Role of endogenous regucalcin in the regualtion of Ca2+-ATPase activity inrat liver nuclei

The role of endogenous regucalcin in the regulation of Ca2+-ATPase, a Ca2sequestrating enzyme, in rat liver nuclei was investigated. Nuclear Ca2+-AT Pase activity was significantly reduced by the addition of regucalcin (0.1- 0.5 mu M) into the enzyme reaction mixture. The presence of anti-regucalcin monoclonal antibody (25 or 50 ng/ml) caused a significant elevation of Ca2 +-ATPase activity; this effect was completely abolished by the addition of regucalcin (0.1 mu M). The effect of anti-regucalcin antibody (50 ng/ml) in increasing Ca2+-ATPase activity was completely prevented by the presence o f thapsigargin (10(-6) M), an inhibitor of Ca2+ sequestrating enzyme, N-eth ylmaleimide (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 Ca2+-ATPase activity. Meanwhile, the effect of anti-regucalcin antibody (50 ng/ml) was significantly enhanced by the a ddition of calmodulin (5 mu g/ml), which could increase nuclear Ca2+-ATPase activity. In addition, the effect of antibody (50 ng/ml) was significantly reduced by the presence of trifluoperazine (20 mu M), an antagonist of cal modulin. These results suggest that the endogenous regucalcin in liver nucl ei has a suppressive effect on nuclear Ca2+-ATPase activity, and that reguc alcin can inhibit an activating effect of calmodulin on the enzyme. J. Cell . Biochem. 78:541-549, 2000. (C) 2000 Wiley-Liss, Inc.