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 Mg^sup 2+^ and absence of Ca^sup 2+^ retain significant ATP hydrolytic activity that can be attributed to the Ca^sup 2+^-ATPase protein. At neutral pH and the presence of 5 mM Mg^sup 2+^, 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 Mg^sup 2+^ 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 Ca^sup 2+^-free medium is increased by Mg^sup 2+^ but decreased by vanadate when Mg^sup 2+^ is present. It is suggested that MgATP hydrolysis in the absence of Ca^sup 2+^ requires some optimal adjustment of the enzyme cytoplasmic domains. The Ca^sup 2+^-independent activity is operative at basal levels of cytoplasmic Ca^sup 2+^ or when the Ca^sup 2+^ binding transition is impeded.[PUBLICATION ABSTRACT]