990 DIGESTION 



substances, such as the disaccharides, it will be found that they re- 

 quire several enzymes to convert them into the monosaccharides. 

 Maltose has its own specific enzyme maltase, and lactose a similarly 

 specific enzyme lactase. We also note that a single secretion, such as 

 the pancreatic juice, may harbor a number of ferments, which act sepa- 

 rately upon different foodstuffs. To be sure, this specificity is also 

 displayed by other catalyzers, but not quite so definitely as by the 

 enzymes. Thus, it is a well-known fact that the oxidation of hydriodic 

 acid by bromic acid may be effected by means of potassium bichromate 

 but not by iodic acid. Quite similarly, the oxidation of potassium 

 iodid by potassium persulphate may be quickened by copper salts, 

 but not the oxidation of sulphur dioxid by potassium persulphate. 



In many cases, these ferments exist within the cell in an inactive 

 form and do not unfold their characteristic properties until they have 

 been discharged into the secretory medium. This antecedent body 

 is known as the proferment or zymogen, and is usually stored in the 

 form of granules. Its activation may be accomplished by inorganic 

 or organic means. In the former instance the intermediary substance 

 is known as an activator and the latter as a kinase.'^ 



The Manner of Action of the Ferments. — Catalysis is a common 

 phenomenon in nature and many chemical means and substances may 

 be employed to bring it about. Thus, the disaccharides may be made 

 to undergo hydrolysis into the monosaccharides by simply heating 

 them under pressure to 110°C., and cane-sugar may be inverted into 

 dextrose and levulose by the addition of a weak acid. Either means 

 serves to accelerate the reaction, which otherwise would not take place 

 at all or only with extreme slowness. It is for this reason that cata- 

 lyzers have been compared to the oil by means of which machinery may 

 be made to run smoothly, i.e., while they do not initiate a certain proc- 

 ess, they are in a position to vary its velocity. Consequently, the 

 essential difference between ordinary catalyzing agents and ferments 

 lies in the fact that the latter effect catalysis much more rapidly at 

 moderate temperatures and impart to it a more specific character. In 

 analogy with ordinary catalyzing agents the ferments may cause: 



(a) Hydrolysis. — This change involves a taking up of water and a conversion of 

 the substance into simpler molecules. As an example of this process might be 

 mentioned the decomposition of the disaccharides, such as maltose, into monosac- 

 charides, one molecule of water being taken up and two molecules of the latter 

 substance being produced. The conversion of fats into fatty acids and glycerin 

 requires three molecules of water. The reverse process is dehydration. As an 

 example of this kind might be mentioned the building up of the amino-acids into 

 polypeptides and the complex proteins of the cells. 



(h) Deamination. — Many tissues possess the power of splitting off an NH2 

 group from amino-acid as ammonia and replacing it by H or OH. The reverse 

 process is continuously going on in plants which synthetize proteins from ammonia 

 and a carbohydrate. Some evidence is also at hand to show that this reversion 

 may be effected by animals. 



(c) Decarboxylation. — This process involves the loss of a molecule of carbon di- 



^ Samuely, Handb. der Biochemie, 1908. 



