Activation of the Leukocyte NADPH Oxidase by Protein Kinase C in a Partially Recombinant Cell-free System

The leukocyte NADPH oxidase is an enzyme present in phagocytes and B lymphocytes that when activated catalyzes the production of O⨪2 from oxygen at the expense of NADPH. A correlation between the activation of the oxidase and the phosphorylation of p47 PHOX, a cytosolic oxidase component, is well recognized in whole cells, and direct evidence for a relationship between the phosphorylation of this oxidase component and the activation of the oxidase has been obtained in a number of cell-free systems containing neutrophil membrane and cytosol. Using superoxide dismutase-inhibitable cytochrome c reduction to quantify O⨪2 production, we now show that p47 PHOX phosphorylated by protein kinase C activates the NADPH oxidase not only in a cell-free system containing neutrophil membrane and cytosol, but also in a system in which the cytosol is replaced by the recombinant proteins p67 PHOX, Rac2, and phosphorylated p47 PHOX, suggesting that neutrophil plasma membrane plus those three cytosolic proteins are both necessary and sufficient for oxidase activation. In both the cytosol-containing and recombinant cell-free systems, however, activation by SDS yielded greater rates of O⨪2 production than activation by protein kinase C-phosphorylated p47 PHOX, indicating that a system that employs protein kinase C-phosphorylated p47 PHOX as the sole activating agent, although more physiological than the SDS-activated system, is nevertheless incomplete.