The NADPH:O2 oxidoreductase of human neutrophils. Stoichiometry of univalent and divalent reduction of O2.
The ratio of superoxide production to oxidation of NADPH affected by the NADPH:O2 oxidoreductase of human neutrophils is strongly influenced by pH, NADPH substrate concentration, aging of the enzyme, or its exposure to excess deoxycholate. Freshly prepared enzyme exhibited a Km for NADPH of 52 microM as determined by assaying NADPH oxidase activity, or approximately 33 microM by measurement of superoxide formation. In the range of 100-150 microM NADPH at pH 7.6 and in the presence of 0.06% deoxycholate, the univalent flux of electron equivalents given up by NADPH to O2 was 99%. Following storage of the oxidoreductase overnight on ice, its Km for NADPH rose to 125 microM as determined by monitoring oxidation of NADPH but was unaltered when measured in terms of superoxide production. Concomitantly, its capacity to affect univalent reduction of O2 fell approximately 20-30% under the same assay conditions. Univalent flux rates of less than 40% were observed with exposure of the enzyme to concentrations of deoxycholate in excess of 0.1% or to pH values below 6.0 or above 8.0. The capacity of the enzyme to carry out univalent reduction fell with increasing NADPH concentrations in a manner resembling that previously reported with increasing concentrations of xanthine in the case of xanthine oxidase (Fridovich, I. (1970) J. Biol. Chem. 245, 4053-4057). The reduced form of the neutrophil oxidoreductase, like xanthine oxidase, thus appears to be capable of conducting both 1- and 2-electron transfer steps in reducing O2. Its capacity to affect univalent reduction of O2 depends upon the concentration of electron donor (NADPH) supplied as well as conditions of storage and assay of the native enzyme.