Platelets Thrombosthenin

Actin and myosin molecules, and thrombosthenin, are contractile proteins that enable platelets to contract. 

Thrombosthenin, the Contractile Protein from Blood Platelets and Its Relation to Other Contractile Proteins M. Bettex-Galland and E. F. LOscher... 

THROMBOSTHENIN, THE CONTRACTILE PROTEIN FROM BLOOD PLATELETS AND ITS RELATION TO OTHER CONTRACTILE PROTEINS... 

F. Thrombosthenin, the Contractile Protein from Blood Platelets... 

This review will deal primarily with this contractile protein from human blood platelets, which we have named thrombosthenin in view of its origin and function. A brief review on blood platelets in general and their role in hemostatis will be included. The role of thrombosthenin for platelet function will be discussed and its properties compared with those of other contractile proteins. 

In 1959 Bettex-Galland and Liischer succeeded in extracting from human blood platelets such a (iontractile protein, which was subsecpiently named thrombosthenin. Its solubility properties, as well as its dependence for activity on the presence of ATP and metal ions, soon led to its classification as a member of the actomyosin group. Work on thrombosthenin has since continued, both with respect to its properties as a complex protein with enzymatic activity and to its biological significance. 

Thrombosthenin is extracted from a concentrated suspension of washed blood platelets obtained by any of the described methods of isolation (Maupin, 1954b). Bettex-Galland and Liischer (1961), starting with 50 liters of freshly (collected citrated human blood isolated by differential centrifugation in the (told, from the buffy layers, 20 to 30 ml of a highly concentrated suspension of washed platelets. Since the isolation of thrombosthenin is based on its solubility properties, special ce,re must be taken to eliminate the leucocytes their content of deoxyribonuclcoproteids... 

Thrombosthenin from Pig Platelets. The method described by Grette (1962) for the extraction of the contractile protein from pig platelets uses butanol for the lysis of platelets, and the precipitation is effected at an ionic strength of 0.2 y in the presence of Mg++ ions. 

It is noteworthy that, in spite of the differences in origin and in experimental procedure, the yield and the (Karacteristic properties of the active material from pig platelets are the same as for thrombosthenin of human origin. 

This difference in the effects of the two metal ions explains why the ATP loss suffered by the blood platelets in model systems containing only Mg++ ions is not as pronounced as the disappearance of the nucleotide during normal blood coagulation (Born, 1958). It has been mentioned before that ADP production during VM of the platelets was of biological importance. This production most likely is due mainly to the activation of the thrombosthenin ATPase by the influx of plasmatic Ca++ ions their pronounced activator role therefore appears of particular significance. 

The findings reported above may be summarized as follows Inhibitors of the ATPase activity of thrombosthenin always are inhibitors of superprecipitation, whereas they do not necessarily affect manifestations of the contractile activity of the whole platelet, such as clot retraction, to the same extent. 

Ultracentrifugation of thrombosthenin as well as of thrombosthenin M led to conflicting results. As is known from the contractile proteins of muscular origin, poorly defined complexes of variable size may form very easily, making molecular weight determinations difficult. The same conditions were encountered with the contractile protein from human platelets (Bettex-Galland and von Tavel, unpublished observations, 1963). 

The Relaxing Factor of the Blood Platelets—a Natural Inhibitor of Thrombosthenin Activities... 

Recently, Grette (1963) has reported the extraction from pig platelets of a material with properties comparable to the relaxing factor from muscle (Marsh, 1951). Solutions of this factor will inhibit superprecipitation as well as ATPase activity of thrombosthenin of porcine origin. Calcium ions, on the other hand, were found to inhibit this relaxing effect. 

Today thrombosthenin is the only contractile protein of nonmuscular origin available in larger quantities. This alone makes it an interesting material from the point of view of comparative physiology. On the other hand, the high content of thrombosthenin in the blood platelets also suggests an important function of this material in physiological platelet activity. 

One last important function of thrombosthenin remains to be mentioned in relation to hemostasis, namely its ATPase activity. ADP arising from this activity must be considered of importance for platelet aggregation, and its production by the platelets during VM is perhaps an essential step in the arrest of hemorrhage as well as thrombus formation (Kaser-Glanzmann and Liischer, 1962). 

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. 

Thus, thrombosthenin may prove not only interesting with respect to the functions of the blood platelets but also with respect to the general importance of the elucidation of the many remaining problems in the field of the contractile proteins. 

The platelets contain a contractile protein (throm-bosthenin), which has solubility properties similar to those of the actomyosin group of proteins. The activity of the protein requires the presence of ATP and metallic ions. The protein has been isolated from human platelets, and like actinomycin, the contractile protein possesses ATPase activity. Electron microscopic examination of isolated thrombosthenin revealed a microfibric structure 80-100 A wide with a possible periodic structure [28]. In the presence of ATP, the contractile protein dissociates into an actinlike protein (thrombosthenin A) and a myosinlike (thrombosthenin M) moiety. ATPase activity is nonexistent in thrombosthenin A and weak in thrombosthenin M. When tested alone, neither thrombosthenin A or M is ATP sensitive, but the mixing of A and M thrombosthenin restores ATP sensitivity and ATPase activity. 

The exact physiological role of thrombosthenin is not known, but the contractile protein is assumed to play a role in the retraction of the aggregated platelets and in clot retraction. Because of its ATPase activity, thrombosthenin may split ATP and yield ADP, which is known to stimulate platelet aggregation. 

Bettex-Galland, M., Luscher, E. Thrombosthenin, the contractile protein from blood platelets and its relation to other contractile proteins. Advanc. Protein Chem. 20, 1-35 (1965)... 

Zucker-Franklin, D., Nachman, R.L., Marcus, A.J. Ultrastructure of thrombosthenin, the contractile protein of human blood platelets. Science 157, 945-946 (1967)... 

The activation of the contractile system of the platelet. Up to 15% of the platelet s protein are accounted for by an actomyosin-like, contractile protein, termed thrombosthenin, which depends for its activation on Ca2+-ions and a regulatory system closely related to the one of muscle (27). It is thought that contractile activity is essential for the formation, in the case of arterial thrombosis, of a tight platelet aggregate, capable of withstanding the eroding forces of arterial circulation. 

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