Studies of Protein Retention and Turnover Using Nitrogen-15 as a Tag

Sharp, George S.; Lassen, Sven; Shankman, S.; Hazlet, John W.; Kendis, Merwin S.

doi:10.1093/jn/63.1.155

Cited by 31 publications

(7 citation statements)

References 7 publications

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“…In another subject there was evidence of a high basal energy requirement (50 kcal/kg per d to maintain weight). Nitrogen balance can be altered by such variables as age (26), sex (27), basal metabolic rate (28), body temperature (29), previous dietary intake (30), and changing energy expenditure during refeeding (31). Since somatomedin-C appears to be a very sensitive indicator of nutritional status, it is probable that some of these variables can modify the somatomedin-C response to dietary manipulation.…”

Section: Resultsmentioning

confidence: 99%

Dietary components that regulate serum somatomedin-C concentrations in humans.

Isley¹,

Underwood²,

Clemmons³

1983

J. Clin. Invest.

418

140

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Section: Resultsmentioning

confidence: 99%

Dietary components that regulate serum somatomedin-C concentrations in humans.

Isley¹,

Underwood²,

Clemmons³

1983

J. Clin. Invest.

418

140

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“…In an early study of human protein metabolism with [lSN]glycine, it was found that protein turnover in young subjects (age 24 years) was one-and-one-half times that in older ones (ages 59-70 years) (Sharp et al 1957). Winterer et al (1976) studied protein metabolism in healthy human subjects, aged 65-91 years, using the [15N]glycine infusion method of Picou & Taylor-Roberts (1969).…”

Section: (D) Whole-body Protein Turnovermentioning

confidence: 99%

Protein Synthesis and Degradation During Aging and Senescence

Makrides

1983

Biological Reviews

171

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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 a...

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“…At least three factors may account for these varied outcomes. First, some studies have relied on small sample sizes, often comparing eight or fewer people grouped as "young" and "old" (7,11,12,14,15,23,33,45,49,51). Unless the variability among people is small, there may not be adequate power to detect a difference between age groups.…”

mentioning

confidence: 99%

Age and aerobic exercise training effects on whole body and muscle protein metabolism

Short

Vittone

Bigelow

et al. 2004

American Journal of Physiology-Endocrinology and Metabolism

393

311

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.-Aging in humans is associated with loss of lean body mass, but the causes are incompletely defined. Lean tissue mass and function depend on continuous rebuilding of proteins. We tested the hypotheses that whole body and mixed muscle protein metabolism declines with age in men and women and that aerobic exercise training would partly reverse this decline. Seventy-eight healthy, previously untrained men and women aged 19-87 yr were studied before and after 4 mo of bicycle training (up to 45 min at 80% peak heart rate, 3-4 days/wk) or control (flexibility) activity. At the whole body level, protein breakdown (measured as [13 C]leucine and [ 15 N]phenylalanine flux), Leu oxidation, and protein synthesis (nonoxidative Leu disposal) declined with age at a rate of 4-5% per decade (P Ͻ 0.001). Fat-free mass was closely correlated with protein turnover and declined 3% per decade (P Ͻ 0.001), but even after covariate adjustment for fat-free mass, the decline in protein turnover with age remained significant. There were no differences between men and women after adjustment for fat-free mass. Mixed muscle protein synthesis also declined with age 3.5% per decade (P Ͻ 0.05). Exercise training improved aerobic capacity 9% overall (P Ͻ 0.01), and mixed muscle protein synthesis increased 22% (P Ͻ 0.05), with no effect of age on the training response for either variable. Fat-free mass, whole body protein turnover, and resting metabolic rate were unchanged by training. We conclude that rates of whole body and muscle protein metabolism decline with age in men and women, thus indicating that there is a progressive decline in the body's remodeling processes with aging. This study also demonstrates that aerobic exercise can enhance muscle protein synthesis irrespective of age. leucine; phenylalanine; amino acid kinetics; resting metabolic rate; fractional synthesis rate; aging AGING IN HUMANS is associated with alterations in body composition, leading to a loss of lean tissue and a corresponding gain in body fat (34,35). This shift in body composition is considered a risk factor for disease and disability, yet the mechanisms for lean tissue loss in older persons are not yet fully understood. The quantity of lean tissue mass depends on protein turnover and the balance between protein synthesis and breakdown. Protein kinetics at the level of the whole body, body regions, or individual tissues can be readily measured in vivo with the use of labeled amino acid tracers (24). Although these methods have been used to determine whether protein metabolism is altered with aging, several questions remain.Several studies have compared whole body protein turnover data in younger and older people (Table 1) but have provided widely mixed results. At least three factors may account for these varied outcomes. First, some studies have relied on small sample sizes, often comparing eight or fewer people grouped as "young" and "old" (7,11,12,14,15,23,33,45,49,51). Unless the variability among people is small, there may not be adequate power to detect a ...

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Studies of Protein Retention and Turnover Using Nitrogen-15 as a Tag

Cited by 31 publications

References 7 publications

Dietary components that regulate serum somatomedin-C concentrations in humans.

Dietary components that regulate serum somatomedin-C concentrations in humans.

Protein Synthesis and Degradation During Aging and Senescence

Age and aerobic exercise training effects on whole body and muscle protein metabolism

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