Lipid digestion in nonruminants Flashcards
Digestion steps
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
Emulsification
-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
Phases of lipid digestion in small intestine
-emulsification
-hydrolysis
-absorption
-re-esterification
-package into chylomicrons
Gastric lipase
-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)
Gastric lipase role in stomach
-break down of TAG to DAG and finally MAG
-promotes some emulsification and contributes to it through grinding and mixing
What stimulates gastric lipase release?
-gastric motility, cholinergic stimuli, gastrin
Gastric lipase release in humans vs. dogs vs. cats
-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
Gastric lipase bond hydrolization
-breaks down (stereospecific) sn-3
-not sn-2
Pancreatic lipase
-can hydrolyze ester bonds at sn-1 and sn-3
-not sn-2
Fat digestion in small intestine
-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)
Bile acid secretion in small intestine
-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
Bile production
-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
Bile secretion
-conjugated with either glycine or taurine before secretion
>Pigs: glycine or taurine
>poultry, cats, dogs: taurine
-facilitated emulsification
Bile reabsorption
-95% reabsorbed through the ileum by using a Na co-transport system
>enterohepatic circulation
Absorption of lipid digestion products by enterocytes
-most efficient in proximal duodenum
-unstirred water layer presents a barrier
Bile acid micelles role in overcoming the unstirred water layer
-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
Lipid digestion in the lumen of small intestine
-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
Exocrine pancreas secretions
-Secretes pancreatic lipase, procolipase, pancreatic lipase related polypeptide 2 (PLRP2) carboxy ester lipase, prophospholipase A2
Pancreatic lipase
-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
Colipase
-secreted by pancreas as procolipase
-activated by trypsin
-needed for the interaction between pancreatic lipase and bile acid emulsified droplets
Phospholipase A2
-catalyzes the hydrolysis of the sn-2 FA to form lysophospholipids and FAs
Mechanisms of absorption
-passive diffusion
-carrier facilitated
Passive diffusion
> driven by concentration gradient and maintained by rapid intracellular reesterification
products dissolve in lipids of the brush border membrane
Carrier facilitated
> carrier proteins have been identified for fatty acid digestion products (except monoacylglycerol)
Lipid metabolism within the enterocyte
-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
TAG and cholesterol ester formation
-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
Absorption of different size FA chains
-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
Chylomicron release
-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
Lipid density
-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)
FA delivery from chylomicrons
-delivered primarily to adipose tissue, heart, and muscle (80%) while about 20% goes to liver
TAGs hydrolyzation from chylomicrons
-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
CM remnants
-CM remnants after TAG hydrolyzation is removed by recycling back to the liver through receptor mediated endocytosis
Problems with determining digestibility
-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)
Steatorrhea
-abnormal amounts of lipids in the feces due to poor malabsorption
>many different reasons for malabsorption. Ex. due to low pancreatic enzymes
