440 THE CHEMICAL COMPOSITION OF THE BODY [CH. XXVIII. 



The enzymes which bring about the digestion of food in the 

 alimentary canal may be classified as follows : 



1. Amylolytic -those which convert polysaccharides (starch, 

 glycogen) into sugar with intermediate dextrins. Examples: the 

 diastase of vegetable seeds, and the ptyalin of saliva. 



2. Inverting those which convert disaccharides into mono- 

 saccharides. Examples : Invertase of yeast cells ; invertase of 

 intestinal juice ; these convert cane sugar into equal parts of 

 dextrose and laevulose. 



3. Lipolytic those which split fat into fatty acids and 

 glycerin. An example, lipase, is found in pancreatic juice. 



4. Proteolytic or proteoclastic those which split proteins into 

 proteoses, peptones, polypeptides, and finally amino-acids. Examples: 

 the pepsin of gastric, and the trypsin of pancreatic juice. 



5. Peptolytic those which split proteoses and peptones into 

 polypeptides and amino-acids, e.g., the erepsin of intestinal juice. 



But in addition to the digestive enzymes there are others to be 

 mentioned, for instance : 



The enzymes in the foregoing list produce hydrolysis; that is, 

 water is added to the substrate, which then splits into simpler 

 molecules, as for instance in the inversion of cane sugar by invertase. 



C 12 H 22 O n + H 2 = C 6 H 12 6 + C 6 H 12 6 . 



[Cane sugar.] [Water.] [Dextrose.] [Laevulose.] 



Coagulative enzymes those which convert soluble into 

 insoluble proteins; the best example of this class is thrombin or 

 fibrin-ferment, which comes into play in blood-coagulation, convert- 

 ing the soluble protein in blood -plasma called fibrinogen into fibrin. 

 Eennet or rennin, found in the gastric juice, is another ; it converts 

 the soluble caseinogenate of milk into casein. 



Oxidases ; these are not hydroly tic, but are oxygen carriers and 

 produce oxidation : they are mainly found as intracellular enzymes, 

 and are important in tissue respiration. 



Reductases; these are the counterpart of the oxidases, and 

 produce reduction in the tissues. 



Deamidases ; these remove the ammo-group from amino- 

 compounds. 



Intracellular or Autolytic Enzymes. These come into play 

 during cell life, and are important in the metabolic or intracellular 

 chemical changes which occur in protoplasm; they also may be 

 subdivided into proteolytic, peptolytic, lipolytic, etc., according to 

 the substrate upon which they act. After death their activity 

 continues, and so they produce self-digestion or autolysis of the 

 cells in which they are situated, if the tissue or organ is kept at 

 an appropriate temperature and under aseptic conditions. 



