Muscle sarcoplasmic proteins

LeBlanc, E.L., Singh, S., and LeBlanc, R.C. 1994. Capillary zone electrophoresis of fish muscle sarcoplasmic proteins. J. Food Sci. 59, 1267-1270. 

The sacroplasmic proteins myoglobin and hemoglobin are responsible for much of the color in meat. Species vary tremendously in the amount of sacroplasmic proteins within skeletal muscle with catde, sheep, pigs, and poultry Hsted in declining order of sarcoplasmic protein content. Fat is also an important component of meat products. The amount of fat in a portion of meat varies depending on the species, anatomy, and state of nutrition of the animal. The properties of processed meat products are greatiy dependent on the properties of the fat included. Certain species, such as sheep, have a relatively higher proportion of saturated fat, whereas other species, such as poultry, have a relatively lower proportion of saturated fat. It is well known that the characteristic davors of meat from different species are in part determined by their fat composition. 

Louis, C.F., Saunders, M.J., and Holroyd, J.A. (1977) The cross-linking of rabbit skeletal muscle sarcoplasmic reticulum protein. Biochim. Biophys. Acta 493, 78-92. 

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. 

Radicals generated during peroxidation of lipids and proteins show reactivity similar to that of the hydroxyl radical however, their oxidative potentials are lower. It is assumed that the reactive alkoxyl radicals rather than the peroxyl radicals play a part in protein fragmentation secondary to lipid peroxidation process, or protein exposure to organic hydroperoxides (DIO). Reaction of lipid radicals produces protein-lipid covalent bonds and dityrosyl cross-links. Such cross-links were, for example, found in dimerization of Ca2+-ATPase from skeletal muscle sarcoplasmic reticulum. The reaction was carried out in vitro by treatment of sarcoplasmic reticulum membranes with an azo-initiator, 2,2/-azobis(2-amidinopropane) dihydrochloride (AAPH), which generated peroxyl and alkoxyl radicals (V9). 

The Ca -ATPase piays an essential role In the pumping of calcium out of cells, and in the control of its cytosolic concentration. In the heart, the role of the pump is minor with respect to that of the sodium-calcium exchanger, but is most probabiy predominant in skeletal and smooth muscle. The pump is encoded by four independent genes, showing different patterns of tissue-specific expression and alternative splicing of the primary transcripts. The intracellular Ca pump proteins from skeletal muscle sarcoplasmic reticulum (SR), cardiac SR and brain microsomes are similar. Thapsigargin is a potent inhibitor, also lanthanum salts inhibit the pump at most sites. 

The answer is c. (Murray, pp 238-249. Scriver, pp 2367-2424. Sack, pp 159-175. Wilson, pp 287-317.) A variety of agonists activate the plasma membrane-bound enzyme phospholipase C, which hydrolyzes the phosphodiester bond of phosphatidyl inositol 4,5-bisphosphate and consequently releases diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3). Phospholipase C is also known as phosphoinositidase and as polyphosphoinositide phosphodiesterase. Both DAG and IP3 are second messengers. DAG activates protein kinase C, which is important in controlling cell division and cell proliferation. IP3 opens calcium channels and allows the rapid release of the calcium stores in endoplasmic reticulum (in smooth muscle, sarcoplasmic reticulum). The elevated levels of calcium ion stimulate smooth-muscle contraction, exocytosis, and glycogen breakdown. 

Actomyosin. Solubility. Studies have dealt with changes in the solubility of proteins during frozen storage of fish muscle or solutions of isolated actomyosin (33,51,52). Analysis by gel filtration of the salt extracts has shown that the actomyosin fraction decreases in solubility during frozen storage whereas the sarcoplasmic proteins remain essentially unchanged (53). 

Angsupanich and Ledward (1998) showed that oxidative stability of lipids in Atlantic cod (Gadus morhud) muscle decreased after treatment at a pressure higher than 400 MPa dne to the release of metal ions from complexes. Of the major muscle proteins, myosin denatured at 100-200 MPa, whereas actin and most sarcoplasmic proteins denatured at 300 MPa. High-pressure treated hsh was harder, chewier, and gummier than both the raw and cooked prodncts. 

Basically, there are three major groups of proteins in muscle tissue (a) the sarcoplasmic proteins of the muscle cell cytoplasm, (b) the myofibrillar proteins, soluble at high ionic strengths, that make up the myofibril or contractile part of the muscle, and (c) the stromal proteins comprised largely of the connective tissue proteins, collagen, and elastin. The myofibrillar proteins and the stromal proteins are fibrous and elongated they form viscous solutions with large shear resistance. These properties coupled with other lines of indirect evidence indicate that the physical properties of the myofibrillar and stromal proteins are directly related to the texture and tenderness of meat (34). 

Tappel (99) reported on the hydrolytic activity of various enzymes. They found that the relative hydrolysis of muscle proteins increased in the following order pepsin, Rhozyme A-4, ficin, papain, bromelain, protease 15, Rhozyme P-11, and trypsin. Wang et al. (100) observed that papain was twice as active as ficin towards elastin, a minor component of connective tissue. Ficin and bromelain had equal enzyme activity towards collagen, a major component of connective tissue. Kang and Rice (101) studied the effects of various tenderizing enzymes on water-soluble sarcoplasmic proteins, salt-soluble myofibrillar proteins, and the insoluble stromal proteins. Table VII tabulates the results of some of these studies. 

Two examples of membrane proteins physically associated to lipids will be discussed bovine erythrocyte acetylcholinesterase and skeletal muscle sarcoplasmic reticulum Ca -Mg " ATPase. 

The Ca -Mg ATPase is the major protein constituent of the skeletal muscle sarcoplasmic membranes. This enzyme can be solubilized... 

Sebranek (1988) has reviewed the effects of heat on denaturation of the proteins. Dehydration by heat denatures the muscle proteins, particularly the sarcoplasmic proteins. This induces a rather dramatic change in meat color. The heme pigments, which provide most of the color of fresh meat, serve as a general indication of doneness or temperature history. In the case of cured products, heme pigments react to form nitric oxide hemochro-mogen, which contributes the characteristic pink cured meat color (Pearson and Tauber, 1984). 

The majority of sarcoplasmic proteins are enzymes participating in cell metabolism, such as the anaerobic energy conversion from glycogen to ATP. If the organelles within the muscle cells are broken, this protein fraction may also contain the metabolic enzymes localized inside the endoplas-matic reticulum, mitochondria, and lysosomes. 

CE has also been successfully apphed to the study of muscle proteins, and some of these applications have been recently reviewed. - - Methods for the determination of muscle proteins were based on CZE, SDS-CE, or isoelectric focusing (CEIF). Meat species identification was carried out by analyzing sarcoplasmic or myofibrillar proteins by a replaceable polymer-filled SDS-CE method (Table 30.8). However, the analysis of sarcoplasmic protein profiles allowed better differentiation among beef, pork, and turkey meat (Figure 30.8). The importance of sample preparation in the established method was highlighted since sarcoplasmic proteins extracted by simply homogenizing meat with cold bidistilled water were most useful for meat species identification when protein profiles were examined by linear discriminant analysis. On the other hand, myofibrillar proteins extracted with 0.6 M NaCl/0.01 M phosphate buffer with 0.5% polyphosphates (pH 6.0) were not useful for raw meat species identification, although they may be of importance in the identification of heat-processed meats. ... 

Factors that affect texture include moisture content, composition, variety or species, pH, product history (maturation or age), and sample dimensions. Texture is also dependeut on the method of dehydration and pretreatments. Purslow [55] stated that meat texture is affected by the structure of the solid matrix. He coucluded that it is important to have a fundamental understanding of the fracture behavior of meat and how it relates to the structure of the material. Stanley [83] stated that many researchers now believe that the major structural factors affecting meat texture are associated with connective tissues aud myofibrillar proteins. Moreover, two other components muscle membranes and water also deserve consideration not because of their inherent physical properties, but rather as a result of the indirect influence they have on the physical properties. It should be noted that sarcoplasmic proteins may be important for the same reason, although little information on their role is available. He suggested that these structures merit particular attention. 

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. 

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