Proteins of Other Origin

A great variety of contractile proteins of the actomyosin type have been described. They are found in many different animal species and in different tissues. No attempt will be made here to give a complete list this seems the more permissible as a detailed review on the subject has been published (Perry, 1960). 

Loewy (1952) has reported the extraction from the plasmodia of myxo-mycetes of a contractile protein with the typical properties of an acto- 

The following discussion will deal mainly with the properties of the actomyosins from striated and smooth muscles, and those properties will be considered which appear pertinent to a comparison with thrombosthenin. 

Most of our knowledge about contractile proteins comes from the extensive studies of the actomyosin from striated muscle. For more detailed accounts the reader is referred to one of the recent reviews on the subject (Weber, 1957 Perry, I960 Szent-Gyorgyi, 1960). 

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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