DIGESTION 191 



Pepsin is a proteolytic enzyme, secreted by the central cells of the 

 gastric mucous membrane, especially by those of the fundus. The 

 cray-fish secretes its pepsin in the mouth just as men produce ptyalin 

 there, but use it largely in the stomach's undisturbed fundus. Pepsin 

 is present in the stomach of the child at birth, but in many animals, 

 especially the carnivora, for example the cat and the dog (Moriggia), 

 it makes its appearance only two weeks or so after birth. It differs 

 from most enzymes in requiring an acid medium for its activity. In an 

 alkaline or even in a neutral medium pepsin is rapidly destroyed. Indeed 

 some have supposed that the really active proteolytic agent is pepsin- 

 hydrochloric-acid or even the acid alone. There is no proof that this 

 is a true compound, while Maly has shown that other acids may serve 

 in place of the hydrochloric acid. Nitric acid is the best substitute, 

 but lactic, phosphoric, and three or four other common acids also serve 

 experimentally this function of giving pepsin its " required acid medium." 

 Maly's work suggests that part of the action of the acid may be to swell 

 up and soften the proteid, and those acids which are most active in this 

 respect are those which are also most effective in combination with the 

 pepsin. The most recent theory is that the pepsin acts solely as a coupler 

 between the protein and the acid, the latter doing the hydrolyzing 

 work. This enzyme acts most rapidly at a temperature 2 or 3 above 

 the normal temperature of the body, but is destroyed at about 80 C. 

 (dry at 100), a degree of heat about 12 higher than that which destroys 

 ptyalin. As the temperature falls action becomes slower and quite ceases 

 at zero. The pepsin of commerce is a gastric extract, containing usually 

 either lactose or starch. 



The function of pepsin has long been said to be to hydrolyze proteids 

 and albuminoids so that they may be absorbed into the circulation and 

 thus feed the organism. Pepsin's solvent power over different forms of 

 proteid varies exceedingly. Casein is perhaps the most easily hydrolyzed 

 of all the proteids, raw meat more easily than cooked meat, beef more 

 readily (Cummens and Chittenden) than fish, and animal proteids more 

 easily than those of vegetal origin. 



Pepsin hydrolyzes proteids into peptone, probably into polypeptids, and 

 possibly further into the amido-acids even, as the final products of its 

 action. The intermediate substances produced are variously named 

 and as variously described by different chemists, and must be considered 

 as yet uncertain. Even less is known about the molecular structure 

 of proteids and albuminoids than about that of starch, and here even 

 more than there are the way-products undetermined. Two of these 

 way-products, however, are fairly well known, namely, the albuminate 

 syntonin (acid-albumin) and proteose ("propeptone," albumose, globu- 

 lose, vitellose, gelatinose, elastose, etc.). It is very possible that syntonin 

 may not be formed from all the proteid undergoing digestion, but usually 

 the proteid swells all through owing to the combined action of the acid 

 and pepsin, the softened mass then dissolving. By hydrolysis (absorp- 

 tion of water and molecular cleavage) the syntonin then splits into soluble 

 proteoses. The debated question is largely as to how many different 



