Glutamate pyruvate transaminase (GPT)
Alanine aminotransferase (ALT) catalyzes the transfer of the alpha amino group of alanine to alpha-ketoglutaric acid, resulting in the formation of pyruvic and glutamic acid. ALT is mostly located in the cytoplasm (with small amounts in mitochondria) and like AST, requires cofactor P5P for maximal enzyme activity.
It is useful as a specific indicator of hepatocellular injury in dogs and cats. ALT is not a useful indicator of liver disease in large animals, and pigs, due to low enzyme activity in liver tissue of these species. SDH and GLDH are viable alternatives to ALT in all species.
Serum half-life is 59 hours in dogs and 2.8-4.4 hours (n=3 after injection of liver homogenate, Nilkumhang and Thornton 1979) in cats. Following acute hepatic injury, serum enzyme activity peaks at about 48 hours and then begins to decrease. Increases in the enzyme occur due to cell damage (increased membrane permeability, blebbing or necrosis), which may be transient or permanent (reversible or irreversible).
ALT is found in the liver, muscle (cardiac and skeletal), kidneys, and erythocytes (in some species). ALT is fairly liver specific in dogs, cats, rabbits, rats and primates. Some increases are possible in severe muscle diseases of the dog and cat due to release of enzyme from this tissue (in this situation, higher increases in AST are typically seen).
The following colorimetric method is used at Cornell University:
This two step process commences with ALT catalyzing the reaction between α-ketoglutarate and L-alanine to form L-glutamate and pyruvate. ALT catalyzes this reaction to equilibrium and the increased levels of pyruvate are quantified by its subsequent reduction by NADH in the presence of lactate dehydrogenase (LDH). Decreased levels of NADH are measured photometrically and are proportional to the rate of formation of pyruvate, which correlates to ALT activity. Reaction is shown below:
L-alanine + α-ketoglutarate ALT > pyruvate + L-glutamate
Pyruvate + NADH + H+ LDH > L-lactate + 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. The conversion formula is shown below:
U/L x 0.0167 = µkat/L
Serum and plasma
The stability of ALT activity in human serum and plasma samples is as follows (per product information sheet): 3 days at 15-25 °C, 7 days at 2 – 8 °C, and more than 7 days at (-60)-(-80) °C.
- Lipemia/Turbidity, icterus: No significant interferences.
- Hemolysis: In some species, such as cats and pigs, intravascular or in vitro hemolysis may increase activity due to the ALT release from erythrocytes. In pigs, hemolysis of >100 units increased ALT by a median of 8 U/L (di Martino et al 2015).
- Drugs: Anticonvulsants (primidone, phenobarbitone, dilantin) can increase ALT activity up to 4 x normal. Although these drugs were thought to induce ALT synthesis, increases in ALT activity are mostly thought to be secondary to hepatocellular necrosis. Corticosteroids increase ALT to approximately 2-3 x normal. Activity is higher in dogs with steroid hepatopathy (where actual hepatocellular injury occurs). This is currently attributed to altered permeability and not due to increased synthesis. Any drugs that can cause hepatotoxicity can result in increased ALT activity, e.g. tetracycline in cats, caparsolate in dogs, acetaminophen. Certain drugs may decrease ALT (and AST) activity, by impairing activation of vitamin B6 to P5P, e.g. cephalosporin, cyclosporin, isoniazide.
Increased ALT 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. Quantification of the degree of change is variable among clinicians and clinical pathologists.
- Artifact: Intravascular or in vitro hemolysis may cause increased levels in the cat or pig. Cats have a high RBC to plasma ALT ratio. In contrast, hemolysis (intravascular or in vitro) has a minimal effect on ALT in cattle, horses, and dogs.
- Liver disease: Increases in ALT activity can be seen with both primary and secondary hepatic disease, if altered cell membrane permeability or necrosis occur, and are not specific for the cause of hepatic disease. Highest increases are seen in necrotizing or inflammatory conditions and some diseases, e.g. hepatocellular carcinoma, hepatic insufficiency, advanced cirrhosis, chronic hepatitis, may not be associated with any increases in ALT activity if active hepatocyte injury is not occurring. Usually ALT activity exceeds AST activity in liver disease. Bile duct obstruction or other causes of cholestasis may increase ALT (and AST) activity due to the toxic effects of retained bile salts on hepatocytes. With cholestasis, the fold increases in cholestatic enzymes (ALP, GGT) and bilirubin are usually higher than the fold increases in liver leakage enzymes, which can help identify the primary pathologic process as cholestasis, which is then leading to liver injury. When liver injury causes cholestasis, the fold increases in leakage enzymes will be higher than that of the induction enzymes and total bilirubin concentrations may be normal or only mildly increased. An example of the latter scenario is chronic hepatitis in dogs, in which higher fold increases in ALT (7.5x) than ALP (5.5x) are seen and increases in ALT activity is considered one of the earliest biochemical markers for chronic hepatitis (particularly an unexplained increase that is present for >2 months), with increases in cholestatic enzymes (GGT, ALP) occurring later in the condition (Webster et al 2019). Trauma will often increase ALT activity, even without morphologic evidence of cell injury.
- Muscle disease: In large animals, ALT activity will increase with muscle injury and is not more useful than AST in this regard so it is not included on large animal chemistry panels. In horses, the highest ALT activity is seen in homogenates of heart and muscle with lower activity in liver. By virtue of tissue mass, the main source of ALT activity in horses is skeletal muscle (Johnson et al 2019). In small animals with severe muscle injury (ischemic myopathy in cats, muscular dystrophy in dogs, where CK activity is usually > 10,000 U/L), ALT will increase with CK and AST activities. However, the increases in ALT activity are usually less than increases in AST activity in primary muscle disease and SDH values should be normal (unless there is concurrent liver injury) (Valentine et al 1990).
- Hyperthyroidism: Cats with hyperthyroidism often have mild increases in ALT, AST and ALP activities.