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

Abstract

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 +, Ca 2+, Co 2+, Ni 2+ 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 E m -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.