Aspartate Aminotransferase Activity in Human Serum. Factors to Be Considered in Supplementation with

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 tiis enzyme. The extent of the reactivation did not de-pend on the presence of the substrates, L-aspartate or 2-oxoglutarate. Reactivation was greatest with 110 zmol of added pyridoxal phosphate present per liter during a preincubation for 7 mm in tris(hydroxymethyl)methylamine buffer with serum volume fractions ranging from 0.017 to 0.267. In comparison with measurements performed 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 phos-phate-supplemented system were two- to eightfold higher and (b) progress curves were nonlinear when L-aspartate rather than 2-oxoglutarate was used as the starting sub-strate. Aspartate aminotransferase measurement with pyridoxal phosphate supplementation was slightly more precise than without. The increase in aspartate aminotransferase (ASAT)3 activity in serum (1) and erythrocytes (2) after in vitro supplementation with pyridoxal phosphate has proven useful for detecting nutritional deficiency of vitamin B6. This finding was the major argument for the proposal by Hamfelt a decade ago (3) that pyridoxal phosphate should be added when ASAT is measured in serum, and subsequent reports (1, 4-9) support this view