Among various enzymatic secretions, gastric juice is released in stomach whereas the bile, pancreatic juice and the intestinal juice are the secretions released into the small intestine. Pancreatic juice and bile are released through the hepato-pancreatic duct. Gastric juice contains hydrochloric acid and proenzyme - pepsinogen and prorennin, HCl maintains a strongly acidic pH which converst these proenzymes into pepsin and rennini (in infants)
respectively. These enzymes act on proteins and convert them into simpler form, peptones.
The pancreatic juice contains inactive enzymes trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases and nucleases.
Trypsinogen is activated by an enzyme, enterokinase, (secreted by the intestinal mucosa) into active trypsin, which in turn activates the other enzymes in the pancreatic juice. The bile released into the duodenum contains bile pigments (billirubin and billiverdin), bile salts, cholesterol and phospholipids but no enzymes.
Bile helps in emulsification of fats, i.e., breaking down of the fats into very small micelles. Bile also activates lipases. The secretions of the brush border cells of the mucosa along with the secretions of the goblet cells constitute the intestinal juice or succus entericus.
This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc. The mucus along with the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium (pH 7.8) for enzymatic activities. Sub-mucosal glands (Brunner's glands) also help in this process.
Various reactions involved in this process are as follows
(i) Pepsinogen Pepsin
Proteins Peptones
(ii) Peptones Dipeptides
Dipeptides Amino acids
(iii) Carbohydrates Disaccharides
Maltose Glucose + Glucose
Lactose Glucose + Galactose
Sucrose Glucose + Fructose
(iv) Fats Diglycerides Monoglycerides
Di and Monoglycerides Fatty acids + Glycerol
(v) Nucleic acids Nucleotides
Nucleosides Sugars + Bases
