Mechanism of the inhibition of protein synthesis by vasopressin in rat liver

A recent study reported that protein synthesis was inhibited in rat livers perfused with medium containing vasopressin (Chin, K.-V., Cade, C., Brostrom, M. A., and Brostrom, C. O. (1988) Int. J. Biochem. 20, 1313-1319). The inhibition of protein synthesis caused by vasopressin was associated with a disaggregation of polysomes, suggesting that peptide chain initiation was slowed relative to elongation. In contrast, Redpath and Proud (Redpath, N. T., and Proud C. G. (1989) Biochem. J. 262, 69-75) recently reported an inhibition of peptide chain elongation by a calcium/ calmodulin-dependent mechanism. Therefore, the question remained whether only peptide chain initiation was inhibited or both initiation and elongation were affected by vasopressin. In the present study, vasopressin was found to inhibit protein synthesis in both nerfused rat livers and isolated rat hepatocytes. Ribosomal half-transit times in isolated hepatocytes averaged 1.9 ± 0.1 min with or without vasopressin present in the media, demonstrating that the rate of peptide chain elongation was unaffected by vasopressin. Instead, the inhibition of protein synthesis induced by vasopressin was manifested at the level of peptide chain initiation. Vasopressin treatment resulted in both a 2-fold increase in the number of free ribosomal narticles and a greater than 50% decrease in the amount of [³⁵S]methionine bound to 43 S preinitiation complexes. In addition, the activity of eukaryotic mitation factor (eIF) 2B in crude extracts from perfused vers was reduced to 53% of the control value in response to vasopressin. The inhibition of eIF-2B activity was associated with an increase in the proportion of the α-subunit of eIF-2 in the phosphorylated form from 9 6% in control livers to 30.7% in livers perfused with medium containing vasopressin. The results demonstrate the novel finding that the inhibition of protein synthesis in vasopressin-treated livers is caused by a reduction eIF-2B activity due to an increase in phosphorylation of eIF-2α.