Glutamate oxaloacetate transaminase (GOT)
Aspartate aminotransferase (AST) 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 (which 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.
The enzyme half life is about 22 hours in the dog, 77 minutes in the cat, 7-8 days in horses and around 1 day in cattle.
AST is not organ specific. Skeletal muscle contains the highest concentration, followed by liver and cardiac muscle. Erythrocytes contain enough to raise activity in serum when intravascular or in vitro hemolysis occurs (or RBC membranes are unstable causing leakage without overt hemolysis as occurs in some aged or stored samples). AST is also found in renal epithelial cells and brain tissue. It is located in the cytoplasm and mitochondria as different isoenzymes. Increases in the cytoplasmic AST isoenzyme requires only mild injury (and compared to ALT, AST activity 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.
In the first reaction AST catalyzes the reaction of α-ketoglutarate and L-aspartate which results in the production of L-glutamate and oxaloacetate. This reaction is carried out to equilibrium. Oxaloacetate produced in the first reaction is then reduced by NADH to L-malate under the catalytic action of malate dehydrogenase (MDH). The declining rate of NADH is measured photometrically and is proportional to the rate of formation of oxaloacetate and thus AST activity. The reaction is shown below:
L-aspartate + α-ketoglutarate AST > oxaloacetate+ L-glutamate
Oxaloacetate + NADH + H+ MDH > L-malate + NAD+
Units of measurement
Enzyme activity is measured in U/L (U = international unit) and µkat/L (SI units), which is defined as the amount of enzyme that catalyzes the conversion of 1 µmol of substrate per minute under specified conditions. Conversion formula is shown below:
U/L x 0.0167 = µkat/L
Serum and plasma
Li-heparin or K2-EDTA
The stability of AST in human serum and plasma samples is as follows (per product information sheet): 24 hours at 15 – 25 °C and 7 days at 15 – 25 °C (with P-5-P activation).
- Lipemia/Turbidity: A turbidity index > 150 units may affect results.
- Hemolysis: Will increase AST activity with severe hemolysis (>200 hemolysis index), due to release of intra-erythrocyte AST. Studies in pigs show that values may increase with mild hemolysis (median, 10 U/L) (di Martino et al 2015).
- Icterus: An icteric index > 60 units (about 60 mg/dL) may affect results.
- Drugs: Anticonvulsants may cause an increase in AST activity, which is thought to be secondary to hepatocellular injury in dogs. Corticosteroids generally do not result in increased AST activity, unless they cause hepatocellular injury (in dogs).
Increased AST activity
As a guideline to degree of increase, a 2-3 fold increase (compared to the upper reference limit) is mild, 4-5 fold increase is moderate, and >10 fold increase is marked. Degree of change is variable among clinicians and clinical pathologists.
- Artifact: Intravascular or in vitro hemolysis or leakage from cells can cause erroneously high activity (the enzyme is present in RBC).
- Physiologic: 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.
- 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 (particularly in horses), increases in AST persist for longer than increases in CK activity. Therefore, in chronic muscle disease, AST may be increased, whilst CK activity may be normal. When there is active muscle disease, both CK and AST activities are usually increased (and CK will decline more rapidly as the injury resolves due to the shorter half). In dogs, the degree of increase in AST is proportionally higher than that of ALT activity with muscle injury potentially helping to discriminate between hepatic and muscle sources of ALT activity increases in dogs with severe muscle injury (Valentine et al 1990).
- Liver disease: AST activity will increase in liver disease that causes hepatic injury as for ALT. Increased activity seen with hepatocellular injury often are not as high as those seen with muscle damage. CK activity are normal unless there is concomitant muscle disease. Other liver specific enzymes (SDH or GLDH) would also be increased in activity. In cats, AST appears to be a more sensitive marker of liver injury (activity is often mildly increased with normal ALT activity in conditions such as pyogranulomatous hepatitis secondary to feline infectious peritonitis virus infection). In one study in 7 horses with carbon tetrachloride-induced hepatocellular injury, increases in AST activity lagged behind increases in SDH or LDH5, being above baseline at 24 hours versus 4 hours post treatment. The activity also remained increased for up to 6 days after administration, likely due to the long half-life in horses (Bernard and Divers 1989). In horses with experimental equine parvovirus infection, AST activity does increase just after peak viremia but increases are less consistent and are of lower magnitude than increases in SDH or GLDH activities (Tomlinson et al 2020)
- Hyperthyroidism: Just like ALT, AST activity may be increased in cats with hyperthyroidism. Increases in both enzymes (and ALP) are generally mild.