Allosteric modification by K+ of the (Ca2+ + Mg2+)-dependent ATPase of sarcoplasmic reticulum. Interaction with Mg2+.

The K’ activation profile of the (Cap+ + My’+)-dependent ATPase of sarcoplasmic reticulum vesicles is markedly changed by the free magnesium concentration. At 0.2 mM Mg’+, a complex dependence on the K+ concentration is observed which changes to a simple hyperbolic dependence when Mg’+ is raised to 3 mM. The degree of activation by K+ is always higher at 0.2 rnH than at 3 mM MgY+. At 0.2 mM Mg”+, the degree of maximal activation is markedly affected by the Mg. ATP concentration, being significantly higher at 1 mM Mg.ATP than at Mg. ATP concentrations of 0.1 mM or lower. This effect of Mg. ATP is not seen at 3 mM MgZ+. In media devoid of K+, the increase of the Mg”+ concentration activates the reaction. This effect is more pronounced at Mg. ATP concentrations above 0.1 mM. Roth K+ and Mg’+ modify markedly the profile of the saturation curves for Mg. ATP and CaZ+. At 0.2 mM Mg’+ and in the absence of K+, both curves are almost hyperbolic (with Hill coefficient values of 0.86 for Mg. ATP and 1.09 for CaY+). The Hill coefficient for Mg. ATP diminishes upon the addition of 100 mM K+ (to a value of 0.59) or upon the increase of Mg’+ to 3 rnM (to a value of 0.40). Simultaneously, in each condition, the value of the Hill coefficient for Ca’+ increases to about 1.6. The addition of 100 mM K+ to media containing 3 mM Mg”+ does not modify further the observed values. It is proposed that the K+ activation of the ((;a”+ + Mg’+)dependent ATPase involves two sites, one of which can also interact with Mgy+, bringing about an allosteric modification of the enzyme. The second site is not affected by Mg’+ up to 3 mM. The possibility of the existence of two extreme functional states of the enzyme is discussed.