740 PHYSIOLOGY OF DIGESTION AND SECRETION. 



to the fats, ethyl-but3a-ate, and showed that hpase causes not 

 only an hydrolysis of this substance into ethyl-alcohol and butyric 

 acid, but also a synthesis of the two last-named substances into 

 ethyl-butyrate and water. The reaction effected by the hpase is 

 therefore reversible and may be expressed as : 



C3H7COOC2H5 + H2O ^ C3H7COOH + C2H5OH. 



Ethyl-butyrate. Water. Butyric acid. Ethyl-alcohol. 



Lipase is capable of exerting probably a similar reversible reaction 

 on the fats in the body. Assuming the existence of such an action 

 in the body, it is possible to explain not only the digestion of fats 

 but 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 subcutaneous 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. 



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 disaccharids, 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 that of the dilute acids or alkalies which hydrolyze many dif- 

 ferent substances without indication of any specificity. It has 

 become customary to speak of the substance upon which an 

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



