Effect of calcium-binding protein on the activation of phosphorylase a in rat hepatic particulate glycogen by Ca2+.

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Abstract
The effect of a calcium-binding protein (CaBP) isolated from rat liver cytosol on phosphorylase a activity in the hepatic particulate glycogen was investigated. The particulate glycogen phosphorylase a activity was significantly increased by addition of Ca2+ in the range of 1.0-103 .mu.M. This increase was not prevented by the presence of N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7 100 .mu.M), an inhibitor of calmodulin. The elevation of phosphorylase a activity by 10 .mu.M Ca2+ addition was significantly inhibited by the presence of CaBP at a concentration of greater than 3.0 .mu.M. This inhibition was complete at 7.0 .mu.M CaBP. CaBP itself had no effect on the enzyme activity. Of various metals (20 .mu.M) used, addition of Zn2+ caused a significant decrease of the particulate glycogen phosphorylase a activity, while Mg2+, Mn2+ and Cd2+ had no effect on the enzyme activity. The presence of 3.5 .mu.M CaBP did not block the inhibition of the enzyme activity by Zn2+ addition, indicating that CaBP uniquely reverses the Ca2+ effect. The present study suggests that CaBP regulates the effect of Ca2+ on phosphorylase a activity in the hepatic particulate glycogen of rats.