Stimulation of Plasmalemma Adenosine Triphosphatase from Oat Roots by Inorganic and Organic Cations: Concentration-Dependence, Selectivity, and Sites 1

AUTOR(ES)

Håvarstein, L. Sigve

RESUMO

K+-stimulated ATPase activity of a plasmalemma-enriched fraction from excised roots of oat was triphasic in the range 5 to 80 millimolar KCl. The phases obeyed Michaelis-Menten kinetics and were separated from each other by jumps or sharp breaks at about 10 and 20 millimolar. Stimulation by 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 to 80 millimolar appeared monophasic rather than biphasic. Substitution on the quaternary nitrogen 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.

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