Thrombosthenin ATPase activity

At low ionic strength, Mg ions up to a concentration of 10 M increasingly activate the thrombosthenin ATPase. Still higher concentra-... 

Fig. 7. Influence of magnesium ion concentration on ATPase activity of thrombosthenin at low (left) and high ionic strength (right). ATP, 10 Af temperature, 20°C. From Beltex-Galland and Luscher (1961).

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. 

Tephorine and Diparcol, both synthetic antihistamines known to inhibit clot retraction (Bounameaux, 1957 Hugues, 1959), only partially inhibit ATPase activity and superprecipitation of thrombosthenin. Finally, monoiodoacetate, a powerful inhibitor of clot retraction, inhibits neither phenomenon. 

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. 

Of particular interest were the ATPase activities of the fragments and of the recombined complexes. Thrombosthenin M is a considerably weaker ATPase than thrombosthenin. Thrombosthenin A, as mentioned before, is completely inactive, but it is capable of potentiating the enz3unatic... 

ATPase Activities of the Recombined Dissociation Product of Thrombosthenin and... 

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. 

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). 

It has generally been assumed that the energy required for muscular contraction was derived directly from the ATP split in the course of the reaction. The establishment of accurate relationships, however, has been hampered by the fact that the actomyosin ATPase continues to degrade the nucleotide long after contraction has reached an optimum. The availability of thrombosthenin with its much lower ATPase activity might be an interesting material for a comparative investigation of this basically important mechanism. 

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. 

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