Diphenylene iodonium as an inhibitor of the NADPH oxidase complex of bovine neutrophils

Abstract

Diphenylene iodonium (Ph21), a lipophilic reagent, is an efficient inhibitor of the production of 0; by the activated NADPH oxidase of bovine neutrophils. In a cell-free system of NADPH oxidase activation consisting of neutrophil membranes and cytosol from resting cells, supplemented with guanosine 5'-[y-thio]triphosphate, MgC12 and arachidonic acid, or in membranes isolated from neutrophils activated by 4P-phorbol 1Zmyristate 13-acetate, addition of a reducing agent, e. g. NADPH or sodium dithionite, markedly enhanced inhibition of the NADPH oxidase by Ph21. The membrane fraction was found to contain the PhJ-sensitive component(s). In the presence of a concentration of PhzI sufficient to fully inhibit 0; production (around 10 nmol/mg membrane protein), addition of catalytic amounts of the redox mediator dichloroindophenol (C121nd) resulted in a by-pass of the electron flow to cytochrome c, the rate of which was about half of that determined in non-inhibited oxidase. A marked increase in the efficiency of this by-pass was achieved by addition of sodium deoxycholate. The C12-Ind-mediated cytochrome c reduction was negligible in membranes isolated from resting neutrophils. At a higher concentration of Ph21 (100 nmol/mg membrane protein), the C1,Ind-mediated cytochrome c reductase activity was only half inhibited, which indicated that, in the NADPH oxidase complex, there are at least two Ph21 sensitive components, differing by their sensitivity to the inhibitor. At low concentrations of Ph21 (< 10 nmol/mg protein), the spectrum of reduced cytochrome b558 in isolated neutrophil membranes was modified, suggesting that the component sensitive to low concentrations of Ph21 is the heme binding component of cytochrome b558. Higher concentrations of Ph21 were found to inhibit the isolated NADPH dehydrogenase component of the oxidase complex. A number of membrane and cytosolic proteins were labeled by [1251]Ph21. However, the radiolabellng of a membrane-bound 24-kDa protein, which might be the small subunit of cytochrome b558, responded more specifically to the conditions of activation and reduction which are required for inhibition of 0; production by Ph2T. The 0;-generating form of xanthine oxidase was also inhibited by Ph21. Inhibition of xanthine oxidase, a non-heme iron flavoprotein, by Ph21 had a number of features in common with that of the neutrophil NADPH oxidase, namely the requirement of reducing conditions for inhibition of 0, production by Ph2I and the induction of a by-pass of electron flow to cytochrome c by C1,Ind in the inhibited enzyme, suggesting some similarity in the molecular organization of the two enzymes.The neutrophil NADPH oxidase is an 0, -generating enzyme complex, which is dormant in circulating neutrophils and becomes abruptly active when the neutrophils are stimulated by particulate or soluble specific ligands. The activated oxidase complex is located in the plasma membrane of neutrophils. It is thought to consist of two membrane-bound proteins with a redox function, namely an NADPH flavoCor...

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