aminotransferases

Last reviewed 01/2018

Aminotransferases are enzymes involved with amino acid metabolism. They act in the process of transamination to convert an amino acid into its respective oxoacid by transfer of an amino (-NH2) group to another oxoacid. Aminotransferases require pyridoxal phosphate as a coenzyme.

In hepatocytes, the two clinically relevant aminotransferases have different subcellular localisation:

• aspartate aminotransferase (AST) is found in cytoplasm and mitochondria
• alanine aminotransferase (ALT) is cytoplasmic

AST is abundant in muscles e.g. cardiac and skeletal, kidney, pancreas and erythrocytes.

ALT is predominantly hepatic.

The clinical picture usually permits localisation of the source of enzyme activity. AST is measured most frequently in clinical practise. Where there is doubt about the tissue of origin, ALT or other enzymes e.g. creatine kinase, may be measured.

AST and ALT are released from liver when hepatocytes are damaged or destroyed. Serum activities are increased in virtually all cases of viral hepatitis (the increase may begin up to 14 days before the onset of jaundice) and in cases of hepatocellular damage due to alcohol or other toxic substances.

Cholestasis will increase ALT and AST when associated with hepatocellular death.

In general, increases in AST and ALT are higher when hepatocytes are damaged by viral hepatitis or toxic substances than in biliary obstruction.