THE BIOCHEMICAL PROCESSES OF DIGESTION 487 



ess is the same for each of these enzymes. Indeed, there is considerable 

 evidence that pepsin does not, like the other enzymes, break up the long 

 chain of amino acids that are linked together to compose the polypep- 

 tides, but that it only splits the big molecule of albumin or globulin 

 into several large groups, each of which is composed of long amino-acid 

 chains. Its action appears to be analogous with that of amylase on 

 starch, by which, it will be remembered, the big polysaccharide mole- 

 cule is split into smaller polysaccharide molecules, which then become 

 attacked by the dextrinase and split into disaccharide molecules (see 

 page 656). The evidence in support of this view is: (1) that pepsin is 

 unable to digest polypeptides, and (2) that it is able to digest certain 

 proteins upon which erepsin (see page 490) has no action. 



The hydrolytic splitting of large into smaller protein molecules, like 

 that by which the chains of amino acids in the polypeptides are subse- 

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

 (NHCO) (see page 598), 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 Avas 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. 



*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. By determining the increased acidity by 

 titration with alkali, an estimate is obtained of the number -of amino groups. (See page 599.) 



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

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

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



