Effect of calcium-binding protein regucalcin on hepatic protein synthesis: inhibition of aminoacyl-tRNA synthetase activity.

The effect of regucalcin, a calcium-binding protein isolated from rat liver cytosol, on in vitro protein synthesis in the 5500 g supernatant fraction of rat liver homogenate was investigated. Addition of Ca2+ up to 5.0 microM in the reaction mixture caused a significant decrease in protein synthesis. This decrease was saturated at 10 microM Ca2+. The Ca2+ effect was not reversed by the presence of regucalcin (2.0 microM); the protein caused a remarkable decrease in hepatic protein synthesis, and it enhanced significantly the Ca2+ effect. Meanwhile, calmodulin (2.5-20 micrograms/ml), a calcium-binding protein, did not have an appreciable effect on the Ca2+ (10 microM)-induced decrease in hepatic protein synthesis. [3H]Leucyl-tRNA synthetase activity in the 105000 g supernatant fraction (cytosol) of liver homogenate was markedly decreased by addition of Ca2+ (1.0-50 microM). This decrease was not reversed by the presence of regucalcin (2.0 microM); the protein (1.0-2.0 microM) caused a remarkable decrease in the enzyme activity. The present results suggest that regucalcin can regulate protein synthesis in liver cells.