Plasmalemma Redox Chain and H⁺ Extrusion

Ferricyanide reduction by Elodea densa leaves is associated with a release of protons in the cytoplasm, a fraction of the increase in protons being then extruded by the ATP-driven proton pump (20). The data presented here show that ferricyanide induces a marked increase in O(2) uptake, additive to that induced by fusicoccin plus K(+), and here interpreted as depending on the utilization of ATP by the H(+) pump. Glucose 6-phosphate and malate levels are markedly increased by fusicoccin plus K(+). The simultaneous presence of ferricyanide reduces by about 50% the increase of malate, while it completely suppresses that of glucose 6-phosphate. The ferricyanide-induced decrease of malate is interpreted as due to the acidification of the cytosol associated with ferricyanide reduction, while the more marked decrease of glucose 6-phosphate might depend in part on the pH change and in part on a faster oxidation of this substrate. In fact, ferricyanide reduction is accompanied by a marked decrease of the incorporation into RNA ribose of C-1 as compared with C-2 of [(14)C]glucose. This suggests a stimulation of the release of C-1 as CO(2) at the level of the glucose 6-phosphate oxidation pathway, as expected if NADPH was the electron donor for ferricyanide reduction. These results are interpreted as confirming that the H(+) efflux associated with ferricyanide reduction depends on the activation of the ATP-driven plasmalemma H(+) pump. They also suggest that NADPH is used as an electron donor to some initial component of the plasmalemma redox system.