Inhibition of leucine decarboxylase by thiol-binding reagents

Abstract

Leucine decarboxylase from Proteus vulgaris was partially purified. It is inhibited by iodoacetate. The coenzyme, pyridoxal phosphate (PLP), protects the enzyme from inhibition if added prior to, or simultaneously with the inhibitor. It cannot reverse inhibition once established. Analogs of the coenzyme lacking either the 4-CHO group or the 5-CH 2O.H 2PO 3 group cannot protect the enzyme against iodoacetate. Substrates of the enzyme tend to potentiate the action of the inhibitor. There are indications that iodoacetate inhibits competitively with respect to the coenzyme, noncompetitively with respect to the substrates. p-Chloromercuribenzoate (PCMB) also inhibits leucine decarboxylase, but the characteristics of this inhibitor are markedly different from those of iodoacetate. The relation of these findings to the mechanism of action of this enzyme is discussed.