Skeletal muscle proteins

In humans, skeletal muscle protein is the major nonfat source of stored energy. This explains the very large losses of muscle mass, particularly in adults, resulting from prolonged caloric undernutrition. 

Arihara K, Nakashima Y, Mukai T, Ishikawa S, Itoh M. (2001) Peptide inhibitors for angiotensin I-converting enzyme from enzymatic hydrolysates of porcine skeletal muscle proteins. Meat Sci 57 319-324. 

Hackman, P., Vihola, A., and Haravuori, H. (2002). Tibial muscular dystrophy is a titinopathy caused by mutations in TTN, the gene encoding the giant skeletal-muscle protein titin. Am.J. Hum. Genet. 71, 492-500. 

Twelve ACE-inhibitory peptides have been identified from sardine muscle hydrolysate, revealing that a dipeptide, Val-Tyr, acts as a key inhibitor (Matsufuji et al., 1994). Of the identified ACE-inhibitory peptides, the tripeptides (Leu-Arg-Pro, lie-Val-Tyr) and the dipeptide (Val-Tyr) show strong inhibitory activity. Moreover, two inhibitory peptides (myopentapep-tides A and B) have been purified from a thermolysin digest of porcine skeletal muscle proteins. The sequences were found in the primary structure of the myosin heavy chain (Arihara et al., 2001). 

Also, HPLC methods with electrochemical or fluorescent detection are used (H19, M3). In proteins, dityrosine can be estimated by immunochemical methods employing dityrosine-specific antibodies (K5). Measurements of o,o -dityrosine and o-tyrosine levels in rat urine express dityrosine contents in skeletal muscle proteins, and have been proposed as the noninvasive oxidative stress test in vivo. One should be aware, however, that A-formylkynurenine, also formed in protein oxidation, has similar fluorescence properties as dityrosine (excitation 325 nm, emission at 400-450 nm) (G29). Also, oxidation of mellitin when excited at 325 nm produces an increase in fluorescence at 400—450 nm, despite the fact that mellitin does not contain tyrosine. Oxidation of noncontaining Trp residues ribonuclease A and bovine pancreatic trypsin inhibitor with "OH produces loss of tyrosine residues with no increase in fluorescence at 410 nm (S51). There are also methods measuring the increased hydrophobicity of oxidized proteins. Assays are carried out measuring protein binding of a fluorescent probe, 8-anilino-l-naphthalene-sulfonic acid (ANS). Increase in probe binding reflects increased surface hydrophobicity (C7). 

L12. Leeuwenburgh, C., Wagner, P., Holloszy, J. O., Sohal, R. S., and Heinecke, J. W., Caloric restriction attenuates dityrosine cross-linking of cardiac and skeletal muscle proteins in aging mice. Arch. Biochem. Biophys. 346, (1997). 

Smith, C.K., Durschlag, R.P. Layman, D.K. (1982) Response of skeletal muscle protein synthesis and breakdown to levels of dietary protein and fat during growth in weanling rats. 

Using casein as substrate, cyclic AMP has been measured directly in crude tissue extracts by the stimulation of the rate of phosphorylation of casein catalysed by skeletal muscle protein kinase [158]. By using high concentrations of casein and [y- P]ATP, the interference with the protein kinase activity by materials present in the tissue extracts is minimised and preliminary purification is not necessary. The phosphory-lated casein is isolated on filter paper discs. As little as 0.5 pmol of cyclic AMP can be measured. The assay is rapid and simple. 

Anabolic steroids decrease catabolism and increase skeletal muscle protein synthesis. Whether this results in muscular hypertrophy or hyperplasia, or a combination of these, is unclear and probably depends upon the muscle studied. Different muscle types contain different cytosolic receptor numbers and, therefore, the response to anabolic steroids varies. Anabolic steroids initiate an increase in RNA polymerase activity and the synthesis of either structural or contractile proteins. In some muscles, anabolic steroids may increase the ratio of fast twitch to slow twitch fibers (Nimmo et al 1982, Snow et al 1982). Increased activity of enzymes involved in energy metabolism may also occur. However, the total glycogen content may remain unchanged (Hyyppa et al 1997). The effects are most profound in females and castrated males (Snow 1993). 

Cooney RN, Kimball SR, Vary TC. Regulation of skeletal muscle protein turnover during sepsis Mechanisms and mediators. Shock 1997 7 1-16. 

G7. Giometti, C. S., Anderson, N. G., and Anderson, N. L., Muscle protein analysis. 1. High-resolution two-dimensional electrophoresis of skeletal muscle proteins for analysis of small biopsy samples. Clin. Chem. 25, 1877-1884 (1979). 

Preedy, V. R., Duane, P., and Peters, T. ]., Comparison of the acute effects of ethanol on liver and skeletal muscle protein syn thesis in the rat, Alcohol Alcoholism, 23, 155, 1988. 

Skeletal muscle is specialized to perform intermittent mechanical work. As described previously, the energy sources that provide ATP for muscle contraction depend on the degree of muscular activity and the physical status of the individual. During fasting and prolonged starvation, some skeletal muscle protein is degraded to provide amino acids (e.g., alanine) to the liver for gluconeogenesis. 

Okita M, Watanabe A, Tsuji T. 1988. Effect of branched-chain amino acid on 15N incorporation into liver and skeletal muscle proteins following [15N]-ammonium chloride administration to carbon tetrachloride-intoxicated rats. J Nutr Sci Vitaminol 34(1) 85-96. 

Fig. 42.3. Interorgan amino acid exchange after an overnight fast. After an overnight fast (the postabsorptive state), the utilization of amino acids for protein synthesis, for fuels, and for the synthesis of essential functional compounds continues. The free amino acid pool is supported largely by net degradation of skeletal muscle protein. Glutamine and alanine serve as amino group carriers from skeletal muscle to other tissues. Glutamine brings NH4 to the kidney for the excretion of protons and serves as a fuel for the kidney, gut, and cells of the immune system. Alanine transfers amino groups from skeletal muscle, the kidney, and the gut to the liver, where they are converted to urea for excretion. The brain continues to use amino acids for neurotransmitter synthesis.

The brain is glucose dependent, but, like many cells in the body, can use BCAA for energy. The BCAA also provide a source of nitrogen for neurotransmitter synthesis during fasting. Other amino acids released from skeletal muscle protein degradation also serve as precursors of neurotransmitters. 

The importance of the gut in whole body nitrogen metabolism arises from the high rate of division and death of intestinal mucosal cells and the need to continuously provide these cells with amino acids to sustain the high rates of protein synthesis required for cellular division. Not only are these cells important for the uptake of nutrients, but they maintain a barrier against invading bacteria from the gut lumen and are, therefore, part of our passive defense system. As a result of these important functions, the intestinal mucosal cells are supplied with the amino acids required for protein synthesis and fuel oxidation at the expense of the more expendable skeletal muscle protein. 

In these hypercatabolic states, skeletal muscle protein synthesis decreases, and protein degradation increases. Oxidation of BCAA is increased and glutamine production enhanced. Amino acid uptake is diminished. Cortisol is the major hormonal mediator of these responses, although certain cytokines may also have direct effects on skeletal muscle metabohsm. As occurs during fasting and metabolic acidosis, increased levels of cortisol stimulate ubiquitin-mediated proteolysis, induce the synthesis of glutamine synthetase, and enhance release of amino acids and glutamine from the muscle cells. 

Fig.5.2. Post-translational modification of histidine residues in skeletal muscle proteins, and the release of 3-methyl histidine after proteolysis.

Prostaglandins have also been suggested as important regulators of skeletal muscle protein breakdown. Enhanced protein catabolism and synthesis of PGE2 is seen in skeletal muscle of rats injected with endotoxin " . Indomethacin reduced leukocyte-pyrogen-induced proteolysis of skeletal muscle in vitro " however, in a canine model of E. coli sepsis, ibuprofen had minimal effects on protein catabolism . 

Three protein phosphatase activities have been isolated from rabbit skeletal muscle protein phosphatase I (mol. wt. 3 x 10 , an active histone phosphatase) exhibited only little glycogen synthase phosphatase-1 and -2, phosphorylase... 

Inspired by the skeletal muscle protein titin, Guan and coworkers have reported a biomimetic concept, which exploits a reversibly unfolding modular cross-linker (Figure 15). Titin s combination of high-strength... 

Drummond MJ, Dreyer HC, Fry CS et al. (2009) Nutritional and contractile regulation of human skeletal muscle protein synthesis and mTORC 1 signaling. J Appl Physiol 106, 1374-1384. 

Fujita S, Rasmussen BB, Cadenas JG et al. (2006) Bffect of insulin on human skeletal muscle protein synthesis is modulated by insulin-induced changes in musde blood flow and amino acid availability. Am J Physiol Endocrinol Metab 291, B745-B754. 

Fujita S, Glynn BL, Timmerman KL et al. (2009) Supraphysiological hyperinsuhnaemia is necessary to stimulate skeletal muscle protein anabolism in older adults evidence of a true age-related insulin resistance of musde protein metabolism. Diabetologia 52, 1889-1898. 

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