Kinetic Studies on the Effects of Divalent Cations on the ATPase Activity of the Fragmented Sarcoplasmic Reticulum of Rabbit Skeletal Muscle

The effects of divalent cations, , and on the total ATPase activity of the fragmented sarcoplasmic reticulum isolated from rabbit skeletal muscle were investigated. The inhibitory effects of the cations on the enzyme activity increased as the concentrations of the ions increased with the order of efficiency of > > > > in the concentration range between 10 and 500M. The 50% inhibition for each ion was almost identical with the inhibition constant (Ki) value for each ion. The Ki's were 10, 30 130, and 350M for , respectively. seemed to be an activator at lower concentrations and an inhibitor at higher concentrations. The presence of the cations did not change the Km values, suggesting that the ions act as a reversible noncompetitive inhibitor on the FSR ATPase. The energy of activation of the enzyme was aproximately 19 Kcal/mole. The presence of the ions decreased the value slightly. A possible mechanism for the reversible noncompetitive inhibitory effect of the cations was discussed.