THE INDIVIDUAL GROUPS OF PROTEINS 439 



Thrombiri has been considered to be a ferment or enzyme, but 

 the observations of Howell and Rettger show that a solution of 

 thrombin, if obtained almost free from protein, is stable to heat. 

 Enzymes are generally recognised to be more or less easily destroyed 

 by heating. 



The following scheme represents the processes which occur in the 

 formation of fibrin : 



Prothrombinj Thromb g en ] 



( Thrombokmase ! . I 



Calcium salt J JFibrin + (Soluble globulin ?) 



FibrinogenJ 



For the elucidation of the above factors in the scheme of blood clot- 

 ting, it was necessary to prevent the blood from clotting, to prepare 

 fibrinogen in a pure state from non-coagulated blood, to prepare a 

 solution of fibrin ferment or thrombin and to determine the factors 

 leading to the formation of thrombin. 



III. Prevention of Clotting of Blood. 



Blood may be prevented from clotting by collecting it in various 

 salt solutions when it is drawn, e.g. sodium sulphate, magnesium 

 sulphate, potassium oxalate, sodium fluoride, sodium citrate. Salt 

 plasmas are thus obtained. 



Clotting may also be hindered or prevented by keeping the drawn 

 blood at a low temperature (o), or by adding leech extract to it im- 

 mediately after it is drawn. 



If peptone be injected into the circulation, or if leech extract or the 

 active principle of leech extract, termed hirudin, be injected, the blood 

 when drawn does not coagulate. 



Preparation of Salt Plasmas. 



(a) Sodium sulphate. i part of blood is collected in I part of saturated 

 sodium sulphate solution (500 c.c. to 500 c.c.). 



(b) Magnesium sulphate. 3 parts of blood are collected in i part of 

 saturated magnesium sulphate solution (750 c.c. to 250 c.c.). 



(c) Oxalate. 9 parts of blood are collected in i part of potassium 

 oxalate solution (i per cent.) (900 c.c. to 100 c.c.). 



(a] fluoride. 9 parts of blood are collected in i part of sodium 

 fluoride solution (3 per cent.) (900 c.c. to 100 c.c.). 



During the mixing each is well shaken. The plasma is then separated 

 from the corpuscles by centrifugalising. All these plasmas should be quite 

 free from blood corpuscles, as the stromata of these may serve as the mother 

 substance of the ferment. It is difficult, however, to obtain them quite free 

 from haemoglobin, which gives them a reddish colour. 



Fibrinogen. 



Fibrinogen can be prepared from the salt plasmas by precipitation with 

 sodium chloride or ammonium sulphate. 



(a) From sodium sulphate plasma. 



A precipitate, the plasmine of Denis, is formed when sodium sulphate 

 plasma is saturated with sodium chloride. This precipitate consists of 

 fibrinogen and serum globulin. 



