Functional Approach to the Catalytic Site of the Sarcoplasmic Reticulum Ca2+-ATPase: Binding and Hydrolysis of ATP in the Absence of Ca2+

Isolated sarcoplasmic reticulum vesicles in the presence of Mg2+ and absence of Ca2+ retain significant ATP hydrolytic activity that can be attributed to the Ca2+-ATPase protein. At neutral pH and the presence of 5 mM Mg2+, the dependence of the hydrolysis rate on a linear ATP concentration scale can be fitted by a single hyperbolic function. MgATP hydrolysis is inhibited by either free Mg2+ or free ATP. The rate of ATP hydrolysis is not perturbed by vanadate, whereas the rate of p-nitrophenyl phosphate hydrolysis is not altered by a nonhydrolyzable ATP analog. ATP binding affinity at neutral pH and in a Ca2+-free medium is increased by Mg2+ but decreased by vanadate when Mg2+ is present. It is suggested that MgATP hydrolysis in the absence of Ca2+ requires some optimal adjustment of the enzyme cytoplasmic domains. The Ca2+-independent activity is operative at basal levels of cytoplasmic Ca2+ or when the Ca2+ binding transition is impeded.