Choline acetyltransferase of the calf caudate nucleus was studied, with particular emphasis on its mode of inhibition by styrylpyridine analogs.Product-inhibition studies suggest a sequential, Theorell-Chance type of mechanism for this enzyme.CoA was competitive with the substrate acetyl CoA, and acetylcholine was competitive with choline.Inhibition by styrylpyridine analogs was noncompetitive with both acetyl CoA and choline and was initially reversible, but partial irreversibility was observed when the enzyme and inhibitor were preincubated at 37° before substrate additionAcetylcholinesterase was also inhibited to various degrees by most of the styrylpyridine analogs, and with this enzyme.N-methyl-4-(1-naphthylvinyl)pyridinium iodide (NVP+) blocked the enzyme (E) deacylation step (E-acetyl + H2O ← E + acetic acid), which is thought by other workers to involve an imidazole moiety on cholinesterase.With choline acetyltransferase in the presence of 14C-labeled acetyl CoA, an apparent 14C-labeled acetyl CoA-enzyme intermediate was detected by gel filtration.The relative amount of this intermediate was diminished when choline was present.NVP+ did not prevent its formation, but did prevent its disappearance in the presence of choline.This effect with the acetyltransferase may be analogous to the blocking of deacylation observed with acetylcholinesterase.The inhibition of choline acetyltransferase by NVP+ was not reversed by dithiothreitol, although this sulfhydryl compound completely reversed inhibition by Hg2+ and by the sp. sulfhydryl reagent, allyl 2-propene-1-thiosulfinate (allicin).Inhibition by Cu2+, which may be expected to interact with imidazole groups as well as others, tended to protect the enzyme from NVP+.It is proposed that NVP+ may interfere with the functioning of a catalytic imidazole moiety during enzyme deacylation.
