Oxygen metabolism in plant/bacteria interactions: effect of DPI on the pseudo-NAD(P)H oxidase activity of peroxidase.

Diphenyleneiodonium (DPI) has been used frequently as a specific inhibitor of NADH oxidase activity in studies of plant/pathogen interactions. The present study reports the effect of DPI on the pseudo-oxidative activity of horseradish peroxidase. DPI, like other phenolics, is able to catalytically stimulate NADH oxidation in the presence of exogenous H2O2. The stimulated NADH oxidation has an acidic pH optimum and has an apparent Km of 111 microM NADH. The NADH oxidation rate is linearly proportional to [DPI] and the amount of NADH oxidized is proportional to [H2O2]. Once exogenous H2O2 is depleted, the NADH oxidation is abruptly halted until additional H2O2 is supplied. In some respects DPI appears to mimic the effects of certain anti-oxidants that use HRP to scavenge H2O2 and NAD(P)H as a reductant.