Liver Chemistry Tests - AST, ALT, Alkaline Phosphatase and Bilirubin Metabolism AST and ALT: • AST: liver > skeletal muscle, cardiac muscle > kidney, brain. In liver, 80% of AST is in mitochondria (mAST), Rest cytoplasmic (CAST) • ALT: primarily within liver Alkaline phosphatase: • Synthesis: in hepatocyte canalicular membrane (NOT within bile duct cells) • Other locations: bone, intestine, placenta • Increased levels: due to increased synthesis, not reduced clearance - AP synthesis stimulated by bile acid accumulation in hepatocyte - Elevation usually occurs several days after initial insult Bilirubin metabolism: • Breakdown of heme (80% Hgb, rest myoglobin, cytochromes) • Unconjugated circulates with albumin in serum • Released from albumin upon hepatocyte entry • UGT1A1 responsible for conjugation in hepatocyte • MRP2 transports conjugated bilirubin into bile canaliculi (ATP-dependent process) • MRP3 transports back into plasma where most undergoes reuptake into hepatocyte via OATP1B1 + B3 - Dr. Hersh Shroff @HershShroff #Liver #Chemistry #Tests #Laboratory #Metabolism #ASTALT #AlkPhos #Bilirubin #Hepatology #Pathophysiology #Diagnosis #LFTs