434 ANIMAL BIOCHEMISTRY 



duodenum, other portions of the intestine being incapable of stinui- 

 hiting the pancreas. 



The duodenum supplies yet another type of activator called entero- 

 kinase, though there is now some doubt that the material is an enzyme. 

 Enterokinase is secreted into the chyme and as a result comes in 

 contact with pancreatic juice. The latter fluid contains certain enzyme 

 precursors whose conversion to active enzymes is initiated by the 

 enterokinase. For example, enterokinase changes trypsinogen to 

 trypsin, the active form. Once some trypsin appears it catalyzes its 

 own production, the phenomenon of autocatalysis. 



Pancreatic juice supplies enough sodium bicarbonate to neutralize 

 the acid of chyme and bring the intestinal contents to pH 8 to 9 for 

 the effective action of intestinal and pancreatic enzymes. Pancreatic 

 juice also contains a number of enzymes, several of them very exten- 

 sively studied, and taken together they contribute to the digestion of 

 proteins, fats, and carbohydrates. 



Trypsinogen and chymotrypsinogen are converted from the zymo- 

 gens to the corresponding active forms. It now seems that there is 

 a family of chymotrypsins, all similarly active but of slightly different 

 sizes, which result from the removal of different peptide fragments 

 during the activation steps. Both trypsin and chymotrypsin are pro- 

 teolytic enzymes and attack different kinds of peptide bonds from 

 those attacked by pepsin. Thus the proteolysis is carried farther, and 

 any proteins previously escaping gastric digestion are partly degraded 

 by trypsin and chymotrypsin. Hydrolysis of proteins to amino acids 

 is continued by carboxypeptidase, activated from procarboxypeptidase 

 by trypsin. Carboxypeptidase has little action on proteins but attacks 

 polypeptides by removing amino acids one at a time from the free 

 carboxyl end of the chain. Since some types of peptide bonds are not 

 rapidly hydrolyzed, additional enzymes are needed. These occur in 

 intestinal juice, and they are discussed in that connection on page 437. 



Pancreatic amylase (perhaps two separate enzymes) converts starch 

 and the soluble degradation products from the stomach into maltose. 

 As maltose is formed, maltase splits it into glucose. Likewise lactose, 

 so important in the diets of young animals, is hydrolyzed by pan- 

 creatic lactase. Again these enzymes are supplemented by similar 

 catalysts in intestinal juice. A lipase (steapsin) also occurs in pan- 

 creatic juice and induces hydrolysis of neutral fats. Pancreatic lipase 

 appears to be the major enzyme of fat digestion. 



Bile 



Among the many important functions of the liver, one is the forma- 

 tion of bile. Liver secretes bile continuously, although the amount 



