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Protein turnover is the balance between protein synthesis and protein degradation. More synthesis than breakdown indicates an anabolic state that builds lean tissues, more breakdown than synthesis indicates a catabolic state that burns lean tissues. According to D.S. Dunlop, protein turnover occurs in brain cells the same as any other eucaryotic cells, but that "knowledge of those aspects of control and regulation specific or peculiar to brain is an essential element for understanding brain function."
Protein turnover is believed to decrease with age in all senescent organisms including humans. This results in an increase in the amount of damaged protein within the body. It is unknown if this is a cause or consequence of aging but it seems likely that it is both. The damaged protein results in a slower protein turnover which then results in more damaged protein causing an exponential increase in damage to all protein within the body and to aging.
Four weeks of aerobic exercise has been shown to increase skeletal muscle protein turnover in previously unfit individuals. A diet high in protein increases whole body turnover in endurance athletes.
Some bodybuilding supplements claim to reduce the protein breakdown by reducing or blocking the number of catabolic hormones within the body. This is believed to increase anabolism. However if protein breakdown falls too low then the body would not be able to remove muscle cells that have been damaged during workouts which would in turn prevent the growth of new muscle cells.
When older proteins are broken down in the body, they must be replaced. This concept is called protein turnover, and different types of proteins have very different turnover rates. Protein synthesis occurs during the process of translation on ribosomes. Protein breakdown occurs generally in two cellular locations:
- Lysosomal proteases digest endocytosed proteins
- Cytoplasmic complexes, called proteasomes, digest older or abnormal proteins that have been tagged with ubiquitin for destruction.
- Toyama, B. H.; Hetzer, M. W. (2013). "Protein homeostasis: Live long, won't prosper". Nature Reviews Molecular Cell Biology 14 (1): 55–61. doi:10.1038/nrm3496. PMC 3570024. PMID 23258296.
- Eden, E; Geva-Zatorsky, N; Issaeva, I; Cohen, A; Dekel, E; Danon, T; Cohen, L; Mayo, A; Alon, U (2011). "Proteome half-life dynamics in living human cells". Science 331 (6018): 764–8. doi:10.1126/science.1199784. PMID 21233346.
- [Aerobic Exercise Training Increases Skeletal Muscle Protein Turnover in Healthy Adults at Rest by Pikosky et. al http://jn.nutrition.org/content/136/2/379.long]
- [Level of dietary protein impacts whole body protein turnover in trained males at rest by Gaine, et. al http://general.utpb.edu/fac/eldridge_j/kine4364/protein%20intake.pdf]
- Dietary protein intake impacts human skeletal muscle protein fractional synthetic rates after endurance exercise by Bolster et. al
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