[Kinetic interpretation of the original pH-dependence of enzymatic activity of "basal" Mg(2+)-ATPase of the smooth muscle sarcolemma].

It was demonstrated in experiments made on a fraction of plasma membranes of the uterine smooth muscle cells that PH-dependence of enzymatic activity of the "basal" (Ca(2+)-independent) Mg(2+)-ATPase obtained under the conditions of determining the initial velocity of ATP hydrolysate is not bell-shaped but is characterized by linearity in the range of the values of hydrogen index 6.0-8.0. A kinetic model of Mg(2+)-dependent enzymatic hydrolysis of ATP has been suggested and analyzed; the model explains the linearity of the above pH-dependence. Results of kinetic analysis prove that the cause of linear pH-dependence of enzymatic activity of the "basal" Mg(2+)-ATPase is that the proton H+ is a competitive inhibitor of the given enzyme: the increase of protons concentration leads to a decrease of the affinity of Mg(2+)-ATP substrate for the enzyme, but it has no effect on the number of circulations of the latter. Thus the work gives a kinetic substantiation of the possible regulatory role of protons H+ as the factor of original negative inverse relation which controls the enzymatic activity of the basal Mg(2+)-ATPase "producing" protons in the myometrium cells; the concentration of protons in the near-membrane regions of the myoplasma being increased the ATPase activity decreases, and the former being decreased the latter increases. It is not excluded that owing to its original linear pH-dependence the studied ATP-hydrolase system serves as an important element of the control of proton homeostasis in the smooth-muscle cells.