00 DIGESTION 



Pepsin is a proteolytic enzyme that acts only in acid media. 

 A piece of fibrin, for example, when subjected to an artificial 

 gastric juice swells up and finally passes into solution. It is 

 changed into a more diffusible form of protein called peptone, 

 but this conversion takes place through a number of inter- 

 mediate steps. There is, first, the formation of an acid albumin, 

 which has been named syntonin. Upon neutralization of the 

 medium, syntonin is precipitated, and upon further addition 

 of the alkali is converted into alkali albumin, which again 

 passes into solution. Syntonin is by hydrolysis changed 

 to a series of bodies known as primary proteases. These in 

 turn undergo cleavage with the formation of secondary pro- 

 teoses or deuteroproteoses. The latter finally become, by the 

 further action of the ferment, peptones. 



The original protein molecule has undergone a series of 

 hydrolytic cleavages, so that there is obtained a large number 

 of much smaller, more soluble molecules whose molecular 

 weights are perhaps only 250 or so instead of 5000 or 7000, 

 the weight of the original protein molecule. The peptones 

 are a group of compounds which, while still showing the pro- 

 tein (biuret reaction), are not coagulated by heat nor precipitated 

 by saturation with ammonium sulphate. There is evidence 

 that along with peptone, or in place of it, the further action 

 of pepsin gives rise to certain simpler bodies which no longer 

 give the biuret test and are not precipitable by phospho- 

 tungstic acid. These apparently are polypeptids. In addi- 

 tion, finally, amino-acids and nitrogenous bases may be found. 



Rennin. The gastric juice contains an enzyme that coagu- 

 lates milk. The latter, if left undisturbed at the proper 

 temperature, sets into a solid clot, which shrinks and presses 

 out a clear, yellowish liquid called whey. The curd of human 

 milk is not a solid, but is deposited in loose flocculi. Rennin 

 acts upon caseinogen, giving rise to a modification known 

 as paracasein. The latter unites with calcium salts and is 

 precipitated as the curd. If soluble calcium salts are removed 

 from milk by the addition of oxalate solutions it will not curdle 

 under the influence of rennin. Addition of lime salts restores 

 the coagulating power. Casein is precipitated by an excess 

 of acid. This takes place in the souring of milk. Here the 

 bacteria, acting upon milk sugar, produce lactic acid, which, 



