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 02 uptake, additive to that induced by fusicoccin plus K+, and here interpreted as depending on the utilization of ATP by the HI 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 ['4CJglucose. This suggests a stimulation of the release of C-1 as CO2 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 HI effiux 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.Evidence was presented in a previous paper (20) leading us to conclude that ferricyanide reduction by the plasmamembrane redox chain and the associated release of H+ into the medium depend on the operation of two distinct mechanisms: the redox chain transporting electrons to ferricyanide and the ATP-driven H+ pump extruding the H+ released in the cytoplasm during the transport of electrons from an intracellular substrate to the extracellular electron acceptor. According to this view, the ATPdriven H+ pump would be activated by both the depolarization of Em and the decrease in intracellular pH associated with ferricyanide reduction (20; see also 13, 21, 28).In the present paper we studied the relationship between the observed intracellular acidification associated with Fe(CN)3-reduction and the eventual changes in the levels of malate and Glu6P,l two metabolites apparently influenced by cytoplasmic pH changes (16,19 pumping is driven by ATP hydrolysis. We also attempted a first evaluation of the possibility that ferricyanide reduction affects the levels of Glu6P, the rationale of this being that the rate of Glu6P oxidation (via the PPP), depends on the value of the NADPH/NADP + ratio, (1)(2)(3)29 (20).

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