Effect of K+ on Phosphorylation of the Sarcoplasmic Reticulum ATPase by Either Pi or ATP

The effect of K+ on phosphorylation of the Ca2+-dependent ATPase of the sarcoplasmic reticulum by either Pi or ATP was studied using a millisecond mixing and quenching device. Equilibrium levels of phosphoenzyme formed by Pi were progressively decreased in the presence of increasing K+ concentrations. This effect was more pronounced in empty vesicles than in vesicles previously loaded with calcium. Potassium did not modify the initial rate of enzyme phosphorylation by Pi but increased the rate of phosphoenzyme hydrolysis 4- to 6-fold. Using low ATP concentration (50 micro M), the steady state level of phosphoenzyme was decreased by the addition of K+. This effect disappeared when the ATP concentration was raised to 1 mM.