Relation of Thrombosthenin to Other Contractile Mechanisms

The discussion of the similarities and dissimilarities of thrombosthenin and other contractile proteins will be restricted to the group of actomyosin-like proteins. As already mentioned, contractile substances of other types have not yet been isolated in a purified form and most of our knowledge about them comes from studies of glycerol-extracted cell models, which have not yet been prepared from blood platelets. Lastly, there can be little doubt that the mode of action of these other systems must be quite different from thrombosthenin. 

The structure of the striated muscle and in particular the distribution of actin and myosin within the myofibril is well established today. This concerns not only the topography but the functional state as well and is the result of a whole series of admirable observations involving more recently high-resolution electron microscopy on ultrathin sections (see review by Huxley and Hanson, 1960). The striated muscle is composed of fibers consisting of giant multinucleated cells. These fibers contain the myofibrils in a longitudinal arrangement which is characterized by a transversal 

Although many micrographs of thin sections from blood platelets have been published, no evidence for the presence of orientated or filamentous 

A Comparison of the Enzymatic Activities of Different Actomyosin-Like Proteins 

In Table IV an attempt has been made to summarize the available data, as far as they appear comparable, on the enzjnnatic activities of the different actomyosins. In the presence of Mg++ ions and at a low ionic strength, the actomyosin from striated muscle is by far the most powerful ATPase, in fact about ten times as active as the corresponding activity of 

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