The intimate and mysterious relationship between proton channels and NADPH oxidase

Abstract

Voltage gated proton channels and NADPH oxidase function cooperatively in phagocytes during the respiratory burst, when reactive oxygen species are produced to kill microbial invaders. Although these molecules are distinct entities, with no proven physical interaction, their presence and activity in many cells appears to be coordinated. We describe these interactions and discuss several types of mechanisms that might explain them.

Figure 1

Involvement of proton channels in the phagocyte respiratory burst. Phosphorylation enhances the opening of proton channels and phosphorylation of cytosolic subunits results in assembly of the NADPH oxidase complex. Each NADPH molecule donates two electrons, which cross the membrane through a redox pathway contained within gp91^phox and reduce two O₂ molecules to superoxide anion, O₂·^-. Two protons are effectively left behind. Electron flux depolarizes the membrane, opening proton channels, which extrude the bulk of the protons. (From Ref. 49).

Figure 2

Inhibiting NADPH oxidase reverses the slowing of proton channel tail current decay that occurs in the enhanced gating mode. This human eosinophil was voltage-clamped in perforated-patch configuration and stimulated with PMA. Illustrated is the current during a pulse from -60 mV to +40 mV in the presence of PMA, and then after addition of DPI. DPI eliminated ∼6 pA of inward current at -60 mV, which indicates this was electron current generated by NADPH oxidase activity. The proton current activation kinetics and amplitude were not affected; the outward current was shifted upwards by the loss of electron current, which is inward up to +200 mV [52]. However, the tail current decay was greatly accelerated.

Figure 3

The effect of PMA on the g_H-V relationship in a human monocyte (■, □), a human basophil (◆, ◇), and a HEK-293 cells transfected with the human proton channel gene, H_v1 (●, ○). In each case, the open symbol is after PMA. The g_H values were calculated from the current at the end of 8-s pulses using the measured reversal potential.