Regulation of aspartate aminotransferase isozymes by D-erythrose 4-phosphate and glycolaldehyde phosphate, the naturally occurring homologues of D-glyceraldehyde 3-phosphate.

As part of a study of the regulation of aspartate aminotransferase isozymes (EC 2.6.1. l), we have investigated the characteristics of inhibition of the anionic and cationic isozymes by u-erythrose 4-phosphate and glycolaldehyde phosphate, the naturally occurring homologues of D-glyceraldehyde 3-phosphate. Erythrose-4-P has been found to be a time-dependent inhibitor of both the cationic and anionic isozymes. The inhibitor dissociation constants and types of inhibition were determined from residual enzyme activity after equilibration of each isozyme with the inhibitor and one substrate in the preliminary incubation mixtures. Inhibition of both isozymes by erythrose-4-P was completely competitive with respect to a-ketoglutarate and completely noncompetitive with respect to aspartate. The Kd values of the cationic isozyme were 1.44 and 0.135 nil for the pyridoxal and pyridoxamine forms, respectively. The K; values of the anionic isozyme were 3.04 and 0.33 II~M for the pyridoxal and pyridoxamine forms, respectively. Both forms of the cationic isozyme were more sensitive to inhibition than the corresponding forms of the anionic isozyme. Inhibition of aspartate aminotransferase isozymes by glycolaldehyde-P was complete and not time-dependent. Inhibition was competitive with respect to a-ketoglutarate as the variable substrate and uncompetitive with respect to aspartate as the variable substrate. The K, values were 1.0’7 and 0.79 m&z for the pyridoxamine form of the cationic and anionic isozymes, respectively. The characteristics of the inhibition of aspartate aminotransferase isozymes by erythrose-4-P, glyceraldehyde-3-P, and glycolaldehyde-P are not related to number of carbon atoms in these compounds. Kinetic analyses of the binding of glycolaldehyde-P and glyceraldehyde-3-P to the isozymes showed that their binding is mutually exclusive. The results suggest that glyceraldehyde-3-P and its naturally occurring homologues erythrose-4-P and glycolalde-