Ox liver glutamate dehydrogenase. The use of chemical modification to study the relationship between catalytic sites for different amino acid substrates and the question of kinetic non-equivalence of the subunits.

The effect of pyridoxal 5'-phosphate on the activity of ox liver glutamate dehydrogenase towards different amino acid substrates was investigated. Both alanine and glutamate activities decreased steadily in the presence of pyridoxal 5'-phosphate. The alanine/glutamate activity ratio increased as a function of inactivation by pyridoxal 5'-phosphate, indicating that glutamate activity is lost more rapidly than alanine activity. A mixture of NADH, GTP and 2-oxoglutarate completely protected the alanine and glutamate activities against inactivation by pyridoxal 5'-phosphate. The activity of glutamate dehydrogenase towards glutamate and leucine decreased steadily in a constant ratio in the presence of pyridoxal 5'-phosphate. The effect of leucine on the alanine and glutamate activities as a function of inactivation by pyridoxal 5'-phosphate was studied. The results are interpreted to suggest that the subunits of glutamate dehydrogenase hexamer are kinetically non-equivalent with regard to activity towards the two monocarboxylic amino acids as well as glutamate, and that all three substrates share the same active centre. However, leucine is also able to bind at a separate regulatory site.