INHIBITORY EFFECT OF REGUCALCIN ON CA2+ CALMODULIN-DEPENDENT PHOSPHATASE-ACTIVITY IN RAT RENAL-CORTEX CYTOSOL/

The effect of regucalcin, a Ca2+-binding protein, on Ca2+/calmodulin-d ependent phosphatase activity in rat renal cortex cytosol was investig ated. The addition of Ca2+/calmodulin in the enzyme reaction mir;turf caused a significant increase in the dephosphorylation of p-nitropheny lphosphate and phosphotyrosine used as the substrate for phosphatase i n rat renal cortex cytosol. The presence of regucalcin (10(-6)M) in th e enzyme reaction mixture caused a complete inhibition of Ca2+/calmodu lin-dependent phosphatase activity in renal cortex: cytosol, A half ma ximum effect of regucalcin inhibition was seen at 10(-8) M concentrati on. Moreover phosphatase activity of purified calcineurin was signific antly enhanced by the addition of Ca2+/calmodulin. This enhancement wa s completely inhibited by the presence of regucalcin (10(-7) M). The i nhibitory effect of regucalcin was not weakened by increasing concentr ations of CaCl2 (10(-6) to 10(-4) M). The present results suggest that regucalcin can inhibit Ca2+/calmodulin-dependent phosphatase activity in fat renal cortex.