The interaction of magnesium ions with the calcium pump of sarcoplasmic reticulum.

1. In the presence of Ca2+, ATP phosphorylates the Ca2+ pump of sarcoplasmic reticulum at the same site and to the same extent regardless of whether Mg2+ is added or not to the incubation media, the main effect of added Mg2+ being to increase the rate of phosphorylation. 2. When phosphoenzyme is made in Mg2+-containing media it dephosphorylates about 30-times faster than when it is made in the absence of added Mg2+. Addition of Mg2+ after phosphorylation is uneffective in accelerating the hydrolysis of phosphoenzyme even in solubilized enzyme, suggesting that phosphorylation of the Ca2+ pump results in occlusion of the site at which Mg2+ combines to accelerate the release of phosphate. 3. Occlusion of the site for Mg2+ can be partially reversed by trans-1,2-diaminocyclohexonetetraacetic acid (CDTA). Use was made of this property to demonstrate that for the rapid release of phosphate to occur Mg2+ has to be bound to the enzyme. 4. Results seem to indicate that Mg2+ combines with the Ca2+ pump prior to phosphorylation.