280 IMMUNO-CATALYSIS 



c. Enzyme Nature of Thrombin. Schmidt (1892, 1895), one of 

 the earliest investigators of the problem of plasma clotting, termed 

 thrombin a fibrin ferment, and considered it to be a proteolytic enzyme 

 which split fibrinogen, forming fibrin. That thrombin is an enzyme has 

 been established, after many controversies, by the application of rigid 

 criteria of catalysis. It has been found that the amount of fibrin formed 

 is independent of the amount of thrombin present in a system, over a 

 w^ide range. Eagle (1935) reported that a purified preparation of 

 thrombin clotted over 200 times its own weight of fibrinogen. On the 

 other hand, Ferry and Morrison (1947) calculated that thrombin is 

 capable of converting, over 1 00,000 times its own weight of fibrinogen 

 to fibrin. These findings show decisively that the role of thrombin in 

 the conversion of fibrinogen into fibrin is one of enzyme catalysis. 



d. Conversion of Fibrinogen into Fibrin. Measured by double 

 refraction flow, viscosity and osmotic pressure, fibrinogen has been 

 reported to possess dimension of about from 35X700 to 33X900 A and 

 a molecular weight of the order of 300,000 to 500,000 (Edsall, Ferry, 

 and Armstrong, Jr., 1944; Ferry and Morrison, 1947). Measuring the 

 opacity, modulus of rigidity, friability, syneresis and other mechanical 

 properties of fibrin clot. Ferry and Morrison (1947) and Hawn and 

 Porter (1947) studied the mechanism of the conversion of fibrinogen 

 into fibrin. These measurements were intended to determine: (a) the 



literature. The views expressed on certain aspects of these questions are of contro- 

 versial nature. In presenting the above view it is not intended to pass over the others 

 without due notice. The reader is referred to a treatise by Quick (1942) for 

 a comprehensive discussion of the various aspects of the subject, to an article by 

 Ferguson (1943), and to a critical review by Chargaff (1945). Milstone (1948) 

 summarized an extensive study on the three stage analysis of blood clotting with the 

 following scheme: 



thrombokinase (?)-|-Ca+ + 



( 1 ) Prothrombokinase > thrombokinase. 



thrombokinase-[-Ca+ + 



(2) Prothrombin > thrombin. 



thrombin 



(3) Fibrinogen > fibrin. 



According to him all three reactions are enzymatic, and calcium ion conditions the 

 reactions. According to Quick (1947) the reactions that bring about the production 

 of thrombin from the prothrombin complex, thromboplastin and calcium, are chemi- 

 cal and not enzymatic. Thromboplastin acts stoichiometrically, and the relation of 

 calcium concentration to thrombin production is also stoichiometric. According to 

 Loomis and Seegers (1946) there is no evidence of the existence of a prothrombin 

 complex; it is a homogeneous protein. 



