Enzymes

Several enzymes are produced by and concentrated in hepatobiliary tissues, reflecting the varied and high metabolic activity of this organ. Many of these enzymes are also produced by other tissues in the body; however, increased serum activities may be restricted to just a few of these tissues. Alanine and aspartate aminotransferases (ALT and AST, respectively), and sorbitol and lactate dehydrogenases (SDH and LDH, respectively) are contained within the cytoplasm of hepatocytes. A mitochondrial isoenzyme of AST also exists, whereas glutamate dehydrogenase (GLDH) is strictly contained within hepatocyte mitochondria. Alkaline phosphatase (ALP) and gamma-glutamyltransferase (GGT) are found within cell membranes of hepatocytes (particularly canalicular surfaces) and bile duct epithelium, respectively. Some of these enzymes are liver-specific, and others are not. Species variability also exists. Increased enzyme activity in serum is due to leakage from injured cells, increased enzyme production, or both. The magnitude of the increased activity is not closely correlated with the type of disease or whether the lesion is reversible or irreversible.