COMPOSITION OF FOOD AND ACTION OF ENZYMES. 751 



also their formation in the tissues and their absorption from the 

 tissues during starvation. That is, according to the conditions of 

 concentration, etc., one and the same enzyme may cause a splitting 

 up of the neutral fat into fatty acids and glycerin or a storing up of 

 neutral fat by the synthesis of fatty acid and glycerin. In the sub- 

 cutaneous tissues, therefore, fat may be stored, to a certain point, 

 or, if the conditions are altered, the fat that is there may be changed 

 over to the fatty acids and glycerin and be oxidized in the body as 

 food. 



A similar reversibility has been shown for some of the other 

 enzymes of the body (maltase by Hill, 1898), but whether or not all 

 of them will be shown to possess this power under the conditions of 

 temperature, etc., that prevail in the body can only be determined 

 by actual experiments. 



The Specificity of Enzymes. A most interesting feature of 

 the activity of enzymes is that it is specific. The enzymes that 

 act upon the carbohydrates are not capable of affecting the pro- 

 teins or fats, and vice versa. So in the fermentation of closely 

 related bodies such as the double sugars, the enzyme that acts 

 upon the maltose is not capable of affecting the lactose; each re- 

 quires seemingly its own specific enzyme. In fact, there is no clear 

 proof that any single enzyme can produce more than one kind of 

 ferment action. If in any extract or secretion two or more kinds 

 of ferment action can be demonstrated, the tendency at present 

 is to attribute these different activities to the existence of separate 

 and specific enzymes. The pancreatic juice, for example, splits 

 proteins, starches, and fats and curdles milk, and there are assumed 

 to be four different enzymes present, namely, trypsin, diastase, 

 lipase, and rennin. So if an extract containing diastase is also 

 capable of decomposing hydrogen peroxid it is believed that this 

 latter effect is due to the existence of a special enzyme, catalase. 

 It seems quite probable that this specificity of the different enzymes 

 may be related, as Fischer* has suggested, to the geometrical struc- 

 ture of the substance acted upon. Each ferment is adapted to act 

 upon or become attached to a molecule with a certain definite 

 structure, fitted to it, in fact, as a key to its lock. In this respect 

 the action of the so-called hydrolytic enzymes differs markedly from 

 the dilute acids or alkalies which hydrolyze many different substances 

 without indication of any specificity. Attention has been called to 

 the fact that this adaptibility of enzymes to certain specific struc- 

 tures in the molecules acted upon resembles closely the specific 

 activity of the toxins, and many useful and suggestive com- 

 parisons may be drawn between the mode of action of enzymes 

 and toxins. It has become customary to speak of the substance 



* Fischer, "Zeitschrift f. physiolog. Chemie," 26, 71, 1898. 



