MECHANISM OF METABOLISM 205 



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

 well-known process 



C 3 H 5 (C 18 H350 2 )3 + 3 H 2 = C 3 H 5 (OH) 3 + 3 Ci8H 36 O 2 . 



Tristearin Glycerin Stearic acid 



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. Iniertase decomposes 

 saccharose into dextrose and levulose: 



C 12 H 22 On + H,0 = 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 

 compounds 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. Rennet will coagulate many hundred 

 times its weight of casein, and still the whey will contain rennet. Con- 

 sidering 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 248). 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 



