PROTEINS. 637 



best in neutral, others in either acid or alkaline, solution of certain 

 concentration. The action of enzymes is in many cases reversible. 

 Thus, ethyl butyrate is split by lipase into ethyl alcohol and butyric 

 acid ; and under certain conditions lipase will produce the opposite 

 effect and cause the combination of alcohol and butyric acid with the 

 formation of ethyl butyrate. 



As it has been shown that living " organized ferments " owe their 

 activity to the enzymes which they secrete, there is less stress laid 

 now upon the distinction between organized and unorganized fer- 

 ments, and the term ferment is reserved mainly for yeast. 



The chemical composition of the enzymes is not known. As yet, 

 no enzyme has been prepared in the pure state ; they may be extracted 

 from the cells by means of water and glycerin. The solution in 

 glycerin is very stable. When in solution they are easily obtained 

 by precipitation of some other substance from the same solution, the 

 enzyme being carried down with the precipitate. The activity of 

 their solution is generally destroyed by heating to 80 C. (176 F.). 



The quantitative estimation of enzymes is based on the amount of 

 decomposition-products formed, or the amount of material decom- 

 posed in a given time and under certain conditions. 



Enzymes occur widely distributed in animal and vegetable organ- 

 isms, and possess great diversity of function. At present, enzymes 

 are classified according to the nature of the changes they produce ; 

 the more prominent groups are : 



Amylases or amylolytic enzymes, converting starches into simple 

 sugars : ptyalin, amylopsin, malt, diastase. 



Proteases or proteolytic enzymes, converting proteins into peptone or 

 simpler compounds : pepsin, trypsin. 



Steatases or steatolytic enzymes, splitting fats : steapsin. 



Invertases or inverting enzymes, splitting sugar : invertin. 



Coagulases or coagulating enzymes, converting soluble proteins into 

 insoluble forms : rennin. 



Oxidases or oxidizing* enzymes. Oxidases produce oxidation in 

 the presence of oxygen, peroxidases (catalases), only in the presence 

 of a peroxide. Oxygenases are theoretical enzymes, capable of first 

 combining with oxygen and then transferring it to some other sub- 

 stance. 



Of glucoside-splitting enzymes has been mentioned, in Chapter 50, 

 emulsin or synaptase, which decomposes amygdalin, while myrosin 

 acts on sinigrin found in mustard seed. 



Not infrequently the enzymes of the body are secreted in an inac- 



