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.
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