Labilization of the phosphoester linkage in enzyme-inhibitor complexes of aspartate aminotransferase.

Individual enzyme-inhibitor complexes with characteristic absorption spectra have been obtained as a result of the reaction of the apoenzyme of aspartate aminotransferase with Nalpha-(5'-phosphopyridoxyl)-L-glutamic acid, Nalpha-(5'-phosphopyridoxyl)-D-glutamic acid, and Nalpha-(5'-phosphopyridoxyl)-L-pyroglutamic acid. The stability of the enzyme-inhibitor complexes has been investigated under various conditions, viz., reactivation by the coenzyme, denaturation by urea, variations in the pH. It has been shown that the complexes formed by the last two inhibitors are reactivated by pyridoxal-5'-phosphate and that the inhibitor can be released under mild conditions. The enzyme-inhibitor complex formed by Nalpha-(5'-phosphopyridoxyl)-L-glutamic acid, on the other hand, was not reactivated by the coenzyme. Pyridoxylglutamic acid has been isolate in attempts to release the inhibitor. The dephosphorylation of the inhibitor has been associated both with the hydrolysis of a phosphate bond involving the enzyme and with the phosphorylation of aspartate aminotransferase. A 32P peptide containing 13 amino acids has been isolated from the tryptic hydrolysate of the enzyme-inhibitor complex (formed by a 32P inhibitor). The data obtained have been interpreted on the basis of an assumption that the phosphate group of the coenzyme has an active role in the enzymatic transamination reaction.