Plasma enzymes diagnostic significance

Enzymes are found at different locations within cells. AST, ALT, and LD are cytosolic enzymes. As such, they can be released with cell injury, and appear in plasma relatively rapidly. In the case of AST and ALT, there are both mitochondrial and cytosolic isoenzymes in hepatocytes and other cells containing these enzymes. In the case of ALT, the relative amount of mitochondrial isoenzyme is small, and its plasma half-life is extremely short, making it of no diagnostic significance. In the case of AST, the mitochondrial isoenzyme represents a significant fraction of total AST within hepatocytes. In contrast, ALP and GGT are membrane-bound glycoprotein enzymes. The most important location of both enzymes is on the canalicular membrane of hepatocytes. 

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