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

Abstract

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