1 Answer

1. The mode of action of purine nucleotides and amino acids on the activity of ox-liver glutamate dehydrogenase was investigated. 2. The addition of two chemically unrelated activators, at concentrations below saturation levels, enhanced the enzyme activity much more than a twofold concentration of each one separately. No such synergistic activation was observed when a combination of two members of the same group was tested. 3. With saturating concentrations of the activators, the increase in enzymic activity produced by a pair of chemically related effectors was either identical with or even below that achieved by the more active effector. However, the combination of two unrelated activators, at saturating amounts, still yielded a higher enzyme activity than with each one singly. 4. Unlike ADP, l-leucine was incapable of overcoming completely the inhibition produced by GTP. 5. It is suggested that purine nucleotides and amino acids bind to separate group-specific allosteric sites of this enzyme. 6. The possible physiological significance of these findings with regard to the regulation of the cellular functions of this enzyme is discussed.