Mechanisms of action of organophosphorus fungicides. I. Kitazin P, inhibitor of phosphatidylcholine biosynthesis in Pyricularia oryzae.

The effect of organophosphorus fungicide, Kitazin P (IBP, S-benzyl diisopropyl phos-phorothiolate), on lipid biosynthesis of Pyricularia oryzae was investigated. Addition of IBP to the mycelial cells suspension of P. oryzae induced a striking decrease in incorporation of methionine-methyl-14C, S-adenosylmethionine-methyl-14C, and glycerol-1 14C into phos-phatidylcholine, which is the most abundant phospholipid in P. oryzae, but incorporation of choline-methyl-14C into phosphatidylcholine and that of methionine-methyl-14C into simple lipids were not affected. Incorporation of methionine-methyl-14C into phosphatidylcholine is found to be directly proportional to mycelial cells growth of P. oryzae. Enzymes responsible for the biosynthesis from glycerol to phosphatidylcholine through Greenberg's pathway, except phospholipid N-methyltransferase, were not inhibited by IBP. IBP concentration required for 50% inhibition of phospholipid N-methyltransferase was 40 ppm. IBP had no effect on activities of glycerokinase, glycerophosphate acyltransferase, phosphatidic acid cytidyltransferase, phosphatidylserine synthetase, and phosphatidylserine decarboxylase, respectively. Therefore, the specific inhibition of conversion from phosphatidylethanolamine to phosphatidylcholine by the transmethylation of S-adenosylmethionine might be regarded as one of the modes of action of IBP.