Effects of magnesium plus vanadate on partial reactions of the Ca(2+)-ATPase from human red cell membranes

Under conditions in which pretreatment with Mg2+ plus vanadate activate the Ca(2+)-ATPase, the initial rate of phosphorylation of the enzyme increased from 141 to 259 pmol/mg protein per s while the steady-state level of phosphoenzyme lowered from 1.9 to 1.1 pmol/mg protein. The drop in phosphoenzyme level was caused by incubation and washing during treatment rather than by vanadate. The data allowed to estimate a turnover number for the enzyme that raised by 170% after pretreatment. The results show that the activation of the Ca(2+)-ATPase by Mg2+ plus vanadate is due to changes in the kinetic properties of the enzyme.