All have significant role in digestion of fat, except
Correct Answer: Lingual lipase
Description: A lingual lipase is secreted by Ebner's glands on the dorsal surface of the tongue in some species, and the stomach also secretes a lipase. They are of little quantitative significance for lipid digestion other than in the setting of pancreatic insufficiency, however. Most fat digestion, therefore, begins in the duodenum, pancreatic lipase being one of the most impoant enzymes involved. This enzyme hydrolyzes the 1- and 3-bonds of the triglycerides(triacylglycerols) with relative ease but acts on the 2- bonds at a very low rate, so the principal products of its action are free fatty acids and 2-monoglycerides (2-monoacylglycerols). It acts on fats that have been emulsified. Its activity is facilitated when an amphipathic helix that covers the active site like a lid is bent back. Colipase, a protein with a molecular weight of about 11,000, is also secreted in the pancreatic juice, and when this molecule binds to the -COOH terminaldomain of the pancreatic lipase, opening of the lid is facilitated. Colipase is secreted in an inactive proform and is activated in the intestinal lumen by trypsin. Another pancreatic lipase that is activated by bile salts has been characterized. This 100,000kDa cholesterol esterase represents about 4% of the total protein in pancreatic juice In adults, pancreatic lipase is 10-60 times more active, but unlike pancreatic lipase, this bile salt-activated lipase catalyzes the hydrolysis of cholesterol esters, esters of fat-soluble vitamins, and phospholipids, as well as triglycerides. A very similar enzyme is found in human milk. Fats are relatively insoluble, which limits their ability to cross the unstirred layer and reach the surface of the mucosal cells. However, they are finely emulsified in the small intestine by the detergent action of bile salts, lecithin, and monoglycerides.When the concentration of bile salts in the intestine is high, as it is after contraction of the gallbladder, lipids and bile salts interact spontaneously to form micelles. Micellar formation fuher solubilizes the lipids and provides a mechanism for their transpo to the enterocytes. Thus, the micelles move down their concentration gradient through the unstirred layer to the brush border of the mucosal cells. The lipids diffuse out of the micelles, and a saturated aqueous solution of the lipids is maintained in contact with the brush border of the mucosal cellsREF: GANONG&;S REVIEW OF MEDICAL PHYSIOLOGY, KIM BARRETT, HEDDWEN BROOKS, SCOTT BOITANO, SUSAN BARMANTWENTY THIRD EDITIONPAGE NO:457
Category:
Physiology
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