CLOTTING ENZYMES 179 



exists as a body which may be termed caseinogen ; the action 

 of rennet is to break up this substance, a portion of which 

 remains in solution, the remainder being precipitated, together 

 with calcium phosphate, as casein. This theory of the action 

 of rennet derives support from the following experiment : 

 the caseinogen as a whole may be precipitated by acetic acid, 

 washed and dissolved in lime water. On neutralising with 

 phosphoric acid, a milky-looking liquid is obtained which 

 forms a clot on addition of rennet. 



Another method of exhibiting the same phenomenon is 

 to dissolve the caseinogen in acid sodium phosphate, add the 

 rennet to the solution and allow it to act for, say, half an hour. 

 No clotting occurs ; the solution is then boiled to destroy the 

 activity of the enzyme ; on adding calcium chloride, clotting at 

 once takes place. 



This last experiment, which was devised by Hammersten, 

 would indicate that the action of the rennet is simply to 

 break up the caseinogen, the subsequent clotting being due to 

 calcium salts. 



The addition of peptone to milk inhibits the clotting efiect 

 of rennet, probably owing to its affinity for calcium salts. It 

 may be finally mentioned that the optimum temperature for 

 the action of rennet is 40 C., while its activity is destroyed 

 above 70 C. 



Other important clotting enzymes are the fibrin ferment 

 or thrombase, which causes the clotting of the blood, and 

 pectase, which gelatinises fruit juices containing pectin ; in 

 both these cases, as in the case of rennet, calcium salts play 

 an important part. 



The investigation of the action of thrombase indicates 

 that a substance which has been termed fibrinogen occurs 

 in unshed blood. The action of thrombase is to precipi- 

 tate fibrin, which carries down with it the red corpuscles of 

 the blood, leaving a globulin dissolved in the clear serum. 

 This phenomenon does not occur in the absence of calcium 



vl 



