NADPH-dependent superoxide production by the solubilized oxidase of neutrophils was inhibited 36% by diphenylene iodonium at a 1:1 stoichiometry with the enzyme flavoprotein content. Addition of diphenylene iodonium strongly inhibited the NADPH-dependent reduction of both FAD and cytochrome b-245 in steady-state kinetic experiments. Incubation of solubilized enzyme with diphenylene [125I]iodonium resulted in the specific labelling of a polypeptide of Mr 45,000. In the presence of NADPH the amount of label incorporated into the polypeptide was reduced. There was no difference in labelling between enzyme prepared from stimulated or unstimulated cells.

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