40 PROTOPLASM AND THE CELL 



which they undergo. These enzymes are probably organic 

 bodies or compounds of which the exact composition is unknown, 

 although for certain starch-dissolving enzymes (amylases), 

 Mathews concludes that "the indications are that the active 

 part of the molecule is a protein, probably colloidal, and that 

 this active principle is usually combined with a colloidal, car- 

 bohydrate gum" (Physiological Chemistry, p. 330). 



While the chemical composition of enzymes is at present 

 unknown, we have a rapidly accumulating fund of information 

 concerning their classification and activities in animal organ- 

 isms. Thus in the general function of nutrition where the 

 digestive enzymes are hydrolytic agents throughout, we find 

 some that are proteoly tic (i.e., that break down proteins) , some 

 lipolytic (i.e., that destroy fats), and some that are amylolytic 

 (i.e., that break down carbohydrates). 



Some of the more important proteolytic enzymes are: (i) pepsin, which 

 acts in an acid medium to break down proteins into peptones; (2) trypsin 

 which acts in an alkaline medium to break down proteins and to transform 

 products of peptic digestion into amino-acids and polypeptids; and (3) 

 erepsin which splits peptones and polypeptids to amino-acids. Among the 

 fat-transforming enzymes, i.e., enzymes which split fats to form glycerine 

 and fatty acids, are different forms of lipase from stomach, intestines and 

 the pancreas (steapsin). Among the more important amylolytic ferments 

 are the amylases or diastases which convert insoluble starch into soluble 

 sugars, such as ptyalin of the saliva, diastase of the pancreatic juice, and 

 other amylases of the digestive fluids. Here also belong the enzymes 

 which transform disaccharid sugars into monosaccharids, such as maltase 

 of the saliva, which splits maltose; invertase of the stomach and pan- 

 creatic juices, which splits cane sugar; and lactase of the stomach and 

 pancreatic juices, which splits milk sugar. 



Enzymes having to do with the digestion of food substances 

 may act either within the cells of the body (phagocytes, proto- 

 zoa and coelenterates) , or in cavities lined by the cells which 

 secrete them. In addition to these digestive enzymes there are 

 many others in protoplasm which have to do with the various 

 processes of constructive and destructive metabolism, and these 

 always act within the cell, hence they are often called the 



