MECHANISM OF METABOLISM. 



on the decomposition as far as amino-acids and traces of ammonia. 

 Another class recently recognized is ereptase (erepsin) which cannot 

 decompose protein, but readily attacks peptones, decomposing them much 

 in the same way as trypsin. Pepsin, trypsin and erepsin do not break up 

 amino-compounds. 



The presence of proteolytic enzymes in microorganisms is readily 

 tested by cultivation on nutrient gelatin. The proteolytic enzyme 

 secreted by the cells will liquefy the gelatin. Generally, an organism that 

 liquefies gelatin will also decompose the casein of milk and the protein of 

 blood serum. There are some exceptions, however, as is shown in the 

 following table, after Frost and McCampbell. A + sign means proteo- 

 lysis, a sign means no action. 



Apparently not all organisms which liquefy gelatin are able to de- 

 compose egg albumin, and we must conclude that the enzyme liquefying 

 gelatin is different from the proteolytic enzyme dissolving egg-white. 



COAGULATING ENZYMES. The blood-clotting enzyme (thrombase) 

 does not occur in microorganisms. Rennet, however, is found in many 

 species. Rennet is extracted from the stomach of calves and pigs and 

 used to set the curd in milk for cheese making. The enzyme acts upon 

 the casein in milk, decomposing it into paracasein and some soluble 

 protein. The time of coagulation depends upon the temperature of the 

 milk and the concentration of the rennet. This coagulation of milk is 

 quite different from the acid curd, where the insoluble casein is precipi- 

 tated by the acid. If enough acid is added, the milk curdles immediately; 



