transaminase

Transaminases catalyze reversible transamination reactions, transferring an amino group from an amino acid to an alpha-ketoacid, using pyridoxal phosphate (vitamin B6) as a cofactor. ALT converts alanine to pyruvate, while AST converts aspartate to oxaloacetate; both reactions link amino acid metabolism to the Krebs cycle and gluconeogenesis (alanine cycle). Clinically, serum ALT and AST are used as liver function tests, though they actually reflect hepatocellular injury rather than synthetic function. ALT is more liver-specific, while AST is also found in cardiac muscle, skeletal muscle, kidney, and brain, making it less specific for liver pathology. In viral hepatitis, ALT is typically greater than AST. In alcoholic liver disease, AST is typically greater than ALT (often with a ratio >2:1), due to mitochondrial injury and B6 deficiency. Markedly elevated transaminases (in the thousands) suggest acute hepatocellular necrosis from causes like acetaminophen toxicity, ischemic hepatitis, or acute viral hepatitis. Chronic mild elevations are seen in NAFLD, chronic viral hepatitis, and autoimmune hepatitis. AST is also elevated in myocardial infarction (though CK-MB and troponin are more specific) and muscle disorders like rhabdomyolysis. Vitamin B6 deficiency can falsely lower AST/ALT levels due to reduced cofactor availability.