DIGESTION 437 



Intestinal Juice 



This fluid contains the enterokinase mentioned earlier as the 

 activator of trypsinogen. In addition several hydrolytic enzymes 

 occur in the secretion, including the material formerly called erepsin. 

 It is now known that the erepsin consists of the pancreatic enzyme 

 carboxypeptidase and the intestinal enzymes aminopeptidase, dipep- 

 tidase, aminotripeptidase, and prolidase. Of these, carboxypeptidase 

 has been discussed (page 434). Aminopeptidase likewise attacks 

 polypeptides one step at a time but begins at the end carrying the 

 free a-amino group. Aminotripeptidase has a similar activity but is 

 restricted to tripeptides. Dipeptidase hydrolyzes dipcptides, and pro- 

 lidase the dipeptides of proline, as suggested by the respective names. 

 Most of these enzymes, classed as peptidases, require divalent metals 

 for catalytic activity. Carboxypeptidase specifically depends on zinc 

 and prolidase on manganous ion. Aminopeptidase and dipeptidase 

 can function with any of a number of related ions. Taken together, 

 all these enzyme systems cooperate to hydrolyze proteins almost com- 

 pletely into free amino acids. 



Along with the peptidases, intestinal mucosa secretes carbohydrases 

 acting on the disaccharides sucrose, maltose, and lactose. These en- 

 zymes, sucrase (or invertase), maltase, and lactase, yield readily utilized 

 monosaccharides from the otherwise somewhat inert parent sugars. 



Additional digestion is produced by enzymatic cleavage of nucleic 

 acids. Nucleases catalyze the first stages to nucleotides, attacked in 

 turn by nucleotidases, which are then split by nucleosidases and phos- 

 phatases to the purine and pyrimidine bases, pentoses, and phosphate. 



Absorption 



The intestinal tract is composed of three major segments. The 

 first of these, the small intestine, is tubular, beginning at the stomach 

 and extending to the caecum. In turn the small intestine is divided 

 into three parts called the duodenum, jejunum, and ileum. The over- 

 all length of the small intestine varies with the species and the in- 

 dividual, averaging 9 times the body length in horses, 16 times in 

 sheep and cattle, and 1 1 times in swine. The ileocaecal valve sep- 

 arates the small intestine from the caecum and controls the flow of 

 materials. 



The caecum is an enlargement of the alimentary canal and serves as 

 a storage organ for further digestion and fermentation. No digestive 

 juices are secreted here, but the organ is of great importance in spite 

 of this fact. In a way it might be compared with the rumen or paunch 



