8 PHYSIOLOGICAL CHEMISTRY 



the substrate, or substance acted upon, is well known. Emil Fischer 

 investigated this problem of specificity extensively in connection with 

 the fermentation of sugars and reached the conclusion that enzymes, 

 with the possible exception of certain oxidases, can act only upon such 

 substances as have a specific stereo-isomeric relationship to themselves. 

 He considers that the enzyme and its substrate must have an inter- 

 relation, such as the key has to the lock, or the reaction does not occur. 

 Fischer was able to predict, in certain definite cases, from a knowledge 

 of the constitution and stereo-chemical relationships of a substance, 

 whether or not it would be acted upon by a certain enzyme. An appli- 

 cation of this specificity of enzyme action may be seen in the well-known 

 facts that certain enzymes act on carbohydrates, others on fats, and 

 others on protein; and, moreover, that the group of those which trans- 

 form carbohydrates, for example, is further subdivided into specific en- 

 zymes each of which has the power of acting alone upon some one sugar. 



It has been conclusively shown, in the case of certain enzymes, 1 

 at least, that their action is a reversible one and is, in all its main fea- 

 tures, directly analogous to the reversible reactions produced by chem- 

 ical means. For instance, in the saponification of ethyl-butyrate by 

 means of pancreatic lipase, it has been shown that upon the tormation 

 of the end-products of the reaction, i.e., butyric acid and ethyl alcohol, 

 there is reversion 2 and the reaction is stationary. This does not mean 

 there are no chemical changes going on, but simply indicates that 

 chemical equilibrium has been established, and that the change in one 

 direction is counterbalanced by the change in the opposite direction. 

 Pancreatic lipase was one of the first enzymes to have the reversibility 

 of its reaction clearly demonstrated. 3 A knowledge of the fact that 

 lipase possesses this reversibility of action is of extreme physiological 

 importance and aids us materially in the explanation of the processes 

 involved in the digestion, absorption, and deposition of fats in the 

 animal organism (see page 181). 



Euler 4 claims that enzymatic cleavage and synthesis are often brought 

 about by two different components of an enzyme preparation. He 

 would indicate this fact by giving the termination -ese to those enzymes 

 exerting a synthetic function. For example, the enzyme which catalyzes 

 the formation of nitriles Euler would call mtnlese in distinction from 

 nitnlase which splits nitriles. He would further designate as phos- 



1 This is probably a general condition. 



2 The re-synthesis of ethyl-butyrate from its hydrolysis products. This may be indi- 

 cated thus: 



C 3 H 7 COO.C 2 H 5 + H 2 5 C 3 H 7 COOH + C 2 H 5 OH. 



Ethyl-butyrate. Butyric acid. Ethyl alcohol. 



* This principle was first demonstrated in connection with the enzyme maltase (see p. 56). 

 4 Euler: Zeitschrift fur physiologische Chemie, 74, 13, 1911. 



