Aspartate transaminase occurs as cytosolic and mitochondrial isoenzymes and is found in the liver, heart muscle, skeletal muscle, kidney, brain, pancreas, lung, leukocytes, and red blood cells.
It is not as sensitive or specific for the liver as ALT, and the increase in AST can also be seen as secondary to non-hepatic causes.
AST activity in neonates and infants is about twice that of adults, but they decline to adult levels within about six months.