Potassium Stimulation of Corn Root Plasmalemma ATPase : II. H-Pumping in Native and Reconstituted Vesicles with Purified ATPase.

The stimulation by K(+) of the initial rate of H(+)-pumping by ATPase was studied in native plasmalemma (Zea mays L. var Mona) vesicles and in reconstituted vesicles with enzyme purified on a glycerol gradient. In reconstituted vesicles, a very high H(+)-pumping rate (200,000% quenching per minute per milligram protein) was obtained with 9-amino-6-chloro-2-methoxyacridine provided that the pump was short-circuited by K(+)-valinomycin. A constant ionic strength was used to prevent indirect stimulation by the electrostatic effects of K(+) salts. Indirect stimulation of H(+)-pumping by the short-circuiting effect of internal K(+), could be abolished by using the permeant anions NO(3) (-) and Br(-) in native, but not in reconstituted vesicles. In both materials, half-stimulation of the H(+)-pumping by K(+) was observed at about 5 millimolar. The same stimulation was obtained when K(+) was present only in the external solution or when it was present both outside and inside the vesicles. It was concluded that the stimulating effect of K(+) on the H(+)-pumping evidenced in these experiments on both native and reconstituted vesicles was due to a direct effect of the cation on the cytoplasmic face of the ATPase. These results are discussed within the context of the hypothesis of an active K(+) transport driven by the ATPase through a direct H(+)/K(+) exchange mechanism.