Revised Manuscript Received September 14, 1989 abstract: Neutrophil myeloperoxidase, hydrogen peroxide, and chloride constitute a potent antimicrobial system with multiple effects on microbial cytoplasmic membranes. Among these is inhibition of succinate-dependent respiration mediated, principally, through inactivation of succinate dehydrogenase. Succinate-dependent respiration is inhibited at rates that correlate with loss of microbial viability, suggesting that loss of respiration might contribute to the microbicidal event. Because respiration in Escherichiacoli can be mediated by dehydrogenases other than succinate dehydrogenase, the effects of the myeloperoxidase system on other membranedehydrogenases were evaluated by histochemicalactivity stains of electropho-retically separated membrane proteins. Two bands of succinate dehydrogenase activity proved the most susceptible to inactivation with complete loss of staining activitywithin 20 min, under the conditions employed. A group with intermediate susceptibility, consisting of lactate, malate, glycerol-3-phosphate, and dihydroorotate dehydrogenases as well as three bands of glucose-6-phosphate dehydrogenase, was almost completely inactivated within 30 min. The relatively resistant group, including the dehydrogenases for glutamate, NADH, and NADPH and the remaining bands of glucose-6-phosphate dehydrogenase, retained substantial amounts of diaphorase activity for up to 60 min of incubation with the myeloperoxidase system. The differential effects of myeloperoxidase on dehydrogenase inactivation couldnot be correlated with published enzyme contents of flavin or iron-sulfur centers, potential targets of myeloperoxidase-derived oxidants. Despite the relative resistance of NADH dehydrogenase/diaphorase activity to myeloperoxidase-mediated inactivation, electron transport particles prepared from E. coli incubated for 20 min with the myeloperoxidase system lost 55% of their NADH oxidase activity. Generalized direct inactivation of microbial respiratory dehydrogenase is not a satisfactory explanation for myeloperoxidase-mediated microbicidal activity. Myeloperoxidase-derived oxidants may, however, uncouple dehydrogenases from downstream components of the respiratory chain, thereby contributing to an overall loss of respiratory activity.