Activating effect of regucalcin on (Ca(2+)-Mg2+)-ATPase in rat liver plasma membranes: relation to sulfhydryl group

The activating mechanism of regucalcin, a calcium-binding protein isolated from rat liver cytosol, on (Ca(2+)-Mg2+)-ATPase in the plasma membranes of rat liver was investigated. (Ca(2+)-Mg2+)-ATPase activity was markedly increased by a sulfhydryl (SH) group protecting reagent dithiothreitol (DTT; 2.5 and 5 mM as a final concentration), while the enzyme activity was significantly decreased by a SH group modifying reagent N-ethylmaleimide (NEM; 0.5-5 mM). The effect of DTT (5 mM) to increase the enzyme activity was clearly blocked by NEM (5 mM). Regucalcin (0.25-1.0 microM) significantly increased (Ca(2+)-Mg2+)-ATPase activity. This increase was completely blocked by NEM (5 mM). Meanwhile, digitonin (0.04%), which can solubilize the membranous lipids, significantly decreased (Ca(2+)-Mg2+)-ATPase activity. Digitonin did not have an effect on the DTT (5 mM)-increased enzyme activity. However, the effect of regucalcin (0.25 microM) increasing (Ca(2+)-Mg2+)-ATPase activity was entirely blocked by the presence of digitonin. The present results suggest that regucalcin activates (Ca(2+)-Mg2+)-ATPase by the binding to liver plasma membrane lipids, and that the activation is involved in the SH groups which are an active site of the enzyme.