Digestion of Nucleic acids
There are no enzymes to digest nucleic acids in the mouth.
The highly acidic medium in the stomach destabilizes the nucleoprotein structure and the proteolytic enzymes split them to nucleic acids and proteins.
Pancreatic juice contains two enzymes ribonuclease and deoxylribonuclease that can hydrolyze the nucleic acids to mononucleotides.
Depending upon the site of action, nucleases canbe either endonuclease that attacks the interior linkages and exonuclease that attacks the terminal linkages.
The intestinal juice (succus entericus) contains the following two enzymes that digest nucleic acids.
· Nucleotidases that hydrolyze nucleotides to nuclosides and phosphoric acid.
· Nucleosidases that hydrolyzes the nucleosides to their respective sugars and bases.
There are three major gastro intestinal hormones secreted by the gut. They are gastrin, secretin and cholecystokinin. All of them are polypeptides synthesized by the mucosal endocrine cells of the stomach and small intestine.
Gastrin is produced by the mucosal cells of the pyloric region of the stomach and is the most effective activator of gastric acid secretion. Two gastrins, Gastrin I and Gastrin II have been identified. Gastrin I has 17 amino acids and Gastrin II has 14 amino acids. Gastrin secretion increases with age, vagal stimulation, acetyl choline and intake of foods rich in proteins and amino acids particularly glycine. The terminal four amino acids of gastrin are responsible for its hormonal action.
Secretin is a polypeptide with 27 amino acids of which 14 amino acids are identical to that of glucagon. It is formed in the duodenal mucosal cells. The secretion is stimulated by HCl and it increases the secretion of electrolytes and fluid components of pancreatic juice. It is one of the factors that increase the secretion of bile by the liver. It can act like glucagon by increasing the cardiac output and lipolysis.
Cholecystokinin and Pancreozymin are two hormones that stimulate the secretion of pancreatic juice. Pancreozymin also stimulates the pancreas to secrete insulin and glucagons. Due to this action of pancreozymin, insulin secretion is higher when glucose is given orally than intravenously. Out of the 33 amino acids present in the pancreozymin, the eight C terminal amino acids are biologically active.
Cholecystokinin causes contraction of the gall bladder and discharge the bile into the duodenum. Discharge of bile is also stimulated by secretin and bile salts.
Hepatocrinin stimulates the formation of bile, which is low in bile salts. Motilin increases gastric motility. Enterogastrone and gastric inhibitory polypeptide inhibit gastric acid secretion and gastric motility. Enterocrinin stimulates the secretion of enzymes by the intestinal mucosa. Chymodenin stimulates the secretion of chymotrypsin from pancreas.