuronidation: Most important. UDP-Glucuronyltransferase adds glucuronic acid from UDPGA. Bilirubin, morphine, acetaminophen, steroids. Neonates: Immature → neonatal jaundice, kernicterus. Gilbert's syndrome: Low UGT1A1.
  • Sulfation: PAPS (active sulfate) donor; sulfotransferase. Drugs, steroids, tyrosine derivatives.
  • Glutathione Conjugation: GST adds glutathione to electrophiles. Detoxifies reactive intermediates. NAPQI (from acetaminophen overdose) conjugated by GSH → when GSH depleted → hepatotoxicity. N-acetylcysteine (NAC) replenishes GSH.
  • Acetylation: NAT1/NAT2 enzymes. Slow acetylators (genetic): Isoniazid neuropathy risk; Procainamide lupus. Fast acetylators: less drug efficacy but metabolize faster.
  • Methylation: COMT (catechol-O-methyl transferase) inactivates catecholamines; TPMT methylates thiopurines.

First-Pass Effect

Drugs absorbed from GI tract reach liver via portal circulation before reaching systemic circulation. Extensive hepatic metabolism reduces bioavailability (e.g., nitroglycerin, morphine, propranolol). Overcome by sublingual, IV, transdermal, or rectal routes.

Drug Interactions (CYP)

  • CYP Inducers (↑enzyme → ↓drug levels): Rifampin, St. John's Wort, Phenytoin, Carbamazepine, Alcohol (chronic), Griseofulvin. May cause treatment failure (e.g., OCP failure with rifampin).
  • CYP Inhibitors (↓enzyme → ↑drug levels → toxicity): Ketoconazole, Erythromycin, Cimetidine, Grapefruit juice, Fluoxetine. May cause drug toxicity (e.g., statins + antifungals → rhabdomyolysis).

Bilirubin Metabolism

Heme (from RBC breakdown) → Biliverdin → Unconjugated Bilirubin (UCB — lipid-soluble; transported by albumin) → Liver → Conjugated Bilirubin (water-soluble, to bile) → Urobilinogen (in intestine) → Urobilin (urine, yellow) + Stercobilin (feces, brown).