Muscle insoluble protein

On a larger scale, we have ligaments and cartilage, structures that hold joints together and tie muscle to bone. These too are composed of proteins tough, water-insoluble proteins. Principal among them are the collagens. 

Muscle proteins are an important component of meat and can be classified according to solubility as sarcoplasmic (water soluble), myofibrillar (salt soluble), or stromal (insoluble) proteins. The application of CE to the analysis of meat proteins has been predominantly for separation of sarcoplasmic proteins in aqueous extracts from fish, bovine, and chicken muscle. The sarcoplasmic proteins that are present are mainly metabolic enzymes and therefore their separation profiles are useful for the purpose of species identification. Some reports also exist of the simultaneous separation of sarcoplasmic and myofibrillar meat proteins using SDS-CGE. 

Rklationship betwekn Enzymic Activities and Content op Membrane (Insoluble) Protein in Chick Cardiac and Skeletal Muscle Mitochondria ... 

The swelling of insoluble proteins corresponds to the hydration of soluble proteins in that insertion of water between the peptide chains results in an increase in volume and other changes in the physical properties of the protein. For example, the diameter of myofibrils (cf. 12.2.1) increases to 2.5 times the original value during rinsing with l.Omol/L NaCl, which corresponds to a six-fold volume increase (cf. 12.5). The amount of water taken up by swelling can amount to a multiple of the protein dry weight. For example, muscle tissue contains 3.5-3.6 g water per g protein dry matter. 

Muscle proteins in typical mammalian muscle tissue constitute around 20% of the muscle weight The major proportion of muscle is made up of muscle fibre proteins (elongated, threadlike cells) called myofibrillar proteins. Smaller amounts of soluble sarcoplasmatic proteins and insoluble structural proteins from connective tissue are also present (Table 2.16). Myofibrillar and sarcoplasmatic proteins are almost complete (whole) proteins, while the nutritional value of structural proteins is very low as they are almost indigestible. Table 2.17 gives the amino acid composition of some pure animal proteins Table 2.9 presents the amino acid compositions for the main types of meat proteins. 

Globulins. Proteins insoluble in water, soluble in dilute salt solutions. They include such proteins as myosin from muscle, fibrinogen from blood and edcstin from hemp. 

Monomeric G-actin (43 kDa G, globular) makes up 25% of muscle protein by weight. At physiologic ionic strength and in the presence of Mg, G-actin polymerizes noncovalently to form an insoluble double helical filament called F-actin (Figure 49-3). The F-actin fiber is 6-7 nm thick and has a pitch or repeating structure every 35.5 nm. 

Meat proteins comprise a water-soluble fraction (containing the muscle pigment myoglobin and enzymes), a salt-soluble fraction composed mainly of contractile proteins, and an insoluble fraction comprising connective tissue proteins and membrane proteins. As reviewed by Dierckx and Huyghebaert [107], HPLC analysis of meat proteins has been successfully applied to evaluate heat-induced changes in the protein prohle, to detect adulterations (addition of protein of lower value, the replacement of meat from high-value species with meat from lower-value species, etc.), and for specie identification in noncooked products (also for fish sample). 

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. 

As hormone-sensitive lipase hydrolyzes triacylglyc-erol in adipocytes, the fatty acids thus released (free fatty acids, FFA) pass from the adipocyte into the blood, where they bind to the blood protein serum albumin. This protein (Mv 66,000), which makes up about half of the total serum protein, noncovalently binds as many as 10 fatty acids per protein monomer. Bound to this soluble protein, the otherwise insoluble fatty acids are carried to tissues such as skeletal muscle, heart, and renal cortex. In these target tissues, fatty acids dissociate from albumin and are moved by plasma membrane transporters into cells to serve as fuel. 

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%. 

Fibrous proteins - these have linear molecules, are insoluble in water and resistant to alkalis and acids. Collagen (in tendons and muscles), keratin (in nails, hair, horn and feathers) and elastin (in arteries) are all fibrous proteins. 

Conversely, the connective tissues and cartilage are much more resistant to proteolysis and will survive for a longer period of time, although they too will eventually succumb to the effects of putrefaction. Reticulin, epidermis, and muscle protein will resist breakdown for some time, whereas collagen and keratin may survive for longer periods (Linch and Prahlow 2001). Keratin is an insoluble fibrous protein found in the skin, hair, and nails, and its resistance to attack by most proteolytic enzymes (Gupta and Ramnani 2006) is the reason it is often found intact amongst skeletal remains, particularly in burial environments (Macko et al. 1999). 

Lipid oxidation products react with proteins and other amino compounds to form brown substances, similar to melanoidins. The formation of such brown substances was reviewed already at the first Maillard Symposium.150 The pigments formed are partly soluble in chloroform-methanol and partly insoluble, whereas true melanoidins are largely water-soluble. As most brown pigments of fish muscle are soluble in benzene-methanol and only to a lesser extent in water, the implication is that here oxidised lipid-protein interactions are more important than Maillard browning due to ribose-amino acid interactions. 

Scleroproteins. Insoluble in water and neutral solvents and resistant to enzymic hydrolysis. These are fibrous proteins serving structural and binding purposes. Collagen of muscle tissue is included in this group, as is gelatin, which is derived from it. Other examples include elastin, a component of tendons, and keratin, a component of hair and hoofs. 

The helical reference conformation in synthetic polypeptides fortunately has a counterpart in a group of fibrous muscle proteins, for one of which. Pinna nobilis tropomyosin, hydrodynamic and light-scattering evidence indicate a helical form. This protein is classified as a globulin by virtue of its insolubility in pure water and yet Kay (1958) has shown that its structure is most consistently interpreted as a rigid rod 1400 A in length with... 

Actomyosin is generally extracted from fresh rabbit muscles by the use of buffered KCl solutions of an ionic strength of 0.5-0.6 y. (Weber-Edsall solution). The solubility curve of the isolated actomyosin at pH 7 shows an inflection at 0.25 y, above a value of 0.3 u the protein is completely soluble (Hasselbach et al., 1953). At low ionic strengths, actomyosin upon addition of ATP and provided Mg++ ions are present shows superprecipitation. By glycerol extraction, muscle fibers may be prepared to contain essentially only the contractile system. Such fibers will contract normally under the conditions mentioned above for the isolated actomyosin (Weber and Portzehl, 1952). The muscle fibril contains the actomyosin in the insoluble state and in an optimal spatial arrangement (cf. Section IV, A,2). 

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