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Summary 1. The published results on protein synthesis during aging are contradictory. Possible sources of error and variability include: an insufficient number of different animal ages used; use of whole organs that are cytologically highly heterogeneous; different animal strains; neglecting to measure the specific activity of the precursor pool for protein synthesis; and inadequate methodology for measurement of in vivo rates of protein synthesis. 2. In general, protein synthesis rates in mammals have been reported to decline 4–70% with age. In insects and other organisms, greater losses (60–90%) have been observed. 3. Limited evidence indicates that in some systems a decline in the rate of protein synthesis may be due to alterations (as yet of unknown nature) in the initiation components of the protein synthetic apparatus. Futhermore, some studies suggest that in some organisms aging affects the expression of specific parts of the genome. 4. The significance of results on protein metabolism obtained from some studies with nematodes is at present unknown, owing to problems associated with age-synchronization methods. Also, the in vitro fibroblast system for the study of human cellular aging has not been met with universal acceptance; it is generally believed that this system has not yet been established as a valid analogy to mammalian aging in vivo. 5. Failure to detect defective enzymes in many old organisms indicates at least that not all proteins are altered during aging. The complete thermal stability of purified enzymes from old organisms suggests that the observed thermolability of the same enzymes in crude cell extracts is not an intrinsic property of those enzymes. Post-translational modifications (partial denaturation) may constitute the primary mechanism for the production of altered cell polypeptides during aging. 6. The available evidence does not support the concept of an age-dependent decline in translational accuracy. The future purification to absolute homogeneity of an altered enzyme and its ‘young’ (unaltered) counterpart, and their sequencing, should resolve the question of translational errors. 7. Some degree of age-related ribosome loss appears to occur in fixed postmitotic cells. In general, the published polyribosomal profiles may represent artefacts due to insufficiently suppressed ribonuclease activity during extraction. 8. The published studies on protein degradation during aging are also contradictory. Some investigators have neglected the possibility of reutilization of labelled amino acid. It is possible that some of the observed age-related alterations in protein degradation rates are due to altered endocrine status of the animals used, rather than to defects in the protein degradative pathways. The studies utilizing cell culture systems are also contradictory, probably due to different experimental designs. 9. Limited evidence suggests that protein degradation may slow down with age in mammals and nematodes. An inefficient protein degradation system in old organisms could provide an explanation for the accumulation of altered macromolecules in some organisms. Virtually nothing is known about regulatory mechanisms of protein degradation during senescence. 10. There is a need to examine which proteins are synthesized and degraded at selectively different rates as a function of age and what their physiological role is. This approach would be more informative than the study of total protein turnover with age. 11. The physiological significance, and the causes of the observed declines in protein synthesis and degradation rates during aging and senescence, remain to be established.

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