60 PLANT PHYSIOLOGY 



fell to 78 per cent, 4 days later to 61, and at the end of 6 days 

 to 57 per cent with a corresponding increase in the quantity of 

 peptids and amino acids. 



The first of the proteolytic enzymes to be studied were those 

 present in the digestive tract of animals. Two types were estab- 

 lished : pepsin, secreted by the walls of the stomach and causing 

 a rather weak hydrolysis of proteins into the so-called ''pep- 

 tones"; and trypsin, secreted by the pancreas and sphtting 

 proteins and peptones into amino acids. Peptones are quite 

 complex bodies of a colloidal nature, but their molecular weight 

 is less than that of the true proteins. 



It is usually thought that peptones are the first products of 

 the decomposition of the protein molecule; but recently evidence 

 has accumulated which shows that in the formation of peptones 

 the protein molecule itself is not decomposed, but only the 

 larger colloidal aggregates, composed of several molecules, are 

 broken down. At any rate, peptones do not represent definite 

 chemical compounds. In their chemical composition and prop- 

 erties, they are most closely alhed to polypeptides, obtained by 

 the method of Emil Fischer by combining several amino acids. 



Further decomposition of peptones is accomplished by trypsin, 

 as well as by another special enzyme, erepsin, which is distin- 

 guished by its inability to hydrolyze unchanged proteins. Thus 

 proteins are hydrolyzed in the digestive tract of animals by 

 three enzymes: pepsin, trypsin, and erepsin, which supplement 

 each other in their activity. 



In plants, proteolytic enzymes were discovered much later 

 than in animals, and until recently they were comparatively 

 little studied. The best known of the active plant proteases is 

 papain, obtained from the juice of the fruit of Carica papaya. 

 It is used by persons suffering from indigestion due to insufficient 

 secretion of proteases. Extraction of proteases from germinating 

 seeds is more difficult than the extraction of diastase, for they 

 are not soluble in water. Their activity is usually studied 

 through autolysis, i.e., "self-digestion" of protein substances 

 of the seed. The seeds are ground to flour, and water is added 

 in the presence of chloroform, toluol, or some other neutral 

 antiseptic, which precludes the development of microorganisms 

 during the digestion and likewise kills the cells of the seed embryo 

 that may have remained unbroken during the grinding. 



