346 VEHICLES OF CHEMICAL CORRELATION 



many enzymes, particularly those of bacterial origin, are known which 

 are not inactivated by heating or in which the inactivation is tem- 

 porary or reversible. Moreover it is not really certain that any pure 

 enzymes are thermolabile, since, with one exception, no pure enzymes 

 have ever been prepared. The one exception is that afforded by the 

 Laccase or oxidizing enzyme of Medicago sativa, which has been shown 

 by Euler to be a mixture of calcium salts of aliphatic hydroxy-acids. 

 A synthetic mixture of calcium glycollate, citrate, malate and mes- 

 oxalate has the same oxidizing action as the vegetable enzyme and is 

 unaffected by boiling. 



The evidence afforded by the effects of heating is therefore incon- 

 clusive either for or against the view that thrombin is an enzyme. 

 Much more decisive is the quantitative relationship of the fibrin-yield 

 to the thrombin which has been added to the fibrinogen solution. The 

 following are estimates obtained by Howell: 



0. 05 mgm. of thrombin yielded 10. 75 mgm. of fibrin. 

 0.16 " " " 34.00 " " 



0.25 " " " 36.80 " 



0.64 " " " 42.50 " 



Moreover, a submaximal quantity of thrombin acting upon a solution 

 of fibrinogen will never furnish a full yield of fibrin, no matter how much 

 time is permitted for the reaction to take place. Evidently, therefore, 

 thrombin enters into and determines the final equilibrium which is 

 attained and its action cannot be purely catalytic. 



The action of thrombin upon fibrinogen is specific in the sense that 

 no other protein is similarly modified by thrombin, but it is indifferent 

 whether the thrombin and the fibrinogen are derived from the same 

 or related or even unrelated species of animals. Thus, Howell has 

 found that the fibrinogen of all vertebrates is converted into fibrin by 

 thrombin derived from pigs' blood. 



There remains to consider the part which is played by the various 

 factors which contribute to the formation of thrombin from pro- 

 thrombin. Reasoning from the analogy afforded by the conversion of 

 Trypsinogen into Trypsin by the Enterokinase of the succus entericus, 

 Morawitz supposed that the conversion of prothrombin into thrombin 

 by tissue-extracts was attributable to an enzyme which he designated 

 Thrombokinase. A fact which encouraged this view is that if tissue- 

 extracts be heated to from 56 to 60 Centigrade, they lose their throm- 

 boplastic activity, and it was inferred that, like the majority of the 

 enzymes, the thromboplastic substance was thermolabile. Kephalin, 

 however, which is very active in promoting the conversion of pro- 

 thrombin into thrombin does not lose its thromboplastic powers when 

 it is heated. The solution of this apparent contradiction has been 

 supplied by the investigations of Howell, who has shown that if the 

 coagulum which forms when tissue extracts are heated to 60 be ex- 

 tracted with ether, the dried ether extract has all the thromboplastic 



