SUMMARY: Plasma membranes from osmotically lysed protoplasts of were concentrated 21- to 26-fold (using the recovery of [H]dansyl chloride and ATPase activity at pH 6.5, as criteria). The contamination by mitochondria was approximately 10%. The plasma membrane ATPase was stimulated by K by up to 350% at pH 6.5; Na had a lesser effect, whereas Li had none. The stimulation by K was independent of Cl, NO or CO as accompanying anions. The pH optimum of the ATPase was narrowed in the presence of 80 m-KC1 to a distinct peak at pH 6.5. Under these conditions nucleoside triphosphates other than ATP were hydrolysed at rates less than 5% of that with ATP, and the of the ATPase for ATP was lowered from 1.3 to 0.6 m. Orthovanadate (40 μ) and oligomycin (5 μg ml) inhibited the plasma membrane ATPase by 65% and by 20%, respectively. In contrast, the ATPase activity of the mitochondrial fraction had a sharp optimum at pH 8.5 and was not stimulated by added K.


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