Characterization of a proton pump from pea stem microsomes

The present work deals with the characterization of an ATP‐dependent proton translocation monitored by the ΔpH probe acridine orange. The ATP‐dependent proton translocation has an optimum activity at pH 6.5 and is substrate specific for ATP. It is stimulated by Cl−, HCO3− and Br−, but is insensitive to several monovalent cations. Divalent cations (Mg2+ or Mn2+) are required for proton translocation, while in the presence of Ca2+ no uptake is observed. NO3−, NO2− and citrate strongly inhibit proton uptake. On the contrary, F−, SO42−, malate, pyruvate, succinate, oxalate and acetate have no inhibitory effect. Proton uptake is stimulated by valinomycin and unaffected by molybdate. Two thiols, dithioerythritol and dithiothreitol, are able partially to prevent the FCCP‐abolished proton uptake or partially restore the ATP‐dependent proton translocation in FCCP‐collapsed vesicles. It is suggested that pea stem microsomes possess an electrogenic ATPase, acting as a proton pump, which, on the basis of its characteristics, can be tentatively associated with membranes of tonoplast origin.