The apo-enzyme content of aminotransferases in healthy and diseased domestic animals.

We investigated the apo-enzyme content of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) in clinically normal and ill canine, feline, equine and bovine patients. Aminotransferase activity was measured with and without the addition of exogenous pyridoxal-5'-phosphate (P5P). The amount of apo-enzyme was expressed as the percentage change in aminotransferase activity with the inclusion of P5P. The results of aminotransferase assays without P5P (holo-enzyme activity) were highly correlated to the results obtained with P5P (total enzyme activity) in all four species (Spearman rank correlations > 0.980). The median apo-aminotransferase percentage in clinically normal patients was: 11% ALT and 0% AST in 115 dogs, 7% ALT and -5% AST in 50 cats, 6% AST in 46 horses and 9% AST in 50 cattle. The amount of apo-enzyme did not increase as holo-enzyme activity increased in any of the species. The apo-aminotransferase content was not significantly different in canine, feline and equine patients with diseases affecting the liver, kidney or musculature compared to clinically normal animals. The apo-enzyme content did not differ significantly between specific liver diseases in canine and feline patients.