DIGESTION 



315 



INTESTINAL DIGESTION 



When the chyme enters the duodenum it is mixed with the secretions 

 of the pancreas and bile. 



Pancreatic secretion 



Pancreatic juice has an alkaline reaction, pH 7.1-8.2, and contains 

 a variety of very active enzymes which can attack carbohydrates, pro- 

 teins, and fats. Trypsin and chymotrypsin hydrolyze proteins and poly- 

 peptides to smaller polypeptides and amino acids. The action of the pro- 

 teolytic enzymes depends in part on the sequence of amino acids in the 

 protein or peptide which they attack. Peptides that are the products 

 of one of the proteinases can therefore be hydrolyzed further by another 

 proteinase of different specificity. In model experiments with the syn- 

 thetic peptide, carbobenzoxy-L-glutamyl-L-tyrosyl-glycinamide, it was 

 found that pepsin split the amino linkage of tyrosine, and chymotrypsin 

 split the carboxyl linkage of tyrosine. In the following formulation the 

 site of cleavage is indicated by dotted lines: 



CeHi-CH^-O-CO-NH-CH-CO- 



Carbobenzoxy - 



■NH-CHCO- 



CH2 

 I 



I 

 COOH 



t Pepsin 



Glutamyl- 



-NH'CH2-CO-NH2 

 Glycinamide 



CH2 



C6H4OH 



t 



Chymotrypsin 



Tyrosyl- 



From experiments with other synthetic peptides, it was found that 

 phenylalanine can be substituted for tyrosine. From studies on native 

 proteins, e.g., insulin, it appears that pepsin splits linkages other than 

 those involving the amino group of tyrosine and phenylalanine. For 

 example, the Leu. Val. bond is readily hydrolyzed, and the Ala. Leu. 

 bond to a considerable degree. Pepsin appears to have a much wider 

 range of specificity than chymotrypsin or trypsin. 



Trypsin splits peptides at the carboxyl linkage of either lysine or 

 arginine. Thus 



