Apoenzyme content of serum aminotransferases in relation to plasma pyridoxal-5'-phosphate concentration.

To investigate the considerable variation in stimulation of serum aminotransferase activities by pyridoxal-5'-phosphate added in vitro, we determined the pyridoxal-5'-phosphate content of plasma, using the tyrosine decarboxylase reaction together with the catalytic activities of alanine aminotransferase and aspartate aminotransferase, with and without pyridoxal-5'-phosphate supplementation, within a group of normal human individuals. We found a very significant inverse linear relationship between plasma pyridoxal-5'-phosphate concentration and stimulation of the activities of these enzymes in serum after supplementation with pyridoxal-5'-phosphate. We conclude that the degree of stimulation of the apoenzyme of the two serum aminotransferases clearly depends on the pyridoxal-5'-phosphate concentration in vivo.