Activation of Hepatic Microsomal Ca²⁺-Adenosine Triphosphatase by Calcium-Binding Protein Regucalcin

Abstract

The effect of regucalcin, a calcium-binding protein isolated from rat liver cytosol, on Ca²⁺-adenosine triphosphatase (ATPase) activity in hepatic microsomes was investigated. Mg²⁺-ATPase activity was clearly increased by the presence of 50 μM Ca²⁺. Regucalcin (1.0-4.0 μM) caused a remarkable elevation (about 3-fold) of Ca²⁺-ATPase activity. Also, Mg²⁺-ATPase activity was increased (about 1.6-fold) by the presence of regucalcin (2.0 and 4.0 μM). Guanosine-5'-O-(3-thiotriphosphate) (GTP_rs; 10^-5 and 10^-4 M) and nicotinamide adenine dinucleotide phosphate oxidized form (NADP⁺; 10^-5 to 10^-3 M) or reduced form (NADPH; 10^-4 and 10^-3 M) significantly increased Ca²⁺-ATPase activity. These increases were not enhanced by the presence of regucalcin (2.0 μM). Of various metal ions, a comparatively low concentration of V⁵⁺ (10^-5 M) or Cd²⁺ (10^-6 M) significantly increased Ca²⁺-ATPase activity, while Hg²⁺, Zn²⁺, Cu²⁺ and Mn²⁺ did not have such an effect. Regucalcin (2.0 μM) did not enhance the effect of V⁵⁺ and Cd²⁺ on Ca²⁺-ATPase activity. The present finding, that regucalcin activates hepatic microsomal Ca²⁺-ATPase, suggests a cell physiological role of regucalcin as an activator in the microsomal Ca²⁺-pump activity. This action of regucalcin may not be influenced by other regulators.