Bovine glutamate dehydrogenase. The pH dependence of native and nitrated enzyme in the presence of allosteric modifiers.

The pH dependence of the rate of glutamate oxidation by bovine liver glutamate dehydrogenase has been determined in the absence and presence of allosteric modifiers. In the range of pH 7 to 9 where the enzyme was saturated by substrate, the degree of activation by ADP or inhibition by GTP did not vary substantially with pH. The pH dependence of chicken liver glutamate dehydrogenase measured in the absence and presence of allosteric modifiers was nearly identical with that of the bovine enzyme. In studies of the bovine enzyme, nitration of tyrosine-406 had no significant effect on the activity or the pH dependence of the enzyme activity in the absence of allosteric modifier or in the presence of the activator ADP. However, nitration decreased the inhibitory effect of GTP evenly over the entire pH range and the extent of loss of GTP inhibition was dependent on the extent of nitration. It is concluded that the effect of nitration of tyrosine-406 on the loss of inhibition by GTP is to alter the relationship of the binding site of GTP to the catalytic site in a pH-independent manner. This result is probably a consequence of the introduction of the steric bulk of the nitro group into the tertiary structure of the protein.