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-dependent phosphatase activity in rat renal cortex cytosol was investigated. The addition of Ca2+/calmodulin in the enzyme reaction mixture caused a significant increase in the dephosphorylation of p-nitrophenylphosphate and phosphotyrosine used as the substrate for phosphatase in rat renal cortex cytosol. The presence of regucalcin (10(-6) M) in the enzyme reaction mixture caused a complete inhibition of Ca2+/calmodulin-dependent phosphatase activity in renal cortex cytosol. A half maximum effect of regucalcin inhibition was seen at 10(-8) M concentration. Moreover, phosphatase activity of purified calcineurin was significantly enhanced by the addition of Ca2+/calmodulin. This enhancement was completely inhibited by the presence of regucalcin (10(-7) M). The inhibitory effect of regucalcin was not weakened by increasing concentrations of CaCl2 (10(-6) to 10(-4) M). The present results suggest that regucalcin can inhibit Ca2+/calmodulin-dependent phosphatase activity in rat renal cortex.