Actomyosin content

The figure of 38 % for L-myosin is nearly as high as that obtained by Weber and Meyer (1933) (c/. Table XV) for myosin. This latter on our present knowledge might be thought to include the actin as well, and thus to represent the actomyosin content. Evidently, however, it does not, for the reason that extraction was performed at a pH (8-9) at which, as we now know, the actomyosin becomes dissociated (Guba, 1943 ... 

The actomyosin content of fish muscle, some 70%, appears to be appreciably greater than that of mammals. The difference may well reside in the muscle as a whole rather than in the fiber content. If it is assumed that the stroma protein consists substantially of sarcolemma, connective tissue, and blood vessels, then the difference would be due to... 

Myosin Va is also associated with NFs, with NF-L being the major binding partner (Rao et al., 2002). This association has been shown to be important in regulating the localization and content of a large pool of myosin Va in neurons, in addition to regulating NF number. Association with the actomyosin system provides another potential transport mechanism and an opportunity for more sophisticated modes of regulation within the peripheral nervous system. 

Skeletal muscle moves the bones attached to joints. These muscles are composed of bundles of long, multinucleated cells. The cytoplasm contains a high concentration of a special macromolecular contractile-protein complex, actomyosin (Chap. 5). There is also an elaborate membranous network called the sarcoplasmic reticulum that has a high Ca2+ content. The contractile-protein complex has a banded appearance under microscopy. 

This result also explains why meat products designed to have maximal water content can at the same time appear texturally dry. The protein gel formed, by solubilising actomyosin, or adding polysaccharide water binders, retains water during chewing so successfully that little or none is released. 

Actomyosin denatures also in situ under the influence of hypertonic salt solutions. When cod muscle is immersed in various concentrations of sodium chloride, there is a critical salt content in the fillet (8% to 10% NaCl) at which denaturation occurs together with a rapid loss of water and uptake of salt (Duerr and Dyer, 1952 Fougere, 1952). In herring, however, this critical salt concentration is much lower (3 % NaCl) (Nikkila and Linko, 1954b). The stability of the native configuration appears very variable, but we are unable to explain such differences. The need for a better knowledge of the protein itself is clearly stressed by these researches. Let us now consider the recent progress made in this direction. 

From 1945 on, Dubuisson has examined electrophoretically in the Tiselius apparatus extracts of muscle prepared with extracting media of differing salt content. By this means, he found three myosin components, a, /3 and y. It is almost certain that /3-myosin is identical with L-myosin and a-myosin with actomyosin (Table V). 7-Myosin seems to be identical with contractin (cf. Section IV, 3). The Y-protein may... 

Finally, Buchtal et al. (1949) investigated whether chemical changes take place in F-actomyosin when the threads shrink. Threads of F-actomyosin and also of L-myosin were found to contain, after treatment with 2 X 10 M ATP followed by 7-12 washings, a content of phosphate, adenine and ribose three to five times greater than before. The relative amounts of the different phosphate reactions (c/. Section III, 4a) are not appreciably altered from those already present. The effect is as specific for ATP as shrinkage and contraction, but it probably has no direct connection with the fundamental process of contraction since it also occurs when actomyosin and L-myosin are in the dissolved state (c/. Section III, 5d). 

The muscle filaments can be identified with actomyosin if they are assumed to fill the whole fibril, for actomyosin comprises some 60-70% of the fiber content. (Section IV, 1.)... 

Dubuisson, it is true (see Dubuisson and Mathieu, 1950), regards this explanation as insufficient a- and /S-myosins do not appear in extracts which are made without delay after the ATP has first been washed out, but they do appear in the usual amounts when ATP is added immediately. The a- and /3-peaks in extracts of fatigued muscle, on the other hand, cannot be augmented in this way (Table XVII). The difference can, however, be explained by assuming nothing more than a denatura-tion of the actomyosin when the ATP content is low, leading to a loss of dissociability when ATP is again added. (See Section III, 66.)... 

Structural proteins (actin, myosin, tropormyosin, and actomyosin) which constitute 70%-80% of the total protein content (compared with 40% in mammals). These proteins are soluble in neutral salt solutions of fairly high ionic strength (0.5 M). 

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