THE BIOCHEMICAL PROCESSES OF DIGESTION 521 



quently broken up, consists in a breaking of amino-carboxyl linkings 

 (XHCO) (see page 634), with the consequent liberation of a large num- 

 ber of unattached amino groups. The number of these free amino groups 

 can be determined quantitatively by the formaldehyde titration method 

 of Sorensen.* By this method it can be shown that from the very start 

 of peptic digestion the number of free amino groups increases, and pari 

 passu the power of the digestive products to combine with free hydro- 

 chloric acid. Indeed, when, the experiments are done quantitatively and 

 the digestion allowed to proceed for a considerable time, the increase in 

 the formol titration is practically equal to the decrease in the free acids 

 as determined by the Giinsberg reagent. 



The rate of peptic digestion is usually estimated by the law of Schiitz 

 and Borissow^, according to which the amount of coagulated albumin 

 that is digested in a Mett's tube is proportional to the square root of the 

 amount of pepsin, f 



The pepsin which leaves the stomach in the chyme is not all destroyed 

 in the intestine, as was at one time believed to be the case, for, as we 

 have seen above, some pepsin can be detected in the gastrointestinal con- 

 tents. A part of the pepsin may be absorbed into the blood and carried 

 back to the gastric glands to be used again. This would account for the 

 presence of antipepsin in the blood, and also for the presence of pepsin 

 in the urine. It is probable, however, that most of the pepsin is de- 

 stroyed after it enters the intestine. 



Clotting of Milk in the Stomach 



Besides its pOAver of digesting protein, the gastric juice is also endowed 

 with the property of clotting milk. This action is commonly attributed 

 to the presence of another enzyme besides pepsin, namely, rennin; but 

 in recent years considerable controversy has raged around the question 

 as to whether pepsin and rennin are not the same thing. One strong 

 argument in favor of this view is that all digestive juices that are capable 

 of digesting protein can also clot milk. In any case, when gastric juice 

 acts on milk, it splits *VIP fflfifT 1 ! n ^ *^ A milk into two portions, one of 

 which, called parjicasoin, immediately combines with calcium to form an 

 insoluble f.nllmdpl compound, which is precipitated and, by entangling 

 the fat of the milk, forms the clot; the other protein remains in solution 



*In this method the basic character of the amino acids is destroyed by the formaldehyde, so 

 that a higher decree of acidity develops in the mixture. Bv determining the increased aciditv bv 

 titration with alkali, an estimate is obtained of the number of amino groups. (See page 635.) 



tThe amount of coagulated egg albumin digested is ascertained by measuring the length digested 

 away from the end of a column of coagulated egg white contained in a glass tube (Mett's method). 

 (See Cobb, P. W. : Am. Jour. Physiol., 1905, xiii, 448.) 



Jin the above nomenclature casein is the same as rasc-inogcn, and paracasein the same as casein, 

 of the Knglish physiologists. 



