178 PHYSIOLOGICAL CHEMISTRY. 



The physiologically important constituents, besides free Hydro- 

 chloric acid, are pepsin and rennet. 



Pepsin. This enzyme is found, with the exception of certain 

 fishes, in all vertebrata thus far investigated. 



Pepsin occurs in adults and in new-born infants. This condi- 

 tion is different in new-born animals. While in a few herbivora, 

 such as the rabbit, pepsin occurs in the mucous coat before birth, 

 this enzyme is entirely absent at the birth of those carnivora which 

 have thus far been examined, such as the dog and cat. 



In various invertebrates a fermeut has also been found which has a prote- 

 olytic action in acid solutions. KRUKENBERG has shown that this enzyme, 

 nevertheless is not in all animals, identical with ordinary pepsin. DARWIN has 

 further found that certain plants which feed upon insects secrete an acid 

 juice which dissolves albumin, but it is still doubtful whether these plants 

 contain any pepsin, v. GORUP BESANEZ has isolated from vetch-seed an 

 enzyme which acts like pepsin, but whose identity with pepsin is doubtful. 



Pepsin is as difficult to isolate in a pure condition as other 

 enzymes. The purest pepsin was that prepared by BRUCKE and 

 SUNDBERG ; this gave negative results with most reagents for 

 albumin. Pepsin, therefore, does not seem to be a true albuminous 

 substance. It is, at least in the impure condition, soluble in water 

 and glycerin. It is precipitated by alcohol, but only slowly de- 

 stroyed. It is quickly destroyed by heating its watery solution to 

 boiling ; in the dry state it can, on the contrary, be heated to over 

 100 C. without losing its physiological action. The only property 

 which is characteristic of pepsin is that it dissolves albuminous 

 bodies in acid, but not in neutral or alkaline, solutions with the 

 formation of albumoses and peptones. 



The methods for the preparation of relatively pure pepsin de- 

 pend, as a rule, upon its property of being thrown down with 

 finely-divided precipitates of other bodies, such as calcium phos- 

 phate or cholesterin. The rather complicated method of BRUCKE 

 and SUNDBERG is based upon this property. A relatively pure 

 pepsin solution intended for digestion tests and of effective action 

 may be prepared by the following method as suggested by MALY. 

 The mucous membrane (of the pig's stomach) is treated with water 

 containing phosphoric acid, and the filtrate precipitated by lime- 

 water; the precipitate, which contains the pepsin, is then dissolved 

 in water by the addition of hydrochloric acid, and the salts removed 

 by dialysis, by which means the pepsin which does not diffuse re- 

 mains in the dialyzer. A pepsin solution somewhat impure but 



