alanine aminotransferase

ALT is found predominantly in the cytoplasm of hepatocytes, making it a more liver-specific marker than aspartate aminotransferase (AST), which is also present in cardiac muscle, skeletal muscle, kidney, and red blood cells. When hepatocytes are damaged (due to viral hepatitis, drug-induced liver injury, alcohol use, ischemia, or fatty liver disease), ALT leaks into the bloodstream, causing serum levels to rise.

Clinically, the AST:ALT ratio is diagnostically useful: in alcoholic liver disease, AST > ALT (typically ratio >2) due to mitochondrial injury and pyridoxal phosphate deficiency affecting ALT synthesis, whereas in viral hepatitis and non-alcoholic fatty liver disease (NAFLD), ALT > AST. Marked elevations (>1000 U/L) suggest acute hepatocellular necrosis, as seen in acetaminophen toxicity, ischemic hepatitis, or acute viral hepatitis (especially hepatitis A). Mild-to-moderate elevations are seen in chronic hepatitis, NAFLD/NASH, and hepatotoxic drug reactions (e.g., statins, isoniazid, methotrexate).

ALT requires vitamin B6 (pyridoxal phosphate) as a cofactor, which is relevant in chronic alcoholics who may have B6 deficiency, artificially lowering ALT relative to AST. ALT is used alongside AST, alkaline phosphatase, GGT, and bilirubin to characterize the pattern of liver injury (hepatocellular vs. cholestatic). It's a standard component of liver function tests (LFTs), though technically it reflects hepatocellular integrity rather than synthetic liver function (which is better assessed by albumin, PT/INR).