Kinetic characterization of Mg2+-dependent ATPase of sarcoplasmic reticulum

Kinetic properties of Mg2+-dependent ATPase activity (basic activity) of sarcoplasmic reticulum vesicles are studied in silver-treated preparations. The treated vesicles are shown to have no Mg2+ + Ca2+-dependent ATPase activity (extra activity) and a basic specific activity which is similar to that of untreated vesicles. The basic ATPase activity can be stimulated by different divalent cations. The relative effectiveness of activation is: Ca greater than Mg greater than or equal to MN greater than Cd greater than or equal to Co greater than Zn greater than Ni = Sr. The metal-ATP complex (MgATP or CaATP) is the true substrate of basic ATPase activity while the free ATP is a competitive inhibitor. The Km's found for MgATP and CaATP are respectively 0.32 mM and 0.41 mM. Ki for free ATP is in the range of 2 to 3 mM. The free divalent cations do not affect the ATPase activity. Both MgITP and CaITP are hydrolysed by the enzyme. The Km's for these complexes are respectively 0.67 and 0.73 mM. The Km for the substrate decreases when the pH of the medium increases from 5.5 to 8.5.