Ageing studies in rat liver. II. Patterns of cellular and cell-free protein synthesis from one to ten months of age.

Potential changes in liver protein biosynthesis were investigated in 1 to 10-month-old male Fischer F344 rats by isolation and translation of polysome-derived poly(A)-containing RNA and by incubation of isolated hepatocytes in Swim's S77 medium supplemented with [3H]leucine. Labeled protein products were analyzed by one- and two-dimensional polyacrylamide gel electrophoresis followed by fluorography. The protein profiles revealed that the translation products of poly(A)-containing RNA derived from the livers of 1 -months-old animals lacked a major 22000 dalton band which was present in comparable profiles from livers of older animals. In parallel experiments, gel profiles of products from cultured hepatocytes isolated from 1-month-old animals were missing an 18000 dalton band which was seen in preparations from older animals. With livers from rats between 2 and 10 months of age, however, no age-related changes in the pattern of protein biosynthesis were detected either in the products of poly(A)-containing RNA translated in vitro or in the cellular and secreted proteins of intact hepatocytes. Furthermore, the polysome content (A1(260)cm/g tissue) was constant between 1 and 10 months of age, providing a constant amount of tissue was processed. A procedure for maximizing the yield of polysomes is described. These results suggest that the decline in protein synthesis observed in rat liver and in cultured hepatocytes during the first year of life is not due to changes in mRNA content or the variety of specific products synthesized, but results primarily from an overall decrease in protein biosynthesis, probably at the level of peptide chain elongation.