250 DIGESTION 



likewise very different, though they are all included under the common name 

 proteases. The glutin-forming substances are changed by gastric juice into 

 gelatin, and this into gelatin peptones. 



The same decompositions which proteids suffer in digestion they exhibit also 

 when treated with acids or alkalies or superheated steam, and when they fall 

 under the influence of putrefactive Bacteria. In fact, weak salt solutions have 

 a digestive action on proteids (Dastre). The action of the enzymes is not to 

 be regarded, therefore, as particularly exceptional. 



Loss of the power of coagulation on the part of the blood and other harmful 

 effects which have long been known to follow intravenous injection of albumoses 

 have lately been attributed to other substances e. g., peptozymes mixed with 

 them (Pick and Spiro). Peptozymes, acting mainly in the liver, cause the 

 production of a substance which prevents coagulation (Contejean, Gley). 



C. RENNIN 



It has long been known that milk coagulates by precipitation of its casein 

 when it comes in contact with the mucous membrane itself, or is mixed 

 with an extract of the membrane. Since acids produce the same effect it was 

 supposed that this precipitation was due to the acid reaction of the mucosa. 

 But the investigations of Selmi and Heinz, and especially those of Ham- 

 marsten (1872) and of Alex. Schmidt showed that coagulation takes place 

 in a neutral or alkaline reaction, that the acid is, therefore, quite superfluous, 

 and finally that coagulation of milk is effected by a special enzyme called 

 rennin or chymosin. 



Rennin occurs in the mucous membrane as a precursor, rennin-zymogen, 

 which like pepsinogen is more resistant to alkalies than its enzyme, can be 

 extracted from the mucosa with water, and is transformed into the active 

 enzyme by addition of acids. In its action rennin resembles the other digestive 

 enzymes. One part of the impure enzyme can coagulate 400,000 to 800,000 

 parts of casein. 



In the coagulation of milk produced by rennin, casein first suffers cleavage 

 into paracasein, and whey proteid, a substance resembling albumose; the for- 

 mer, which is the chief product, then precipitates out in solid form, provided 

 Ca salts are present in the solution. If Ca salts are absent, cleavage occurs 

 under the influence of rennin, but no coagulation. 



D. GASTRIC STEAPSIN 



After Marcet, Cash and Ogata had demonstrated the decomposition of neu- 

 tral fat in the stomach, Volhard made further investigations on the subject 

 and established this property of gastric juice beyond doubt with the limita- 

 tion, however, that it acts only on emulsified fats, but on these very powerfully. 



The rule holds for stomach steapsin, as for other enzymes, that its action 

 is proportional to the square root of concentration. It is quickly destroyed in 

 a strongly acid gastric juice. The pure pepsin of Pekelharing has no fat- 

 splitting action, a fact which speaks decisively for the independence of the 

 gastric steapsin. 



