992 DIGESTION 



easily understood, unless it is assumed that the enzyme then destroys 

 itself in part by autolysis. But this retardation does not take place 

 under all circumstances, because certain fermentations which have 

 come to a standstill owing to the large amounts of ferment present, 

 may again be brought under way by diluting the mixture or by remov- 

 ing the products formed in the course of the reaction. 



This ''self -inhibition" is closely allied to the inhibition of ferment 

 action by outside means. ^ Thus, it is a well-known fact that these proc- 

 esses may be greatly retarded and abolished by strong acids, alkalies, 

 alcohol, iodin, potassium cyanide, formaldehyde, and the salts of the 

 heavy metals. In many instances, the cells of the different tissues 

 produce a substance which is called anti-enzyme. For example, if 

 an enzyme is injected into the blood-stream, certain cells are stimu- 

 lated to produce an anti-enzyme of a specific kind, so that the serum of 

 this blood may be mixed with the original enzyme with the result that 

 the latter is then quite unable to unfold its characteristic action. 



As to the manner in which enzymes increase the velocity of the re- 

 action to which they are specifically assigned, few positive statements 

 can be made. In the first place, it may be assumed that the ferment is 

 combined with the substrate in a loose manner — fitted to it as a key in 

 its corresponding lock.^ This simile, no doubt, calls to our minds the 

 interaction between the antigen and the immune bodies, as explained 

 by the side chain theory of Ehrlich. In the second place, it is evi- 

 dent that the ferment is finally removed from the sphere of its action 

 and enters the end-products. This brings the catalysis to an end. 

 These reactions, however, are different from those taking place between 

 various inorganic substances, because the latter are chiefly interac- 

 tions between electrolytes. Thus, the molecules of sodium chlorid 

 are broken up into their cations Na which are charged positively and 

 move toward the cathode or negative pole, and their anions CI which 

 are negative and move toward the anode or positive pole. Since the 

 organic foodstuffs, namely, the proteins, carbohydrates and fats, are 

 not electrolytes, their reactions cannot be regarded as analogous to 

 these almost instantaneous ionic movements. They take place more 

 slowly and are in reality molecular interactions. When only one. 

 substance is being transformed, it constitutes a unimolecular reaction. 

 As an example of this kind might be mentioned the conversion of 

 starch into sugar. The velocity of the reaction is measured in this 

 case in terms of substance transformed, i.e.y in gram-molecules per 

 liter in the unit-time of one minute. But, as the amount of substance 

 acted upon is gradually diminished, the velocity of the reaction must 

 also be reduced in a proportionate measure. In those cases in which 

 two substances are changed simultaneously, as occurs in the decompo- 

 sition of esters under the influence of an alkali, a bimolecular reaction 



1 Porter, Quart. Jour, of Exp. Physiol., iii, 1910, 375. 



2 Emil Fischer, Zeitschr. fiir physiol. Chemie, 1898. 



