In dogs and cats, damage to liver and muscle cells may cause increases in serum alanine aminotransferase (ALT).
ALT is a «leakage» enzyme meaning that the enzyme is intracellular. It leaks out of cells and ultimately into the circulation following damage to the cell membrane.
In other species (e.g., horses, ruminants, pigs, and birds), ALT is less useful because there are not high intracellular levels. Aspartate aminotransferase (AST), sorbitol dehydrogenase (SDH), and glutamate dehydrogenase (GLDH) are the predominant leakage enzyme in these animals (availability is lab-dependent).
The magnitude of increase of leakage enzymes may be deceiving. Severe damage to a healthy liver may result in very high levels, while low levels may be seen when significant atrophy or fibrosis of the liver is present and few cells are left.
Differentiate ALT from muscle vs. liver by evaluation of the clinical picture and other serum biochemistry results. Other muscle enzymes include creatine phosphokinase (CK) and AST; and other liver enzymes are AST, SDH, GLDH, gamma glutamyl transferase (GGT), and alkaline phosphatase (ALP).
Leakage enzymes do NOT provide an estimate of liver function. This is evaluated by measurement of substances dependent on the efficacy of liver function such as bile acids, blood urea nitrogen, albumin, and glucose.
