Liver Isoenzymes: Metabolic, Detox & Synthetic Functions

Liver isoenzymes are tissue-specific variants of enzymes that catalyze hepatic metabolic, detoxification, and synthetic reactions. Key systems:

1. Cytochrome P450 (CYP) Superfamily

• Phase I oxidation: CYP3A4 (50% drugs), CYP2D6, CYP2C9, CYP2C19, CYP1A2.
• Genetic polymorphisms → poor/ultrarapid metabolizers (dosing implications).
• Inducers (rifampin, carbamazepine, St. John's wort) ↑ clearance; inhibitors (ketoconazole, grapefruit) ↓ clearance.

2. Phase II Conjugation

• UGTs (glucuronidation), SULTs (sulfation), GSTs (glutathione), NATs (acetylation), TPMT (thiopurines).
• Increase hydrophilicity for biliary/renal excretion.

3. Metabolic Isoenzymes

• Glycolysis/Gluconeogenesis: Hexokinase IV (glucokinase), PEPCK, FBPase-1.
• Lipid: HMG-CoA reductase (cholesterol synthesis), ACC (fatty acid synthesis), CPT1 (β-oxidation).
• Amino acid: Transaminases (ALT, AST), urea cycle enzymes (CPS1, OTC).

4. Synthetic & Secretory

• Albumin, coagulation factors (II, VII, IX, X, fibrinogen), complement, acute-phase proteins.
• Bile acids (CYP7A1 rate-limiting) → micelles for fat absorption.

5. Clinical Assessment

• Serum ALT/AST (hepatocellular injury), ALP/GGT (cholestasis), bilirubin, albumin, INR, platelet count.
• Child-Pugh, MELD scores quantify synthetic dysfunction.

Liver isoenzyme dysregulation underlies drug toxicity, metabolic disease, and liver failure.