Lipid digestion in nonruminants

Question 1

Digestion steps

Answer 1

1.triglycerides broken down by lingual lipase
2.triglycerides, diglycerides and fatty acids broken down by gastric lipase
3.Emulsification phase I: no lipase; bile present
4. enzymatic digestion phase II: pancreatic lipase

Question 2

Emulsification

Answer 2

-TAG hydrophobic but enzymes are hydrophilic
-combine two immiscible liquids into a single mixture. Process of creating phases in a liquid-liquid mixture that results in creation of emulsions.
»Reduces triglyceride from large fat globule to tiny droplets which increased surface area for enzyme activity

Question 3

Phases of lipid digestion in small intestine

Answer 3

-emulsification
-hydrolysis
-absorption
-re-esterification
-package into chylomicrons

Question 4

Gastric lipase

Answer 4

-acid lipase gene family (including lingual lipase)
-pH 3-6 optimal
-contribute up to 25% of dietary lipid lipolysis
-does not need cofactor for lipolysis (pancreatic and lipoprotein lipase does)

Question 5

Gastric lipase role in stomach

Answer 5

-break down of TAG to DAG and finally MAG
-promotes some emulsification and contributes to it through grinding and mixing

Question 6

What stimulates gastric lipase release?

Answer 6

-gastric motility, cholinergic stimuli, gastrin

Question 7

Gastric lipase release in humans vs. dogs vs. cats

Answer 7

-Humans: secreted with pepsinogen by chief cells in fundus
-Dogs: secreted throughout gastric mucosa with decreasing concentrations from cardia to pylorus
-Cats: secreted throughout with uniform concentrations

Question 8

Gastric lipase bond hydrolization

Answer 8

-breaks down (stereospecific) sn-3
-not sn-2

Question 9

Pancreatic lipase

Answer 9

-can hydrolyze ester bonds at sn-1 and sn-3
-not sn-2

Question 10

Fat digestion in small intestine

Answer 10

-where majority of lipid digestion occurs
-catalyzed by pancreatic enzymes (require neutral pH)
-facilitated by hepatic secretion of bile acids for emulsification and increased pH from bicarbonate secretion (Brunner’s glands)

Question 11

Bile acid secretion in small intestine

Answer 11

-acidic chyme entering the duodenum from the stomach triggers the release of CCK by endocrine cells of the small intestine
-CCK triggers bile release from gallbladder
-bile acids partition into the emulsified lipid droplets

Question 12

Bile production

Answer 12

-made in liver and stored in gallbladder
-mixture of bile salts, phospholipids and cholesterol is transported to the gallbladder where they are concentrated into bile

Question 13

Bile secretion

Answer 13

-conjugated with either glycine or taurine before secretion
>Pigs: glycine or taurine
>poultry, cats, dogs: taurine
-facilitated emulsification

Question 14

Bile reabsorption

Answer 14

-95% reabsorbed through the ileum by using a Na co-transport system
>enterohepatic circulation

Question 15

Absorption of lipid digestion products by enterocytes

Answer 15

-most efficient in proximal duodenum
-unstirred water layer presents a barrier

Question 16

Bile acid micelles role in overcoming the unstirred water layer

Answer 16

-digestion products such as MAGs, FAs, lysopholipids, cholesterol, Vit ADEK are incorporated into the micelles
-bile acid micelles deliver lipids to the brush border membrane, increasing lipid molecule absorption

Question 17

Lipid digestion in the lumen of small intestine

Answer 17

-emulsified lipid droplets formed in stomach due to the action of gastric lipase, mechanical mixing, and grinding actions enter the small intestine and stimulate the release of CCK which then stimulates the release of bile acids
-bile acids cause further emulsification and incorporating phospholipids, cholesteryl esters, DAGs into the mixed micelles

Question 18

Exocrine pancreas secretions

Answer 18

-Secretes pancreatic lipase, procolipase, pancreatic lipase related polypeptide 2 (PLRP2) carboxy ester lipase, prophospholipase A2

Question 19

Pancreatic lipase

Answer 19

-secreted in active form at optimal pH of 8
-catalyzes the hydrolysis of FAs from the sn-1 and sn-3 positions of TAGs and DAGs to form 2-monoacylglycerols and FAs
*carboxyl ester lipase and pancreatic lipase-related peptide 2 (PLRP2) also involved minorly

Question 20

Colipase

Answer 20

-secreted by pancreas as procolipase
-activated by trypsin
-needed for the interaction between pancreatic lipase and bile acid emulsified droplets

Question 21

Phospholipase A2

Answer 21

-catalyzes the hydrolysis of the sn-2 FA to form lysophospholipids and FAs

Question 22

Mechanisms of absorption

Answer 22

-passive diffusion
-carrier facilitated

Question 23

Passive diffusion

Answer 23

> driven by concentration gradient and maintained by rapid intracellular reesterification
products dissolve in lipids of the brush border membrane

Question 24

Carrier facilitated

Answer 24

> carrier proteins have been identified for fatty acid digestion products (except monoacylglycerol)

Question 25

Lipid metabolism within the enterocyte

Answer 25

-Fatty acid binding proteins (FABPs) transport FAs and MAGs to the endoplasmic reticulum for processing. Form TAG and cholesterol esters through one of two pathways

Question 26

TAG and cholesterol ester formation

Answer 26

-Two pathways:
1.Monoacylglycerol pathway (major because large supply of MAGs)
2. glycerol 3-phosphate pathway
-Once esterified, products are incorporated into apoliproprotein B48*(different than ApoB-100 in liver/VLDL) containing chylomicrons
3. apoliproproteins transported to the golgi body for final processing (attachment of ApoA-I)
4. Mature chylomicron leaves the golgi, enters the lymphatic circulation and eventually the systemic circulation

Question 27

Absorption of different size FA chains

Answer 27

-short and medium chain FAs are absorbed into blood via capillaries
-long chain FAs must be formed into triglycerides and transported in chylomicrons into lymph

Question 28

Chylomicron release

Answer 28

-released into lymphatic system via reverse pinocytosis
*too large to enter systemic system directly
-lymph runs into the thoracic duct that drains fat in the rich lymph into the bloodstream at the left subclavian valve
-quickly removed from the blood

Question 29

Lipid density

Answer 29

-CM (lowest density; transport ingested fat and fat soluble vitamins)
-VLDL (transport synthesized glycerides)
-LDL (deliver cholesterol to cells; from VLDL)
-HDL (reverse cholesterol transport; taking cholesterol from cells to liver to be destroyed)

Question 30

FA delivery from chylomicrons

Answer 30

-delivered primarily to adipose tissue, heart, and muscle (80%) while about 20% goes to liver

Question 31

TAGs hydrolyzation from chylomicrons

Answer 31

-TAGs from the chyromicrons are hydrolyzed by liproprotein lipase found on capillary walls
>TAG hydrolyzed to DAG, MAG, and then FFAs and glycerol
»most of FFAs enter the tissues and then some bind to albumin and remain in circulation

Question 32

CM remnants

Answer 32

-CM remnants after TAG hydrolyzation is removed by recycling back to the liver through receptor mediated endocytosis

Question 33

Problems with determining digestibility

Answer 33

-low inclusion levels (use graded levels and regression analysis)
-interaction with other nutrients (FAs and presence of other nutrients can reduce lipid digestibility)
-endogenous (only 48% were FAs)

Question 34

Steatorrhea

Answer 34

-abnormal amounts of lipids in the feces due to poor malabsorption
>many different reasons for malabsorption. Ex. due to low pancreatic enzymes