[Mechanism of action of the inhibition of pyridine-nucleotide-dependent flavine enzymes using the systemic fungicide Dexon].

The actions of Dexon on the NADH-ferricyanide oxidoreductase and the NADPH oxidase system of electron transfer particles (ETP) from beef heart as well as on the NADPH-cytochrome c oxidoreductase from brewer's yeast (Saccharomyces carlsbergensis Hansen) were investigated. The inhibition of the NADH dehydrogenase activity of ETP and that of the yeast enzyme correspond with respect to the following characteristics: 1) increase in the inhibition, 2) enhancement of the Dexon sensitivity by one order of magnitude after preincubation in the presence of NAD(P)H, 3) irreversibility of the inhibition, 4) no detectable changes in the spectral properties and in coenzyme activity of FMN after acid extraction from Dexon-treated enzyme. The inhibition of the NADH dehydrogenase activity of ETP is diminished by both NAD+ and FMN. However, no interaction of Dexon with NAD(P)H or FMN could be detected in the absence of enzyme or apoenzyme. The concentration of half-inhibition by Dexon for the yeast enzyme corresponds with its FMN concentration. It is proposed that both apoenzyme, NAD(P)H and FMN are involved in the interaction with Dexon. Possible mechanisms of binding are both complanar complexations of the ring systems and a triazene formation between FMNH2 and Dexon. The NADPH oxidase activity of the ETP is partly inhibited; the share inhibited by Dexon may represent the pathway via the transhydrogenase reaction.