132 NUTRITION AND METABOLISM. 



The splitting up of a fat molecule into glycerin and fatty acid is also a 

 well-known process 



C S H 6 (C 18 H 35 2 ) 3 +3H 2 = C 3 H 5 (OH) 3 + 3 C 18 H 36 2 



Tristearin Glycerin Stearic acid 



The proteolysis is not so well known and the general supposition that the 

 first stages of protein degradation are hydrolytic is largely based upon 

 analogies. Some of these enzymes which are secreted by the microbial 

 cells act upon soluble compounds. Invertase decomposes saccharose 

 into dextrose and levulose: 



= C 6 H 12 6 +C 6 H 12 6 



Other disaccharides are hydrolyzed in the same way by other enzymes; 

 glucosides are decomposed by emulsin; soluble proteins are changed to 

 peptones. It is not necessary that the enzymes act upon the soluble com- 

 pounds outside the cell since these compounds can diffuse into the cell; 

 these enzymes are found only occasionally within the cell. It may be said, 

 however, that the smaller molecules of the products of enzymic action 

 diffuse more readily than the larger molecules of the original food 

 compound. 



PROPERTIES OF ENZYMES. These secretions of cells are treated in a 

 group by themselves because they differ distinctly in many respects 

 from any other chemical substance. Probably the most notable differ- 

 ence may be discovered in the fact that their action does not follow the 

 law of mass action which supposes that all substances reacting upon each 

 other diminish in quantity. This is not the case with enzymes. Rennet 

 will coagulate many hundred times its weight of casein, and still the whey 

 will contain rennet. Considering that part of the rennet is physically 

 absorbed by the coagulum, the amount of rennet is found to be the same 

 as before, though it has changed a comparatively enormous quantity 

 of casein. The same is true with other enzymes. The enzyme is not 

 destroyed by acting upon other substances. This exceptional quality 

 furnishes a reason for treating enzymes as a separate group or apart from 

 other chemical substances. But there are still other qualities which 

 distinctly separate them from the well-known chemical bodies, and show 

 at the same time their relation to proteins and toxins (page 122). 

 One of these is their sensibility to such outside influences as will destroy 

 life. Enzymes are inactivated by exposure to temperatures above 50 to 

 80, and can, like coagulated albumin, by no means be brought back to 



