Aspartate aminotransferase - AST
Glutamic oxaloacetic transaminase, GOT

Aspartate amino transferase catalyzes the transfer of the alpha amino group of aspartic acid to alpha-ketoglutaric acid, resulting in the formation of oxaloacetic acid and glutamic acid. Both aminotransferases (ALT and AST) require pyridoxal 5' phosphate (P5P) as an essential cofactor for maximum enzyme activity. P5P is the active metabolite of vitamin B6, therefore reduced vitamin B6 (as occurs rarely in animal patients with liver disease or on certain drugs) can result in decreased aminotransferase activity, unless P5P is included in the assay system for the aminotransferases (as done here at Cornell). AST is useful as an indicator of liver and/or muscle injury in large and small animals.

Organ specificity

AST is not organ specific. Skeletal muscle contains the highest concentration, followed by liver and cardiac muscle. Erythrocytes contain enough to raise levels when hemolysis occurs. AST is also found in renal epithelial cells and brain tissue. It is located in the cytoplasm and mitochondria as different isoenzymes. Elevations in the cytoplasmic AST isoenzyme requires only mild hepatocellular injury (and compared to ALT, AST levels may increase less in relatively mild hepatocellular injury), whereas release of the mitochondrial isoenzyme requires (and indicates) more severe cellular injury. Isoenzyme differentiation is not performed in veterinary medicine.
The enzyme half life is about 22 hours in the dog, 77 minutes in the cat and 2 or more days in large animals.

Causes of increased AST
  • Artifact: Hemolysis or leakage from cells can cause erroneously high values (enzyme is present in RBC).

  • Drug effects: Anticonvulsants may cause an increase in AST, which is thought to be secondary to hepatocellular injury in dogs. Corticosteroids generally do not result in increased AST levels, unless they result in a steroid hepatopathy (in dogs).

  • Physiologic effects: In horses, exercise can increase serum activity as much as 30%. In early training, resting levels are 50-100% greater than resting levels of horses not in training.

  • Disease effects
    1) Myopathies: Muscle trauma (including "down" animals), rhabdomyolysis, white muscle disease (vitamin E-selenium deficiency), and infectious myositis (black leg or Clostridial myositis), and muscular dystrophy may result in marked increases. Serum CK activity will also increased. Note that as AST has a longer half life than CK, increases in AST persist for longer than increases in CK. Therefore, in chronic muscle disease, AST may be elevated, whilst CK levels may be normal. When there is active muscle disease, both CK and AST are elevated (and CK will decline more rapidly as the injury resolves).

    2) Liver disease: AST will increase in liver disease with the same causes as for ALT. Increased levels seen with hepatocellular injury often aren't as high as those seen with muscle damage. CK levels are normal unless there is concomitant muscle disease. Other liver specific enzymes (SDH) would also be increased. In cats, AST appears to be a more sensitive marker of liver injury (values are often mildly increased with normal ALT in conditions such as pyogranulomatous hepatitis secondary to feline infectious peritonitis virus infection).
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