Hepatic Calcium-Binding Protein Regucalcin Decreases Ca²⁺/Calmodulin-Dependent Protein Kinase Activity in Rat Liver Cytosol

Abstract

The effect of regucalcin, a calcium-binding protein isolated from rat liver cytosol, on cytosolic Ca²⁺/calmodulin-dependent protein kinase activity was investigated. The increase in cytosolic Ca²⁺/calmodulin-dependent protein kinase activity with passage of incubation time was clearly prevented by the presence of regucalcin (1.0 μM). An appreciable effect of regucalcin was seen at 0.5 μM. The cytosolic Ca²⁺/calmodulin-dependent protein kinase activity was fairly increased by increasing concentrations of added Ca²⁺ (0.25-1.0 mM). This increase was clearly blocked by the presence of regucalcin (1.0 μM). The inhibitory effect of regucalcin on the protein kinase activity was also seen with varying concentrations of calmodulin (2.5-15 μg). In the presence of regucalcin (1.0 μM), trifluoperazin (50 μM), an antagonist of calmodulin, significantly decreased the cytosolic Ca²⁺/calmodulin-dependent protein kinase activity. These results suggest that regucalcin can regulate the Ca²⁺-calmodulin effect in liver cytosol.