The role of histidine residues in glutamate dehydrogenase.

1. Glutamate dehydrogenase was subject to rapid inactivation when irradiated in the presence of Rose Bengal or incubated in the presence of ethoxyformic anhydride. 2. Inactivation in the presence of Rose Bengal led to the photo-oxidation of four histidine residues. Oxidation of three histidine residues had little effect on enzyme activity, but oxidation of the fourth residue led to the almost total loss of activity. 3. Acylation of glutamate dehydrogenase with ethoxyformic anhydride at pH6.1 led to the modification of three histidine residues with a corresponding loss of half the original activity. Acylation at pH7.5 led to the modification of two histidine residues and a total loss of enzyme activity. 4. One of the histidine residues undergoing reaction at pH6.1 also undergoes reaction at pH7.5. 5. The presence of either glutamate or NAD(+) in the reaction mixtures at pH6.1 had no appreciable effect. At pH7.5 glutamate caused a marked decrease in both the degree of alkylation and degree of inactivation. NAD(+) had no effect on the degree of inactivation at pH7.5 but did modify the extent of acylation. 6. The normal response of the enzyme towards ADP was unaffected by acylation at pH6.1 or 7.5. 7. The normal response of the enzyme towards GTP was altered by treatment at both pH6.1 and 7.5.