682 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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. To how complete an extent the idea of the specificity 

 of the different body enzymes may be carried is a matter for future 

 experiments. At present the tendency is to attribute each new kind 

 of activity to a different enzyme, and as a consequence the number 

 of different enzymes supposed to exist in the body is increasing 

 rapidly with the spread of experimental work. 



Definition and Classification of Enzymes (Ferments). On 



the basis of the considerations presented in the preceding paragraphs 



Oppenheimer suggests the following definition: An enzyme is a 



substance, produced by living cells, which acts by catalysis. The 



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



