- Membrane Trafficking Proteins in Autophagosome Formation
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- Determination of Protein Turnover Rate - Creative Proteomics.
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Thank you for posting a review! We value your input. Share your review so everyone else can enjoy it too. Your review was sent successfully and is now waiting for our team to publish it. Additionally, this NBal method is rather inexpensive compared to isotopic methods. Invasive isotopic tracer methods to quantify protein turnover. The advantages and disadvantages of the use of stable isotopes as biological tracers were analyzed by Halliday and Rennie 7.
The major advantage of the use of stable isotopes is the lack of ionizing radiation, being therefore safer and less toxic for humans. Whole-body steady-state tracer techniques assume a simple two-pool model for protein metabolism in which amino acids are either free or protein bound. But there are more invasive methods that involve arteriovenous catheterizations. Moreover, a recent publication by Smith et al.
The lack of consensus reflects the fact that measurement of human protein metabolism in vivo has inherently difficult problems to solve inaccessible compartments, metabolically different compartments. Measurements of whole-body protein turnover from the dilution of tracer amino acids in plasma.
Membrane Trafficking Proteins in Autophagosome Formation
Stable isotopic tracers are administered as a priming dose and then continuous intravenous infusion: 15 N-glycine, 13 C-lysine, 2 H 5 -phenylalanine. Nowadays, the 15 N-glycine method has been gradually replaced with 13 C-lysine and 2 H 5 -phenylalanine. Isotopic enrichment at isotopic steady state is analyzed from a few blood samples see Ref. Approaches to the measurements of tissue protein synthesis using arteriovenous differences.
Stable isotopically labeled tracers have been used to evaluate MPS. This specific technique needs arteriovenous differences and muscle biopsy samples.
The low isotope enrichments of 13 C-lysine and 2 H 5 -phenylalanine perfusions are measured by a combination of gas chromatography and mass spectrometry giving access to MPS and balance However, as observed in human muscle tissues, the metabolic fate and the fractional synthesis rate FSR depend on the choice of the amino acid tracers 1, At present, it is possible to evaluate the distinct muscle fractions myofibrillar, sarcoplasmic, mitochondrial using different separation techniques such as gel electrophoresis and cation-exchange chromatography 1, A new technique based on [5,5,5- 2 H 3 ]-leucine perfusion and SDS-PAGE separation is able to identify muscle fiber myosin heavy chain, and therefore to distinguish slow- and fast-twitch muscle fibers Jaleel et al.
Stable isotope techniques did study the incorporation of labeled amino acids 13 C-proline, 15 N-proline into tendon and ligament to study the kinetics of human collagen 16, Some more recent techniques for the measurement of tissue turnover have been used in humans to precisely identify specific muscle protein modifications during exercise, such as the mRNA activity of different contractile proteins.
However, due to post-translational adaptations, it is difficult to quantitatively assume that the modified mRNA expression is true valuable evidence of specific protein synthesis.
Protein turnover in resting individuals. General view. The term turnover covers both the synthesis and breakdown of protein. Total protein synthesis in adult human subjects is about 3. This degradation rate corresponds to a half-life of approximately 20 days. The optimal intakes of whole protein in the human diet have been a matter of debate for many years see Ref.
The NBal data for healthy adult men and women increased from 0. However, the recommended intake of protein is higher for young children 1. The energy cost of protein turnover in healthy elderly humans years is comparable to that of a young population, and correlates with fat-free mass 18, Vegetarians restrict their diet to plant food and may be at risk of not getting adequate amounts of some indispensable amino acids lysine, methionine, cysteine, threonine because the amount of protein in plant foods compared to animal foods is inadequate. Moreover, plant proteins are generally less digestible than animal proteins.
Nevertheless, available evidence does not support the recommendation of a separate protein requirement for vegetarians who consume complementary mixtures of plant protein 6. We shall now focus our interest mainly on skeletal muscle and collagen tendon tissues, which appear to be quantitatively most important for physically active people. Skeletal muscle tissues.
Skeletal muscle tissue contains a few thousand specific proteins, which could be classified as myofibrillar, sarcoplasmic and mitochondrial fractions. The myofibrillar proteins are different molecules such as myosin heavy and light chains, actin, tropomyosin, troponins T, I, and C , titin, elastin, etc. The rate of myosin synthesis is lower than that of other muscle fractions Table 1. Thus, it appears that the FSR of actin is more or less 2-fold that of the myosin heavy chain. The precise mechanism of this specificity remains unknown. The overall control of the size of human skeletal muscle mass has been elegantly reviewed by Rennie et al.
Thus, the stimulation of protein synthesis depends on the concentration of extracellular rather than intramuscular EAA. Collagen tissues. Extracellular matrix located in tendon tissue ensures a functional link between the skeletal muscle mass and the bone. This unusual amino acid content is imposed by structural constraints unique to collagen molecules. Collagen is the most abundant single protein in most vertebrates humans included , constituting up to nearly one third of the total protein mass. Collagen molecules are not synthesized in muscle tissue but are synthesized in fibroblasts, which are scattered within the tissue.
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Given the importance of collagen to skeletal muscle function, the knowledge of its quantitative synthesis rate was either ignored or estimated to be very slow. Over the last few years, some studies have revealed that FSR of patellar tendon collagen amounted to a mean value of 1. When comparing the FSR of collagen and myofibrillar proteins in humans, there is no major difference as previously assumed, but muscle collagen is not at all responsive to feeding Meanwhile, collagen synthesis is similar in muscle after eccentric and concentric exercise Growth hormone GH and recombinant human GH have no effect on human muscle size or MPS 26 but do have a positive effect on strengthening the collagen matrix in musculotendinous tissue Protein metabolism in exercising individuals.
In , Goldberg et al.
Rennie et al. More information about the development of data may be obtained from a review The measurement of 3-methylhistidine 3-MeHis in urine has been recognized as a useful, indirect, noninvasive technique to assess muscle protein breakdown MPB. The major site of endogenous 3-MeHis is the actin of all muscle fibers and the myosin of white muscle fibers see Ref. However, Rennie and Millward 31 believe that urinary 3-MeHis is not a valid index of skeletal muscle production of 3-MeHis. This is why researchers now use isotopic techniques to evaluate protein turnover in human beings and to distinguish the effects of resistance from endurance exercise and training in skeletal muscle fibers 23, and tendons 17,23, As stated by Kumar et al.
The main operative distinction results in a phenotypic shift towards a population of fibers with greater oxidative capacity with endurance exercise, whereas repeated resistance exercise induces fiber hypertrophy.
Effect of resistance exercise. There are a few review publications related to the regulation of human MPS and MPB during and after resistance exercise 32, We will have to differentiate the results obtained in the fasted postabsorptive condition or fed state, during or after exercise, for MPS or MPB, using different modes of amino acids or protein supplementation.
The data in Table 2 clearly demonstrate that short-term intense resistance exercises induce a greater increase in the amount of mixed skeletal muscle protein at the end of exercise. Resistance exercise seems to be more relevant regarding myofibrillar proteins than sarcoplasmic protein synthesis Moreover, it appears that the increases in MPS persist up to 4 h after the end of exercise. On the contrary, resistance exercise does not seem to have a major effect on MPB.
The effect of feeding a mixed protein meal under resting conditions doubles MPS.
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Several publications have reported the results of resistance exercise on human MPS and breakdown see Table 3. Protein feeding has been applied after the end of exercise. Under most, if not all conditions, MPS has been enhanced by ingestion of amino acids. The increase in MPS is observed in mixed muscle, myofibrillar and sarcoplasmic fractions. It appears that MPS is increased when the amino acids are ingested immediately after the end of the exercise session Rapid aminoacidemia during the post-exercise period enhances MPS and the anabolic signals leading to the increase in muscle protein mass.
Protein targeting - Wikipedia
Moreover, it seems that a bolus dose of 25 g is more efficient than a series of small pulsed drinks 10 x 2. Both myofibrillar and sarcoplasmic proteins may remain stimulated up to h post-exercise or even up to 24 h in young men when the intensity of exercise is high Effect of endurance exercise. Again, it is necessary to differentiate between the effect of aerobic exercise under fasting or fed conditions. Table 4 summarizes the published data.
Endurance-type exercise seems to have less impact on skeletal MPS compared to resistance exercise.
This may be due, at least in part, to post-exercise proteolysis of non-myofibrillar proteins. Apparently, MPS increases after the end of the event and not during prolonged exercise Effect of exercise training on muscle protein synthesis and breakdown in humans. According to Kumar et al. Several investigators have reported an enhanced basal rate of MPS but it seems difficult to have a precise idea about these changes due to the lack of information on the time course of the last bout of exercise sessions during the training schedule. However, an accurate report before and after 10 weeks of training indicated an increase in the basal synthesis of myofibrillar proteins under resistance exercise, while endurance training enhanced basal mitochondrial protein synthesis Furthermore Meredith et al.
Sex differences in muscle protein metabolism under exercise conditions. The literature does not provide significant information about a smaller muscle mass in women compared to men, unless anabolic hormonal injections such as testosterone are administered 45, Vingren et al. They concluded that testosterone plays only a minor role in explaining the difference of muscle mass between women and men.
Moreover, Kumar et al. In addition, using two variable protein intakes, Pannemans et al.