Stimulation of Plasmalemma Adenosine Triphosphatase from Oat Roots by Inorganic and Organic Cations: Concentration-Dependence, Selectivity, and Sites

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Abstract
K+-stimulated ATPase activity of a plasmalemma-enriched fraction from excised roots of oat was triphasic in the range 5-80 mM KCl. The phases obeyed Michaelis-Menten kinetics and were separated from each other by jumps or sharp breaks at about 10 and 20 mM. Stimulation of alkali cations was in the order K+ > Rb+ > Na+ > Cs+ > Li+ or in a closely related sequence. The specificity reflected differences in Vmax, not in affinity (Km-1). Stimulation by the organic cations ethanolamine and choline in the interval 11-80 mM appeared monophasic rather than biphasic. Substitution on the quaternary N of the amino alcohols decreased their effectiveness, as did extension and branching of the chain. Stimulation was maximal at about pH 7 both for K+ and choline. The kinetics of K+ stimulation are multiphasic, not cooperative, as was true of uptake. The ATPase is stimulated by organic cations, but the difference in kinetics indicates the existence of separate sites for stimulation and transition.