Suppressive effect of endogenous regucalcin on guanosine triphosphatase activity in rat liver nucleus

The effect of regucalcin, a regulatory protein of Ca2+ signaling, on guanos ine triphosphatase (GTPase) activity in the nuclei of rat liver was investi gated. GTPase activity was significantly increased by the addition of CaCl2 (50 muM) in the enzyme reaction mixture. This increase was not seen in the presence of trifluoperazine (25 muM) antagonist of calmodulin, which could decrease nuclear GTPase activity, suggesting that nuclear endogenous calmo dulin is involved in an increase in the enzyme activity related to Ca2+ add ition. The presence of regucalcin (0.5 muM) in the enzyme reaction mixture caused a significant decrease in nuclear GTPase activity. The enzyme activi ty was significantly raised in the presence of anti-regucalcin monoclonal a ntibody (25 and 50 ng/ml) in the reaction mixture. This increase was comple tely abolished by the addition of regucalcin (0.5 muM). Also, the effect of regucalcin addition in increasing nuclear GTPase activity was seen in the presence of EGTA (0.1 mM), a chelator of Ca2+. The present study demonstrat es that endogenous regucalcin has a suppressive effect on GTPase activity i n the nuclei of rat liver.