Role of endogenous regucalcin in the regulation of Ca(2+)-ATPase activity in rat liver nuclei.

The role of endogenous regucalcin in the regulation of Ca(2+)-ATPase, a Ca(2+) sequestrating enzyme, in rat liver nuclei was investigated. Nuclear Ca(2+)-ATPase activity was significantly reduced by the addition of regucalcin (0.1-0.5 microM) into the enzyme reaction mixture. The presence of anti-regucalcin monoclonal antibody (25 or 50 ng/ml) caused a significant elevation of Ca(2+)-ATPase activity; this effect was completely abolished by the addition of regucalcin (0.1 microM). The effect of anti-regucalcin antibody (50 ng/ml) in increasing Ca(2+)-ATPase activity was completely prevented by the presence of thapsigargin (10(-6) M), an inhibitor of Ca(2+) sequestrating enzyme, N-ethylmaleimide (1 mM), a modifying reagent of thiol groups, or vanadate (10(-5) M), an inhibitor of phosphorylation of the enzyme by ATP, which revealed an inhibitory effect on nuclear Ca(2+)-ATPase activity. Meanwhile, the effect of anti-regucalcin antibody (50 ng/ml) was significantly enhanced by the addition of calmodulin (5 microg/ml), which could increase nuclear Ca(2+)-ATPase activity. In addition, the effect of antibody (50 ng/ml) was significantly reduced by the presence of trifluoperazine (20 microM), an antagonist of calmodulin. These results suggest that the endogenous regucalcin in liver nuclei has a suppressive effect on nuclear Ca(2+)-ATPase activity, and that regucalcin can inhibit an activating effect of calmodulin on the enzyme.