DIGESTION OF LIPIDS

DIGESTION OF LIPIDS

Lipolytic enzymes present in pancreatic juice are pancreatic lipase, cholesterol ester hydrolase, phospholipase

A, phospholipase B, colipase and bilesaltactivated lipase.

1. Pancreatic lipase

Pancreatic lipase is a powerful lipolytic enzyme. It digests triglycerides into monoglycerides and fatt acids. Activity of pancreatic lipase is accelerated in the presence of bile. Optimum pH required for activity of this

enzyme is 7 to 9. Digestion of fat by pancreatic lipase requires two more factors:

i. Bile salts, which are responsible for the emulsification of fat, prior to their digestion

ii. Colipase, which is a coenzyme necessary for the pancreatic lipase to digest the dietary lipids.

About 80% of the fat is digested by pancreatic lipase. Deficiency or absence of this enzyme leads to excretion of undigested fat in feces (steatorrhea).

2. Cholesterol ester hydrolase

Cholesterol ester hydrolase or cholesterol esterase converts cholesterol ester into free cholesterol and fatty acid by hydrolysis.

3. Phospholipase A

Phospholipase A is activated by trypsin. Phospholipase A digests phospholipids, namely lecithin and cephalin and converts them into lysophospholipids. It converts lecithin into lysolecithin and cephalin into lysocephalin.

4. Phospholipase B

Phospholipase B is also activated by trypsin. It converts lysophospholipids (lysolecithin and lysocephalin) to phosphoryl choline and free fatty acids.

5. Colipase

Colipase is a small coenzyme, secreted as inactive

procoli pase. Procolipase is activated into colipase by

trypsin. Colipase facilitates digestive action of pancreatic

lipase on fats.

6. Bile-salt-activated lipase

Bilesaltactivated lipase is the lipolytic enzyme activated by bile salt. It is also called carboxyl ester lipase or cholesterol esterase. This enzyme has a weak lipolytic action than pancreatic lipase. But it hydrolyses a variety of lipids such as phospholipids, cholesterol esters and triglycerides. Human milk contains an enzyme similar to bile-salt-activated lipase.

DIGESTION OF CARBOHYDRATES

Pancreatic amylase is the amylolytic enzyme present in pancreatic juice. Like salivary amylase, the pancreatic amylase also converts starch into dextrin and maltose.

NEUTRALIZING ACTION OF PANCREATIC JUICE

When acid chyme enters intestine from stomach, pancreatic juice with large quantity of bicarbonate is

released into intestine. Presence of large quantity of bicarbonate ions makes the pancreatic juice highly

alkaline. This alkaline pancreatic juice neutralizes acidity of chyme in the intestine. Neutralizing action is an important function of pancreatic juice because it protects the intestine from the destructive action of acid in the chyme.

MECHANISM OF PANCREATIC SECRETION

SECRETION OF PANCREATIC ENZYMES

Pancreatic enzymes are synthesized in ribosomes, which are attached to the endoplasmic reticulum of

acinar cells in pancreas. The raw materials for the synthesis of pancreatic enzymes are the amino acids,

which are derived from the blood. After synthesis, the enzymes are packed into different zymogen granules by Golgi apparatus and stored in cytoplasm. When stimulated, the acinar cells release zymogen granules into the pancreatic duct. From the granules, the enzymes are liberated into intestine.

SECRETION OF BICARBONATE IONS

Bicarbonate ions of pancreatic juice are secreted from the cells of pancreatic ductules and released into the pancreatic duct.

Mechanism of bicarbonate secretion

1. Carbon dioxide derived from blood or metabolic process combines with water inside the cell to

form carbonic acid in the presence of carbonic anhydrase

2. Carbonic acid dissociates into hydrogen and bicarbonate ions

3. Bicarbonate ions are actively transported out of the cell into the lumen

4. Hydrogen ion is actively transported into blood in exchange for sodium ion

5. Sodium ion from the cell is transported into the lumen, where it combines with bicarbonate to form

sodium bicarbonate

6. Because of the loss of sodium and bicarbonate ions from the blood, there is some disturbance in

the osmotic equilibrium of the blood. To maintain the osmotic equilibrium, water leaves the blood and

enters the lumen of pancreatic duct by osmosis

7. In the lumen, bicarbonate combines with water forming the solution of bicarbonate.

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