Protein turnover in senescent cultured chick embryo fibroblasts

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Abstract
The over‐all rates of protein synthesis, degradation and net accumulation were estimated in rapidly growing young and slowly doubling old cultures of chick fibroblasts. We find that not only the rate of protein synthesis is reduced in senescent cultures, but the average rate of protein degradation is also slowed down considerably. This decrease in the rate of protein breakdown in aging cells stands in contrast with the previously observed acceleration of this process by other conditions (such as serum deprivation or overcrowding) that lead to the cessation of cellular growth. Though the retarded protein degradation may contribute to the accumulation of abnormal proteins in senescen cells, we find that the breakdown of grossly abnormal puromycin peptides proceeds equally rapidly in young and old cultures. The protein content of senescent cells increase by 1.8‐fold as compared to young cells, while the average cell volume is increased even more (almost 5‐fold). By contrast, consideration of the over‐all balance of protein metabolism in these cells indicaes that the average concentration of metabolically turning‐over proteins is somewhat higher in senescent than in young fibroblasts.