DIGESTION 



B. PEPSIN 



After Spallanzani (about 1780) had shown that the gastric juice can 

 produce chemical changes outside the body, Eberle (1834) was the first to 

 demonstrate the same effects with extracts of the gastric mucosa, and Schwann 

 (1836) pointed out that a substance formed in the mucosa, which he named 

 pepsin., is involved in this action. 



Under this name is described the enzyme which acts in acid medium upon 

 proteid, gelatin and connective tissues, causing them to absorb water and to 

 split into simpler compounds. Pepsin has no effect in neutral solution, and 

 is destroyed in soda solution. 



From pure gastric juice of the dog, Nencki and Sieber, also Pekelharing, 

 have prepared by dialysis a very pure pepsin. This is a proteid body, con- 

 taining 51-52 per cent C, 6.7-7.1 per cent H, 14.4 per cent N, 1.5-1.6 per 

 cent S, and 0.5 per cent Cl. and some Fe. On cleavage it yields a pentose, 

 purin bases, and an acid (pepsinic acid, 50.8 per cent C, 7.0 per cent H, 14.4 

 per cent N, 1.1 per cent S). Since in a strongly active preparation no trace 

 of phosphorus could be demonstrated, pepsin cannot be numbered among the 

 nucleoproteids. On the other hand, it is possible that it unites with lecithin 

 to form a compound analogous to jecorin (page 79). 



Pepsin occurs in the mucous membrane only in the preliminary form of 

 its zymogen, pepsinogen. We have seen that pepsin is destroyed by soda. 

 , If however the mucous membrane be extracted with a weak soda solution 

 and the extract be then acidified with HC1, a pepsin-containing fluid of good 

 digestive properties is obtained (Langley). Therefore there must be in the 

 mucosa a substance which is not destroyed by soda, and which is transformed 

 into pepsin by treatment with acids. 



In artificial digestion the quantitative results depend upon the following 

 factors: temperature; the amount f pepsin; the amount and the kind of acid; 

 the kind and the amount of proteid; the presence of the products of digestion; 

 and the presence of certain inorganic salts. 



The quantity of enzyme necessary to produce a very powerful digestive 

 action is very small. Thus in a certain artificial gastric juice of very excellent 

 digestive power, there was found for example only 0.067 per cent of nonvolatile 

 organic matter. 



If, however, the quantity of HC1 and of proteid remaining the same, the 

 quantity of pepsin be increased, the rate of digestion is increased, so that up to 

 a certain limit the action is proportional to the square root of the concentration 

 of the enzyme (Schiitz). The same law holds also for the enzymes contained in 

 the pancreatic secretion (Walther). 



The acidity of the digest is a matter of particular importance. We find: 

 (1) that the optimum acidity is very different for different proteids; (2) that 

 too much or too little acid stops all digestive action. The most powerful action 

 on fibrin, for example, is said to be obtained with an acidity of 0.09 per cent 

 HC1 ; at 0.13 per cent and 0.02 per cent the action is very feeble. But on coagu- 

 lated white of egg the best effect is obtained with 0.16-0.25 per cent HC1. 



In the transformation of proteid under the influence of pepsin a number 

 of different substances arise by cleavage, the composition of which becomes 

 simpler and simpler the farther the cleavage progresses. 



