[Effect of dielectric permeability of culture media on the kinetics of the ATP hydrolysis reaction, catalyzed by Ca2+-transporting ATPase, solubilized from smooth muscle cell membrane].

Transport Ca2+,Mg(2+)-ATPase was solubilized from myometrial plasma membrane and purified by calmodulin-sepharose 4B affinity chromatography; the effect of dielectric permeability of incubation medium on kinetics of ATP hydrolysis by this enzyme was studied. Various concentrations of dimethylsulfoxide, ethanol, acetone, and dioxane (up to 10%) caused various dose-dependent inhibition of enzymatic ATP hydrolysis. The correlation between specific activity (A) of Ca2+,Mg(2+)-ATPase and dielectric permeability of the incubation medium (D) did not depend on the nature of organic solvents and was satisfactory approximated by similar lines in the Laidler-Scatchard coordinate system, i.e., ln(A) versus 1/D. Decrease in polarity of incubation medium (changes in D from 73.05 to 68.80) decreased the initial maximal rate of enzymatic reaction for ATP and Mg2+ and increased apparent Km for these compounds. Decrease in ionic strength of incubation medium from 100 to 25 mM KCl decreased the maximal initial rate of ATP hydrolysis for Ca2+ and increased the apparent Km for this cation. The data are discussed in conjunction with a model of transport Ca2+,Mg(2+)-ATPase active site structure.