1. The steady residual activity of ox liver glutamate dehydrogenase at equilibrium with the reversible inactivator pyridoxal 5′-phosphate was measured in the presence and absence of various protecting agents. 2. NAD+ (up to 15mm) and its 3-acetylpyridine analogue (up to 5mm) both failed to protect, in contrast with NADH. 3. Partial protection was given by glutarate and by succinate. Adipate and pentanoate were much less effective. 4. Correspondingly, whereas succinate and glutarate were both shown to be strong inhibitors of the catalytic reaction, competitive with glutamate, adipate was only weakly competitive, and the still weaker inhibition by pentanoate was non-competitive. 5. When the enzyme was saturated with glutarate, NAD+ became a good, although still partial, protecting agent. In the absence of protection, 1.8mm-pyridoxal 5′-phosphate decreased enzyme activity to 9%, in the presence of 150mm-glutarate to 29%, and with glutarate and 1mm-NAD+ only to 73%. 6. 2-Oxoglutarate also promoted protection by NAD+, but neither pentanoate nor succinate did so. The finding with succinate is remarkable in view of findings 3 and 4 above. 7. It seems possible that substrates or analogues possessing the glutarate structure promote a conformational change that alters the mode of NAD+ binding. This may explain why glutamate is a much better substrate than norvaline or aspartate and why negative interactions in coenzyme binding occur only in the formation of ternary complexes with glutamate or its analogues.
