Plasmalemma Redox Activity and H+ Extrusion

Ferricyanide reduction by Elodea densa leaves, in the dark, is associated with: (a) acidification of the medium; (b) decrease (about 0.2-0.3 units) of intracellular pH (measured in cell sap, cytoplasm, and vacuole); (c) depolarization of the transmembrane potential; (d) net efflux of K(+) to the medium. Ferricyanide-induced acid secretion is markedly increased by the presence of fusicoccin (FC), and this effect is severely inhibited by the proton pump inhibitors erythrosine B and vanadate. In the presence of ferricyanide FC-induced H(+) extrusion no longer requires the presence of K(+) in the medium. The (ferricyanide reduced)/(H(+) extruded) ratio varies from about 2, in the absence of FC, to about 1 when the toxin is present, and to more than 4, when ATP-driven H(+) extrusion is inhibited by erythrosine B or by vanadate. Fusicoccin markedly reduces K(+) release to the medium. The ratio (ferricyanide reduced)/(H(+) extruded + K(+) released) approaches unity under all of the three conditions considered. These results indicate that ferricyanide reduction depends on a plasmalemma system transporting only electrons to the extracellular acceptor, with consequent potential depolarization and cytoplasm acidification. Most of the protons released in the cytoplasm would be secondarily extruded by the ATP-driven pump, stimulated by both intracellular acidification and depolarization. K(+) efflux would depend on potential depolarization.