Myofibrillar protein

In addition to the major proteins of striated muscle (myosin, actin, tropomyosin, and the troponins), numerous other proteins play important roles in the maintenance of muscle structure and the regulation of muscle contraction. Myosin and actin together account for 65% of the total muscle protein, and tropomyosin and the troponins each contribute an additional 5% (Table 17.1). The other regulatory and structural proteins thus comprise approximately 25% of the myofibrillar protein. The regulatory proteins can be classified as either myosin-associated proteins or actin-associated proteins. 

The smdy of tissue protein breakdown in vivo is difficult, because amino acids released during intracellular breakdown of proteins can be extensively reutilized for protein synthesis within the cell, or the amino acids may be transported to other organs where they enter anabohc pathways. However, actin and myosin are methylated by a posttranslational reaction, forming d-methylliistidine. During intracellular breakdown of actin and myosin, 3-methylhistidine is released and excreted into the urine. The urinary output of the methylated amino acid provides a rehable index of the rate of myofibrillar protein breakdown in the musculature of human subjects. 

Jasmer, D.P., Bohnet, S. and Prieur, D.J. (1991) Trichinella spp. differential expression of acid phosphatase and myofibrillar proteins in infected muscle cells. Experimental Parasitology 72, 321-331. 

The slower relaxing component, characterised by a time constant of approximately 100-250 ms, accounting for 5-15% of the relaxation, and in the following referred to as T22, represents water located outside the myofibrillar protein network, i.e., extra-myofibrillar water. 

In summary, the above studies on the relationship between meat structure, composition, and transverse relaxation are consistent with the ascription assignment of the T2i relaxation component to water located in the myofibrillar protein matrix. In addition, the studies confirm that transverse relaxation is an excellent tool for obtaining information about structural features in meat. 

Loss of myofibrillar protein from the diaphragm and intercostal muscles limits the ability to cough, reducing the efficiency of fluid removal from the lungs and bronchioles and so increasing risk of infection. Pneumonia is a likely cause of death. 

In contrast to milk, where samples are primarily derived from cows, meat analysis has to be performed in samples of a widely different animal origin including cattle, lamb, swine, poultry, and fish. Muscle is a complex matrix with a pH of 5.7, composed of muscle fibers, various types of connective tissue, adipose tissue, cartilage, and bones. Sarcoplasmic proteins such as myoglobin, and glycolytic enzymes are soluble in water while the myofibrillar proteins such as myosin and actin are soluble in concentrated salt solutions (14). The connective tissue proteins, collagen and elastin, are insoluble in both solvents. 

The Ca2+-dependent neutral proteases called calpains are found within the cells of higher animals. The 705-residue multidomain peptide chain of a chicken calpain contains a papain-like domain as well as a calmodulin-like domain.328 It presumably arose from fusion of the genes of these proteins. At least six calpains with similar properties are known.329 Some have a preference for myofibrillar proteins or neurofilaments.330 They presumably function in normal turnover of these proteins and may play a role in numerous calcium-activated cellular processes.331-3323... 

The myofibrillar proteins make up 50-60% of the total protein of muscle cells. Insoluble at low ionic strengths, these proteins dissolve when the ionic strength exceeds -0.3 and can be extracted with salt solutions. Analysis of isolated mammalian myofibrils86 shows that nine proteins account for 96% or more of the protein myosin, which constitutes the bulk of the thick filaments, accounts for 43% and actin, the principal component of the thin filaments, 22%. 

Before selecting a method to measure a specific aspect of protein functionality, one must decide on the complexity of the testing matrix. Researchers have used a single purified protein, a crude extract of proteins, a prototype food product, or an actual product to study protein functionality. For meat studies, formulated meat systems, ground muscle, myofibrillar proteins, salt-soluble proteins, actomyosin,... 

Dayton, W., Goll, D., Zeece, M., Robson, R., Reville, W., 1976, A Ca2+-activated protease possibly involved in myofibrillar protein turnover. Purification from porcine muscle, Biochemistry, 15, 2150-2158... 

Crockford, T. and Johnston, I.A. (1993). Developmental changes in the composition of myofibrillar proteins in the swimming muscles of Atlantic herring. Marine Biology 115,15-22. 

MALDI-TOF/TOF MS to successfully study the carbonylation of sarcoplasmic and myofibrillar proteins from fish subjected to metal-catalyzed oxidation (98). 

Morkin, E. 1970. Postnatal muscle fiber assembly localization of newly synthesized myofibrillar proteins. Science 167 1499-1501. 

Groninger (38) succinylated fish myofibrillar protein and examined some of its chemical and functional properties. The fish myofibrillar proteins were succinylated at different levels and the degree of succinyla-tion was related to a functional property such as emulsification capacity. The protein efficiency ratio for succinylated protein was lower than that of untreated fish protein. Grant (39) succinylated wheat flour proteins and analyzed their solubility, viscosity, and chromatographic behavior. The effects of acetylation and succinylation on the physicochemical and functional properties of several plant proteins was reviewed recently by Kinsella and Shetty (6). 

Candido, L. M. B., and Sgarbieri, V. C. 2003. Enzymatic hydrolysis of Nile tilapia (Oreo-chromus niloticus) myofibrillar proteins effects on nutritional and hydrophilic properties. J. Sci. Food Agric., 83, 937-944. 

Saiga, A.,Tanabe, S.,and Nishimura, T. 2003. Antioxidant activity of peptides obtained from porcine myofibrillar proteins by protease treatment. I. Agric. Food Chem.,51, 3661-3667. 

The amount of stroma proteins is less in fish muscles (3-5%) than it is In beef or rabbit muscles (15-18%). This may explain why raw fish fillets are acceptable in Japanese dishes, whereas beef, rabbit and pork are rarely served raw. According to Fennema et al. (9.), tenderness is primarily related to collagen content, while toughness and water-holding capacity are associated with the myofibrillar proteins. Many papers on cooked meat mention both tenderness and toughness, while those on cooked fish note the problems of toughness rather than tenderness. This also might be related to the difference in content of the stroma proteins. 

Actomyosin. At high salt concentrations ( . . 0.6 M KC1), actin and myosin combine to form actomyosin filaments giving a highly viscous solution. Actomyosin retains the ATPase activity of myosin and demonstrates "super-precipitation" on the addition of ATP (24,34). As expected, there are differences between actomyosins of rabbit and fish with respect to solubility (10,22,35,36), viscosity (46) and ultracentrifugal behavior (477. Since actomyosin is the most readily available form of myofibrillar proteins from fish muscle, its behavior relative to deterioration during frozen storage has been most frequently studied. 

Among the above hypotheses, effects of lipids (4-17,59-62, 69-71,155-159), formaldehyde (160-166), and gas-solid interface TMJ appear to be very important in Gadoid fishes. Denaturation of myofibrillar proteins caused by free fatty acids and/or lipid peroxides must occur during frozen storage. To prove this, Jarenback and Liljemark have shown by electron microscopy that, in muscle stored frozen with added linoleic and linolenic hydroperoxides, myosin became resistant to extraction with salt solution (168). 

Myofibrils are composed of several myofibrillar proteins (see Table 21.1). 

Table 21.1. Relative Amounts of Major Myofibrillar Proteins (adapted from Pearson and Young, 1989).

The generation of lactic acid through glycolysis produces a pH drop due to its accumulation in the muscle. The rate of drop may be faster or slower depending on the metabolic status of the muscle. In general, acid pH values (5.6-5.9) may be reached in just a few hours postmortem. Water binding decreases rapidly as pH approaches the isoelectric point of muscle proteins (pi values around 5.0). There is also a tightening of the structure and partial denaturation of myofibrillar proteins. 

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