750 PHYSIOLOGY OF DIGESTION AND SECRETION. 



Reversible Reactions. It has been shown that under proper 

 conditions many chemical reactions are reversible, that is, may 

 take place in opposite directions. For instance, acetic acid and 

 ethyl-alcohol brought together react with the production of ethyl- 

 acetate and water: 



CH 3 COOH + C 2 H 6 OH = CH 3 COOC 2 H 6 + H 2 O. 



Acetic acid. Alcohol. Ethyl-acetate. Water. 



On the other hand, when ethyl-acetate and water are brought 

 together they react with the formation of some acetic acid and 

 ethyl-alcohol, so that the reaction indicated in the above equation 

 takes place in opposite directions, figuratively speaking, a fact 

 which may be indicated by a symbol of this kind: 



CH 3 COOH + C 2 H 6 OH ^ CH 3 COOC 2 H 6 + H 2 O. 



It is evident that in a reversible reaction of this sort the opposite 

 changes will eventually strike an equilibrium, the solution or mix- 

 ture will contain some of all four substances, and this equilib- 

 rium will remain constant as long as the conditions are unchanged. 

 If the conditions are altered, however, if, for example, some of the 

 substances formed are removed or the mixture is altered as to its 

 concentration, then the reaction will proceed unequally in the two 

 directions until a new equilibrium is established. The importance, 

 in the present connection, of this conception of reversibility of reac- 

 tions is found in the fact that a number of the catalytic reactions 

 are also reversible. The catalyzer may not only accelerate a reac- 

 tion between two substances, but may also accelerate the recom- 

 position of the products into the original substances. An excellent 

 instance of this double effect has been obtained by Kastle and 

 Loevenhart in experiments upon one of the enzymes of the animal 

 body, lipase. Lipase is the enzyme which in the body acts upon the 

 neutral fats, converting them into fatty acids and glycerin, a 

 process that takes place as a usual if not necessary step in the diges- 

 tion and absorption of fats. The authors above named* made use 

 of a simple ester analogous to the fats, ethyl-butyrate, and showed 

 that lipase causes not 9nly 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 lipase is therefore reversible and may be expressed as: 



C 3 H 7 COOC 2 H 6 + H 2 ^ C 3 H 7 COOH + C 2 H S OH. 



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 



* Kastle and Loevenhart, " American Chemical Journal," 24, 491, 1900. 

 See also Loevenhart, "American Physiological Journal," 6, 331, 1902. 



