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
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