$ 4 DIGESTION 



The reaction can be accelerated either by a catalyst e.g., H ions 

 as by addition of free hydrochloric or picric acid, or by pancreas 

 lipase. When the concentrations of the reacting substances are 

 appropriately chosen, the same equilibrium point will be reached 

 from either side of the equation i.e., the same percentage of the 

 butyric acid will be converted into the ester if we start with the 

 alcohol and acid, as will remain combined as ester if we start with 

 the amyl butyrate. But the proportion will not be the same when 

 the reaction is acclerated by H + as when it is accelerated by the 

 enzyme. And although it is probable that there is no fundamental 

 difference between the action of the digestive enzymes and that of 

 the inorganic catalysers, it is much too early to dogmatize. 



Not even the m .rkedly specific action of the digestive ferments 

 can be considered an essential distinction. It is true that invertase 

 will act upon dextrose, and not at all upon maltose or lactase. 

 But there are other sugars, e.g., raffinose, a trisaccharide with the 

 formula QgHggOjg, obtained from beet-sugar residues, which it will 

 hydrolyse. Raffinose is made up of one molecule each of dextrose, 

 levulose, and galactose. On heating with dilute acids, it is decom- 

 posed into these substances. Invertase, however, only splits off 

 the levulose molecule, leaving a disaccharide isomeric, but not 

 identical with lactose. Similarly lactase, which is without action 

 upon cane-sugar or maltose, will hydrolyse the /3-galactosides, and 

 maltase, inert as regards cane-sugar or lactose, will hydrolyse the 

 a-glucosides. On the other hand, emulsin decomposes the /3-gluco- 

 sides, to which group most of the natural glucosides belong, as well 

 as the /3-galactosides and lactose. From raffinose emulsin splits 

 off galactose, leaving cane-sugar. Since the a and ft compounds 

 are isomeric, and differ not in their composition but in their struc- 

 ture, it has been concluded that the structure of the molecule of 

 a substance must be related to the structure of the enzyme which 

 can act on it, in some such way as a lock is related to its proper key. 

 Thus the key lactase fits in the lock lactose, but not in the lock 

 dextrose or the lock maltose. Although the same specificity is 

 not to be observed in the action of catalysers as in the action of 

 enzymes, it is not difficult to find many instances in which inorganic 

 substances show a marked limitation of their catalytic effects to 

 particular reactions. Thus hydriodic acid is slowly oxidized in 

 presence of hydrogen peroxide, with formation of iodine and water. 

 This reaction is accelerated by the addition of many substances, 

 e.g., tungstic acid. But tungstic acid has no catalytic effect on the 

 oxidation of hydriodic acid by bromic acid. 



The existence of an optimum temperature for ferment action, 

 above which it rapidly decreases, and eventually comes to a com- 

 plete stop, is also in all probability only a superficial distinction 

 between enzymes and catalysers. For enzymes are easily altered, 



