The interaction of chlorpromazine and butyrophenones with glutamate dehydrogenase

Experiments on the binding of radioactive chlorpromazine to glutamate dehydrogenase, at pH 7.0 and 25° in 100 mM KCl, reveal the presence of two sites for the drug with an apparent dissociation constant, K_Dapp, of 66 μM. In the presence of the substrates α-ketoglutarate and NADH, the affinity of one of the sites for chlorpromazine remains essentially unchanged, whereas that of the other is much lowered (K_Dapp ~ 200 μM). Apparently only the low affinity site of the enzyme is expressed kinetically as K_Dapp for the inhibition of enzyme catalysis (in the direction of glutamate formation) is 136 μM. Each of the two sites in the hexameric enzyme molecule seems to be located in a domain formed by three polypeptide chains. K⁺ and Rb⁺ are competitive inhibitors of NH₄⁺, the third substrate. In the presence of α-ketoglutarate and NADH, with K⁺ replacing NH₄⁺, the two chemically identical domains become conformationally different. Evidence is brought together, from the literature and the present work, to show that the pharmacological effects of phenothiazines and butyrophenones are due primarily to the blocking of dopamine receptors and, marginally at best, to the inhibition of glutamate dehydrogenase.