COMPOSITION OF FOOD AND ACTION OF ENZYMES. 739 



of the inorganic catalyzers will eventually lead to a better under- 

 standing of the underlying physical causes of fermentation. It 

 should be borne in mind, however, that some of the best Ivuown of the 

 ferment actions of the body, such as the peptic or tryptic digestion of 

 protein, fit into this view only theoretically and by analogy. As a 

 matter of fact, albumins at ordinary temperatures do not split up 

 spontaneously into the products formed by the action of pepsin; 

 if we consider that the pepsin simply accelerates a reaction already 

 taking place, it must be stated that this reaction at ordinary 

 temperatures is infinitely slow, — that is, practically does not occur. 

 At higher temperatures, however, similar decompositions of al- 

 bumin may be obtained without the presence of an enzyme. 



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: 



CH3COOH + QHsOH = CH3COOC2H5 + H2O. 



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: 



CH3COOH + C2H5OH ^t CH3COOC2H5 + H,0. 



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 in the digestion and absorption of fats. 

 The authors above named* made use of a simple ester analogous 



* Kastle and Loevenhart, "American Chemical Journal," 24, -191, 1900. 

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



