Evidence that H+ extrusion in Elodea densa leaves is mediated by an ATP-driven H+ pump

Data are presented on the ATP-driven proton extrusion mechanism in Elodea densa leaves, in the absebce of photosynthetic activity. In this material, in the dark, or in the light with photosynthesis inhibited by DCMU, H+ extrusion at pH 6 was very low in the absence of permeating cations and became significant (approx. 1.8 μmol/g fresh wt.) in the presence of K+. Rb+ and, much less so, Na+, could partially substitute for K+ while Li+, Cs+, Ca2+, Co2+, Ni2+ were ineffective. Fusicoccin (FC), a toxin which stimulates the plasmalemma H+ ATPase activity in several plant tissues, strongly increased H+ extrusion only in the presence of K+, Rb+, and, to a much lesser extent, Na+. The capacity of cations to stimulate FC-induced H+ extrusion decreased in the same order as their Em-depolarizing activity (K+ > Rb+ ≫ Na+). Increasing concentrations of FC, between 10-7 and 5 × 10+4 M, showed a saturating effect for H+ extrusion at 10-4 M FC. The stimulating effect of FC on H+ extrusion was accompanied by a hyperpolarization of the transmembrane electrical potential (by approx. 30 mV), larger in the absence than in the presence of K+, and by a marked increase of K+ uptake. FC-induced H+ extrusion was strongly inhibited by orthovanadate and erythrosin B, two inhibitors of the plasmalemma H+ transporting ATPase. The activity of an ATPase and an ATP-dependent H+ transport, both sensitive to vanadate and erythrosin B, were also demonstrated in membrane vesicle preparations. These results are interpreted as indicating the presence in E. densa leaves of an electrogenic H+ extrusion mechanism which appears identical to the K+-dependent, FC-sensitive, ATP-driven proton pump described in several terrestrial plant cells.