The inhibition of glutamate dehydrogenase by some antipsychotic drugs

The phenothiazines chlorpromazine and perphenazine and the butyrophenone haloperidol were shown to be reversible inhibitors of glutamate dehydrogenase (GDH). Inhibition by chlorpromazine was found to be partial, whereas haloperidol and perphenazine would, apparently, give full inhibition at saturating concentrations. Double-reciprocal plots of the difference between activities in the absence and presence of the inhibitor against the inhibitor concentration were linear with chlorpromazine and perphenazine but parabolic when haloperidol was used as the inhibitor. Comparisons between the responses of commercially available preparations of ox liver GDH, which have been shown to have suffered limited proteolysis during purification, and the ox brain enzyme prepared by a procedure which does not result in such proteolysis, revealed the latter preparation to be more sensitive to inhibition because of a higher apparent affinity for these drugs. The apparent dissociation constants for enzyme-drug interactions were, however considerably higher than the concentrations that have been reported to occur in vivo following chronic administration of chlorpromazine and haloperidol. This casts doubt on earlier claims that inhibition of GDH may be involved in the antipsychotic actions of these drugs, although it might be a factor in the side effects associated with the use of such compounds.