Evidence for an electrogenic ATPase in microsomal vesicles from pea internodes

The transmembrane electropotential of microsomal vesicles from pea internode segments, monitored by equilibrium distribution of the permeant anion SCN-, is strongly hyperpolarized when ATP is present in the incubation medium. The stimulation of SCN- uptake by ATP is rather specific with respect to the other nucleoside di- and triphosphates tested: ADP, GTP, CTP and UTP. ATP-stimulated SCN- uptake is strongly inhibited by ATPase inhibitors such as p-chloromercuribenzenesulphonate and N,N'-dicyclohexylcarbodiimide and by 2.5% toluene/ethanol (1 : 4, v/v), the latter being a treatment which makes the vesicles permeable. On the contrary, oligomycin is almost ineffective in influencing ATP-induced SCN- uptake. The proton conductor carbonyl cyanide p-trifluoromethoxyphenylhydrazone strongly inhibits ATP-stimulated SCN- uptake. The effect of ATP on SCN- uptake depends on the pH of the medium, the maximum being reached at about pH 7.0. These data support the view that microsomal fractions from pea internodes contain membrane vesicles endowed with a membrane-bound ATPase coupling ATP hydrolysis to electrogenic transport of ions, probably H+.