Interactions of Divalent Cations in Their Action on ATPases from Cucumber Roots

A microsomal fraction (10,000–30.000 g) was prepared from roots of Cucumis satirus L. (cv. Bestseller Fl) grown in solution culture. The ATPase activity was stimulated by Mg and Mn with optima between pH 6 and 7. Stimulation by Ca was obtained only above pH 7. Activations by Mg and Mn were inhibited by Ca, Mn and Mg interacted, so that Mn appeared strongly inhibitory for activation by Mg and Mg weakly inhibitory for activation by Mn: the simplest interpretation for this would be two separate enzymes. Cucumber accumulates and deposits Ca contrary to wheat and oat, which contain Ca-activated ATPase in the pH region 6 to 7. The Ca data for cucumber are compared with earlier findings from wheat and oat and are tentatively related to known physiological differences.