Regulatory effect of calcium-binding protein isolated from rat liver cytosol on activation of fructose 1,6-diphosphatase by Ca2+-calmodulin.

The role of a calcium-binding protein (CaBP) isolated from rat liver cytosol was investigated in relation to the activation of hepatic fructose 1, 6-diphosphatase by Ca2+-calmodulin. Fructose 1, 6-diphosphatase activity in rat liver cytosol was markedly increased by addition of Ca2+ (1.0-5.0 μM) to the incubation mixture. This increase was completely inhibited in the presence of N-(6-aminohexyl)-5-chloro-1-napthalenesulfonamide (W-7 15 μM), an inhibitor of calmodulin. Added Ca2+ (5.0 μM)-increased cytosolic fructose 1, 6-diphosphatase activity was markedly enhanced by the coexistence of calmodulin (2.5 μg/ml). Further, fructose 1, 6-diphosphatase isolated from rabbit liver cytosol was activated by Ca2+-calmodulin. This activation was completely inhibited by CaBP (20 μg/ml) isolated from rat liver cytosol, though CaBP in the absence of calmodulin had no effect on liver fructose 1, 6-diphosphatase activity. The present data suggest that CaBP can modify the action of Ca2+-calmodulin in liver cells. It is proposed that CaBP, which may regulate Ca2+ effects on liver function, should be named calregulin.