Abstract
A Mg(2+)-dependent, cation-stimulated ATPase was associated with plasma membranes isolated from corn leaf mesophyll protoplasts. Potassium was the preferred monovalent cation for stimulating the ATPase above the Mg(2+)-activated level. The enzyme was substrate-specific for ATP, was inhibited by N,N'-dicyclohexylcarbodiimide, diethylstilbestrol, p-chloromercuribenzoate, and orthovanadate, but was insensitive to oligomycin or sodium azide. A K(m) of 0.28 millimolar Mg(2+)-ATP was determined for the K(+)-ATPase, and the principal effect of potassium was on the V(max) for ATP hydrolysis. Since potassium stimulation was not saturated at high concentrations, a nonspecific role was proposed for potassium stimulation. A nonspecific phosphatase was also found to be associated with corn leaf plasma membranes. However, it could not be determined positively whether this activity represented a separate enzyme.The cation-stimulated ATPase of corn leaves is biochemically similar to other plant plasma membrane enzymes. Thus, the ATPase can serve as a reliable qualitative plasma membrane marker providing its activity is well characterized.