Aspartate aminotransferase activity in human serum. Factors to be considered in supplementation with pyridoxal 5'-phosphate in vitro.

The pyridoxal phosphate reactivation of the apo form of aspartate aminotransferase (EC 2.6.1.1) in human serum has been studied with "normal" and above-normal activity of this enzyme. The extent of the reactionation did not depend on the presence of the substrates, L-aspartate or 2-oxoglutarate. Reactivation was greatest with 110 mumol of added pyridoxal phsophate present per liter during a preinucation for 7 min in tris(hydroxymethyl)methylamine buffer wit;h serum volume fractions ranging from 0.017 to 0.267. In comparison with measurements prformed with no exogenous pyridoxal phosphate present, we found two potential sources of error when this cofactor was added: (a) reagent and sample blanks in the pyridoxal phosphate-supplemented system were two- to eightfold higher and (b) progress curves were nonlinear when L-aspartate rather than 2-oxoglutarate was used as the startin substrate. Aspartate aminotransferase measurement sith pyridoxal phosphate supplementation was slightly more precise than without.