Influence of Pyridoxal-5′-phosphate on the Determination of the Alanine Aminotransferase and Aspartate Aminotransferase of Commercial Test Sera

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Abstract
The influence of pyridoxal-5''-phosphate on the activities of alanine aminotransferase (EC 2.6.1.2) and aspartate aminotransferase (EC 2.6.1.1) from commercial [human, equine and bovine] test sera was investigated. Enhancement of the activity of alanine aminotransferase by the coenzyme is about 20% when 0.1 mmol/l of pyridoxal-5''-phosphate is added to the reaction medium. This concentration seems sufficient for full saturation of the apoenzyme. For aspartate aminotransferase, the enhancement of the enzymatic activity is much higher than for alanine aminotransferase, and 0.1 mmol/l of coenzyme is not sufficient for full saturation in this case. Human sera were freeze dried and the aminotransferase activities determined after reconstitution. A decrease in activity of alanine aminotransferase was observed which could not be restored to the original level by the addition of pyridoxal-5''-phosphate. Freeze drying apparently partially destroys alanine aminotransferase. After freeze drying, the aspartate aminotransferase activity is also somewhat reduced, but the addition of pyridoxal-5''-phosphate restores the activity completely. After freeze drying, the degree of enhancement of the activity by the coenzyme is therefore higher than before freeze drying. Comparing the results obtained from the commercial test sera and the freeze dried human sera, it appears that the freeze drying process might be a reason for the low alanine aminotransferase activities mostly observed in test sera. The freeze drying process could also partly explain why the enhancement of the aspartate aminotransferase activity in commercial test sera by the coenzyme is higher than in normal human sera. Manufacturers of commercial test sera should supply values for aminotransferases which are determined in the absence and in the presence of pyridoxal-5''-phosphate.