Aminotransferases

Aminotransferases are enzymes that catalyze the reversible transfer of an amino group from an amino acid to a keto acid, a central step in amino acid metabolism and nitrogen transport. Most aminotransferases require pyridoxal phosphate (vitamin B6) as a cofactor and function in cellular pathways that connect amino acid catabolism with energy production and gluconeogenesis.

The most widely known aminotransferases in clinical medicine are aspartate aminotransferase (AST) and alanine aminotransferase (ALT). AST catalyzes the transfer between aspartate and oxaloacetate, while ALT transfers an amino group from glutamate to pyruvate to form alanine and alpha-ketoglutarate. ALT is more specific to the liver, whereas AST is found in liver, heart, skeletal muscle, kidneys, brain, and erythrocytes. Both exist in cytosolic forms, and AST has an additional mitochondrial form; ALT is primarily cytosolic with a mitochondrial counterpart in some tissues.

In clinical practice, aminotransferase activities are measured in blood and used as indicators of tissue injury. Elevations of ALT and AST generally reflect hepatocellular injury, but AST is less liver-specific due to its broad tissue distribution. The AST/ALT ratio (the De Ritis ratio) can help narrow the cause of liver injury, with higher ratios suggesting alcoholic liver disease in some contexts. Mild, non-specific elevations can occur with obesity, diabetes, medications, or systemic illness. Sample handling and other lab factors can influence results, so aminotransferase levels are interpreted alongside clinical findings and other tests.