MECHANISM OF CHEMICAL CHANGES 165 



capable of attacking proteins, breaking them down into 

 peptones, and perhaps in some eases into polypeptids and 

 peptids. They require an acid medium. Trypsins are 

 enzymes capable of attacking proteins, breaking them down 

 through the stage of peptones into the peptids, the amino 

 acids, and some cases even to ammonia. They require an 

 alkaline medium. The erepsins resemble trypsins in their 

 end products, but attack only compounds somewhat less 

 complex than the proteins. They will attack, for example, 

 the proteoses and the peptones. The general types of reac- 

 tion brought about may be illustrated by the hydrolysis of 

 a dipeptid into amino acid. 



NH2 NH, 



The amino acids may be regarded as the building stones 

 out of which proteins are constructed. The cement which 

 binds them together is the abstraction of water. Enzymes 

 which attack proteins insert water and the compounds fall 

 apart on the lines of cleavage. 



Microorganisms are known to produce various types of 

 proteases. One of these most commonly observed in the 

 laboratory is gelatinase, the enzyme capable of liquefying 

 gelatin. Bacteria have been described which will liquefy 

 egg white and digest blood serum. These changes are some- 

 times noted as cultural characteristics of microorganisms 

 studied. Many microorganisms, too, are able to digest the 

 casein of milk in which they are grown, either with or with- 

 out the preliminary coagulating action of the so-called lab 

 enzyme or rennet. 



Esterases and Lipases. — Pats, that is, the glycerine esters 

 of the fatty acids, are hydrolyzed by lipases into the corre- 



