Calcium-binding protein regucalcin is an activator of (Ca2+-Mg2+)-adenosine triphosphatase in the plasma membranes of rat liver.

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Abstract
The effect of regucalcin, a calcium-binding protein isolated from rat liver cytosol, on (Ca2+-Mg2+)-adenosine triphosphatase (ATPase) activity in the plasma membranes of rat liver was investigated. 125I-Regucalcin bound to the plasma membranes in the presence or absence of 0.1 mM Ca2+. Regucalcin (0.35 .mu.M) increased the plasma membrane (Ca2+-Mg2+)-ATPase activity about 15% (p < 0.01), and a higher concentration (4.0 .mu.M) showed a remarkable effect. Also, regucalcin (0.35-2.0 .mu.M) markedly increased the plasma membrane Mg2+-ATPase activity in the absence of Ca2+. The effect of regucalcin on (Ca2+-Mg2+)-ATPase activity was not regulated by the presence of guanosine-5''-O-(3-thiotriphosphate) (10-5 and 10-4 M), glucagon (10-6 and 10-5 M) or norepinephrine (10-7-10-5 M), suggesting that the regucalcin effect is not linked to guanosine triphosphate (GTP)-binding protein in the plasma membranes. Vanadium (10 and 10 .mu.M), which inhibits the phosphorylation of liver plasma membrane (Ca2+-Mg2+)-ATPase, decreased the enzyme activity about 20% (p < 0.01). This decrease was completely restored by the presence of regucalcin (1.0 .mu.M). The present results indicate that regucalcin binds to the plasma membranes of liver cells and increases the (Ca2+-Mg2+)-ATPase activity. Regucalcin may be an activator of (Ca2+-Mg2+)-ATPase in the plasma membranes.