[Study of the role of arginine residues in aspartate transaminase from chicken heart cytosol].

Reaction of 1,2-cyclohexanedione with chicken heart cytosolic aspartate transaminase results in loss of enzyme activity complying to first order kinetics up to 70% inactivation. The inactivation rate is markedly decreased in the presence of alpha-ketoglutarate, glutarate or alpha-methylaspartate. The number of arginine residues modified per subunit was approximately two (in enzyme preparations which retained 30% residual activity). The diketone-modified enzyme nearly completely loses affinity for alpha-methylaspartate and glutarate; in contrast, its ability to bind alpha-alanine and catalyze its transamination half-reaction with the bound coenzyme remains unimpaired. From these data it can be inferred that a functional arginine residue is the cationic binding site for the distal carboxyl group of the substrates. The transaminase apoenzyme was inactivated with cyclohexanedione at the same rate as reconstituted holoenzyme. Measurements of circular dichroism showed that the modified apoenzyme is capable to bind pyridoxal-P. No evidence was obtained for the presence of an arginine residue in the coenzyme binding site.