Effects of ligands and pH on the reactions of aspartate aminotransferase with aminooxyacetate and hydroxylamine

Abstract

The bimolecular association and first-order dissociation rate constants for the reactions of aminooxyacetate and hydroxylamine with the cytosolic aspartate aminotransferase (EC 2.6.1.1) of pig heart were estimated from pH 4.8 to pH 9.5. The acidic form of the enzyme (pK = 6.3) was more reactive than the unprotonated enzyme, but the rates of breakdown of the complexes were not affected by pH. The equilibrium dissociation constants, which were of the order of magnitude of 10(-7) M, were thus lowest at acidic pH values. Aminooxyacetate and hydroxylamine reacted similarly, with the former being more reactive. Glutarate and acetate inhibited the rates of formation and dissociation but had no effect on the overall equilibrium constants between enzyme and inhibitor. On the other hand, the substrate analog erythro-beta-hydroxy-L-aspartate, which forms a complex with the enzyme prosthetic group, acted competitively by preventing the inhibitors from reacting. The rate constants for formation of complexes with free pyridoxal phosphate were much less than those for the enzyme and, unlike the enzyme, hydroxylamine was more reactive than aminooxyacetate.