Effects of a Ca2+ gradient and water activity on the phosphorylation of Ca(2+)-ATPase by Pi

The effects of dimethyl sulfoxide (20% v/v) on the phosphorylation of Ca(2+)-ATPase of the sarcoplasmic reticulum by Pi vary depending on whether or not a Ca2+ gradient is formed across the vesicle membranes. In the absence of a Ca2+ gradient the solvent promotes a large increase in the affinity for Pi. This increase is no longer observed after the formation of a Ca2+ gradient. The enzyme affinity for Mg2+ is practically the same in the presence and absence of a gradient. Addition of dimethyl sulfoxide leads to an increase of the enzyme affinity for Mg2+ both in the presence and in the absence of a gradient.