DIGESTION 275 



best in an acid medium and carries the digestion as far as pep- 

 tones only, showing that it is a mixture of pepsinase and albumase. 

 Trypsin, on the other hand, which is produced in the intestine, 

 works best in an alkaline medium and carries the digestion as 

 far as amino acids. It is thus seen to be a mixture of pepsinase, 

 albumase, and peptase. Ereptases (or erepsins), which are secreted 

 by the small intestine, act on the peptones and polypeptids and 

 break them down to amino acids, but they do not digest the higher 

 proteins. They thus consist mostly of peptases. 



The plant proteases, as previously stated, have not been studied 

 so much as those of animals. They are harder to isolate and sep- 

 arate and do not occur so abundantly. Those studied, however, 

 fall into the three classes represented by pepsin, trypsin, and erep- 

 sin, and may be called the peptic, tryptic, and ereptic proteases. 



Among the pepsins are some which have been found in the in- 

 sectivorous plants such as the sundew (Drosera) and the pitcher 

 plant (Nepenthes). The one from Nepenthes, like animal pepsin, 

 acts best in an acid medium, but the one from Drosera seems to 

 act equally well in slightly alkaline, neutral, or acid media. It 

 should also be noted that these enzymes of the insectivorous 

 plants belong to the class of extracellular enzymes since their 

 action is manifested outside of the cells which secrete them. 



The plant trypsins are more common than the pepsins and two, 

 which have been much studied, are especially well known. These 

 are papain from the tropical pawpaw (Carica) and bromelin from 

 pineapple (Ananas). Papain acts in either an alkaline or acid 

 medium, according to both Vines and Mendel (1902), but better 

 in an acid medium. This led to the hypothesis that this enzyme 

 is perhaps composed of at least two separate enzymes, one of 

 which worked better in acid and the other in alkali, but this has 

 not yet been confirmed. It can be obtained at the drug store as 

 an aid to digestion and is often so used. The proteolytic action is 

 so pronounced that the pawpaw leaves are used by the natives to 

 predigest meats and make them tender. It is said that merely 

 wrapping the meat in crushed leaves results in a marked change 

 within a few hours. Bromelin, like papain, is active in both acid 

 and alkaline media but it works best in a neutral or slightly acid 

 environment. 



Plant ereptases are very common in both the lower and higher 

 plants. Not only are they known in bacteria and yeast, but they 



