Myofibrillar

Fish proteia coaceatrates vary widely ia fuactioaal characteristics, ranging from those having high proteia coateat and low water solubiUty, to those having lower proteia coateats but improved water solubiUty. Attempts have beea made to improve fuactioaal properties of fish proteia by enzyme hydrolysis (141), or by modificatioa of the myofibrillar proteia by succiaylatioa (142). 

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. 

Myofibrillar Structural Proteins of Rabbit Skeletal Muscle ... 

The superstructure of smooth muscle actin filaments is differentiated from those of striated muscle by the absence of the troponins and the lateral organization by association of the filaments with dense bodies instead of with the Z-line. How these differences are encoded is again not at all clear. However, the myofibrillar structure and the alignment of the alternating actin and myosin filaments is apparently due primarily to dense bodies and the actin-actinin macrostructures. As the bent dumbbell shaped actins assemble into filaments they are all oriented in the same direction. The S-1 fragments of myosin will bind to actin filaments in vitro and in... 

Figure 2. Muscle stimulation, a) a single nerve impulse (stimulus) causes a single contraction (a twitch). There is a small delay following the stimulus before force rises called the latent period, b) A train of stimuli at a low frequency causes an unfused tetanus. Force increases after each progressive stimulus towards a maximum, as calcium levels in the myofibrillar space increase. But there is enough time between each stimulus for calcium to be partially taken back up into the sarcoplasmic reticulum allowing partial relaxation before the next stimulus occurs, c) A train of stimuli at a higher frequency causes a fused tetanus, and force is maximum. There is not enough time for force to relax between stimuli. In the contractions shown here, the ends of the muscle are held fixed the contractions are isometric.

Calcium levels in the myofibrillar space are usually low, to prevent contraction. The calcium ions are stored in an internal membrane system called the sarcoplasmic... 

Van Der Laarse, W.J., LSnnergren, J., Diegenbach, P.C. (1991). Resistance to fatigue of single muscle fibers from Xenopus related to succinate dehydrogenase and myofibrillar ATPase activities. Exp. Physiol. 76, 589-596. 

Centronuclear myopathy with type 1 fiber hypotrophy is sometimes regarded as a separate entity because many cases show central nuclei only in the hypotrophic type 1 fibers, while the type 2 fibers are morphologically normal. Affected type 1 fibers are even more myotubelike than in other variants of the disorder, with the exception of the severe X-linked form, due to the persistence of a mitochondria-rich core within a peripheral ring of myofibrils. These features are clearly demonstrable using histochemical methods for the localization of SDH activity and myofibrillar ATPase, respectively. 

Figure 14. Normal skeletal muscle showing random distribution of type 1 (dark), type 2A (pale) and type 2B (intermediate) fibers myofibrillar ATPase after pH 4.6 preincubation.

In addition to marked perifascicular atrophy, infarctlike areas are sometimes seen, and are also consistent with a microangiopathy. Muscle fibers which appear normal morphologically may show loss of myofibrillar ATPase activity from the center of the fibers this is also characteristic of muscle subject to ischemia. Such changes may be reversible, but more prolonged ischemia undoubtedly causes irreversible muscle necrosis. 

ADM may evolve over several years, the extent of fiber atrophy provides an important indication of the chronicity of muscle degeneration. Acute muscle necrosis and phagocytosis give some indication as to how active the disease is at the time of biopsy. In most biopsies from ADM patients, the inflammatory cell foci are perivascular and perimysial rather than endomysial and are dominated by B-lymphocytes. The ratio of T4 lymphocytes (helper cells) to T8 lymphocytes (cytotoxic) generally indicates a predominance of the former. As in JDM, this is consistent with humoral mechanisms of cell damage, and vascular involvement is also apparent in the form of capillary endothelial cell abnormalities (tubular arrays) and duplication of basal lamina. Loss of myofibrillar ATPase from the central portions of fibers is a common prelude to muscle necrosis. 

Figure22. (a) Euthyroid(normal)ratmuscleshowingmixtureoftype1 (slow-twitch) and type 2 (fast-twitch) fibers, (b) Hypothyroid rat muscle showing uniformly type 1 (slow-twitch) histochemical profile myofibrillar ATPase after alkaline preincubation.

Biopsy findings show disseminated muscle fiber atrophy which is confined to type 2 fibers, in many instances with type 2B (glycolytic) fibers most affected (Figure 23). Muscle necrosis is not seen, though at ultrastructural level focal myofibrillar disruption and myofilament loss may be evident. The muscle atrophy seems to be due to decreased protein synthesis, and at high doses, to increased catabolism. The reason for the selective effect on phasic, glycolytic fibers is not clear since, although steroids interfere with carbohydrate metabolism and oxidative capacity, there seems to be no overall effect on ATP levels. Nevertheless it has been... 

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. 

Rat (Sprague-Dawley) 6 wk Cardio ad lib (W) 873 M (myofibrillar fragmentation, mitochondrial swelling) Asokan 1974 PbAc... 

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 major relaxation component, characterised by a time constant of approximately 35-50 ms, accounting for 80-95% of the relaxation, and in the following referred to as T2i, represents water trapped within the protein-dense myofibrillar network. 

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. 

Voluntary muscles contain a variety of fibre types which are specialized for particular tasks. Most muscles contain a mixture of fibre types although one type may predominate. All human skeletal muscles are composed of several different muscle fibre types. Up to seven different fibre types have been identified histochemically based on the pH stability of myofibrillar adenosine triphosphatase and on the myosin heavy chain profile. Innumerable fibre type transients exist due to continuing adaptation processes. However, three main... 

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. 

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