Regulatory effect of regucalcin on (Ca(2+)-Mg2+)-ATPase in rat liver plasma membranes: comparison with the activation by Mn2+ and Co2+.

The effect of various metals and regucalcin, a calcium-binding protein isolated from rat liver cytosol, on (Ca(2+)-Mg2+)-ATPase activity in the plasma membranes of rat liver was investigated. Of various metals (Zn2+, Cu2+, Ni2+, Mn2+, Co2+ and Al3+; 100 microM as a final concentration), Mn2+ and Co2+ increased markedly (Ca(2+)-Mg2+)-ATPase activity, while other metals had no effect. When Ca2+ was not added into enzyme reaction mixture, Mn2+ and Co2+ (25-100 microM) did not significantly increase the enzyme activity, indicating that heavy metals act on Ca(2+)-stimulated phosphorylation of the enzyme. Meanwhile, regucalcin (0.25-1.0 microM) caused a remarkable elevation of (Ca(2+)-Mg2+)-ATPase activity. This increase was not inhibited by the presence of 100 microM vanadate, although the effects of Mn2+ and Co2+ (100 microM) were inhibited by vanadate. Also, the inhibition of the Mn2+ and Co2+ effects by vanadate was not seen in the presence of regucalcin. Moreover, regucalcin (0.5 microM) increased significantly the enzyme activity in the absence of Ca2+. This effect of regulcalcin was not altered by increasing concentrations of Ca2+ added, indicating that the regucalcin effect does not depend on Ca2+. The present results suggest that regucalcin activates directly (Ca(2+)-Mg2+)-ATPase in liver plasma membranes, and that the activation is not involved in the Ca(2+)-dependent phosphorylation of the enzyme.