Assessment of the contribution of the cytochrome b moiety of the NADPH oxidase to the transmembrane H+ conductance of leukocytes.

Abstract

Phagocytic cells can kill microorganisms by synthesizing superoxide. Activation of the NADPH oxidase that generates superoxide is accompanied by a large intracellular burst of metabolic acid production. Despite the excess acid generation, cytosolic pH (pHi) remains near neutrality due to the concomitant stimulation of several homeostatic H+ extrusion mechanisms including a recently described H(+)-conductive pathway. Activation of the conductance by phorbol esters is defective in neutrophils of chronic granulomatous disease (CGD) patients lacking the transmembrane cytochrome b subunits of the NADPH oxidase. This finding suggests that the oxidase itself undertakes H+ translocation or that, alternatively, assembly of the oxidase is required to activate a separate H+ conducting entity. To distinguish between these possibilities, the presence of the conductive pathway was assessed in unstimulated normal and CGD cells by manipulating pHi and the transmembrane potential. Using fluorimetric determinations of pHi, a conductive, Zn(2+)-sensitive alkalinization was observed in neutrophils from both normal and cytochrome b-deficient CGD donors. The electrophysiological properties of the conductance were defined in purified blood monocytes using the whole cell configuration of the patch clamp. Depolarizing pulses induced slowly activating outward currents in cells from both normal and cytochrome b-deficient individuals. The elicited currents were potentiated by cytosolic acidification and did not inactivate within the times tested. As in control leukocytes, the reversal potential of tail currents in the CGD cells closely approximated the H+ equilibrium potential and was unaffected by substitution of the major ionic components of the external bathing medium. At all voltages tested, the magnitude of the evoked currents was comparable in normal and CGD cells. The results indicate that, like macrophages and granulocytes, human monocytes display a voltage-gated highly H(+)-selective conductance. More importantly, our findings imply that the conductive pathway is present in cells devoid of cytochrome b. Therefore, the defective activation of the conductive pathway by protein kinase C agonists in CGD cells is not due to the physical absence of the transporter. Instead we propose that the oxidase functions in a regulatory capacity, facilitating the opening of a distinct H+ conductance during cellular stimulation.