Lipid Digestion, Absorption & Transport

Lipid Digestion, Absorption & Transport

Overview

• Typical intake is 120-150 g each day.
• Generally, 95-100% of lipids in each meal are absorbed.
• Lipids include:
  • Triglycerides
  • Phospholipids
  • Cholesterol

Lipid Digestion

Mouth & Stomach

• Minimal digestion of triglycerides (TG) occurs here.
• Lingual lipase is released in the mouth, most active in infants, and its activity decreases with age. It removes the sn-3 fatty acid on TGs containing short or medium-chain fatty acids, producing diacylglycerol (DG) and fatty acids (FA).
• Gastric lipase is released in the stomach; together, lingual and gastric lipase digest ~20% of TG. It also removes the sn-3 FA on TG containing short or medium chain FA producing DG & FA.
• Lipids need to be emulsified in the stomach to allow digestion.
• Shear of stomach churning helps mix.
• Polysaccharides, phospholipids, and digested proteins in chyme act as emulsifiers.

Small Intestine (SI)

• The pancreas is essential for the digestion of long-chain FA from TG.
• Bile, released from the gallbladder, acts as an emulsifier.
• Phospholipids and cholesterol are digested here.
• Chyme enters the SI, releasing:
  • Cholecystokinin (CCK): Contracts the pancreas and gallbladder.
  • Secretin: Contracts the pancreas.
• Bile acids are made in the liver from cholesterol and stored in the gallbladder.
• The surface area in the SI is greatly increased due to emulsification using bile and peristalsis.

Pancreatic Lipases

• Pancreatic zymogens:
  • Procolipase is activated by trypsin to colipase.
  • Prophospholipase A2 is activated by trypsin to phospholipase A2.
• Pancreatic lipases digest ~80% of TG.
• Active pancreatic lipase, with the help of colipase and Ca^{2+}, removes sn-1 & sn-3 FA on TG & DG, producing MG & 2 FA.
• Phospholipase A2 removes the sn-2 FA on phospholipids (mostly lecithin), producing FA & lysophospholipid.
• Esterase (also known as cholesterol esterase, bile salt-dependent lipase, or carboxyl ester lipase) removes FA on cholesterol, Vitamins A & E, and can digest any position on TG, producing FA & free compounds (cholesterol, Vit. A, Vit. E). It requires bile.

Important points

• There are NO brush-border lipases!!
• Lipids are digested to free fatty acids, sn-2 monoglycerides, cholesterol & lysophospholipids.
• These coalesce in micelles with bile acids.
• Micelles also contain fat-soluble vitamins (ADEK).

Review of Lipid Digestion

Large Intestine (LI)

• No digestion or fermentation of lipids occurs here.

Lipid Absorption

• Required for lipid absorption to occur.
• Globular lipids formed by small lipids (digestive products) in polar solutions.
• Micelles contain: fatty acids, monoglycerides, fat-soluble vitamins, cholesterol & bile salts.
• Micelles are absorbed by passive diffusion in the duodenum & jejunum (SI).
• Reesterified into TAGs, phospholipids & cholesterol esters in the smooth endoplasmic reticulum of enterocyte.
• Packaged into chylomicron in the Golgi complex.
• Short & medium-chain FA pass directly into portal blood & taken to the liver.
• Bile not part of micelles is absorbed by facilitated diffusion in the ileum
• Bile is released into the enterocyte and taken back to the liver via the portal vein.

Lipid Malabsorption

• Causes: Impaired digestion or absorption, such as cystic fibrosis, bile acid insufficiency, celiac disease, Crohn’s, etc.
• Signs: Steatorrhea (fatty stools).
• Diagnosis: Fecal fat content, Fecal elastase.
• Treatment: Limit fat intake, Supplement with pancreatic lipase and/or bile salts.
• Orlistat (Xenical/Alli) acts as a pancreatic lipase inhibitor.

Lipid Transport

• Lipids do not like water, so they must have a special carrier called lipoproteins.

Lipoproteins

• Phospholipid exterior (makes it water-soluble).
• Lipid core (TGs, cholesterol, fat-soluble vitamins).
• Apoproteins (protein portion – binds receptors).
• Examples: chylomicrons, very low-density lipoprotein (VLDL), low-density lipoprotein (LDL) & high-density lipoprotein (HDL).

Lipoprotein Structure

• Single layer of phospholipids.
• Integral and Peripheral protein.
• Hydrophobic core of TG, cholesterol ester & vit ADEK

Apoproteins and Their Functions

Lipoprotein Composition

Lipoprotein Transport

1. Chylomicrons are synthesized in enterocytes from dietary fat, contain several apoproteins (including ApoC-II), and travel through the lymphatic system.

2. ApoC-II activates lipoprotein lipase (LPL) in adipose, heart, & skeletal muscle to remove free fatty acids (FFA) from the glycerol backbone.

3. Chylomicron remnants are taken to the liver and are degraded in the lysosome.

4. Dietary lipids (exogenous) + lipids made in the liver (endogenous) are packaged into very low-density lipoprotein (VLDL).

5. ApoC-II on VLDL activates LPL in muscle and adipose, facilitating TG storage.

6. VLDL give up TG to become a TG-depleted and cholesterol-rich LDL (ApoB-100 is major apoprotein).

7. LDL carries cholesterol to macrophages & extrahepatic cells (muscle, adipose, adrenals) by binding the LDL receptor.

   • LDL enters the cell by apoB-100 binding onto the LDL receptor & is internalized by receptor-mediated endocytosis.
   • The LDL receptor buds off & fuses with the membrane again.
   • The endosome fuses with the lysosome.
   • The lysosome digests LDL into cholesterol, phospholipids, amino acids & FA.

8. Nascent HDL is made in the liver & SI & contains lecithin-cholesterol acyltransferase (LCAT).

9. When HDL encounters chylomicrons, VLDL, & macrophages in the blood, LCAT converts cholesterol & phosphatidylcholine from them into cholesterol esters.
   HO - CH2 – O – C – R1 CH – O – C – R2 + O = C = O -> CH2 – O – P – O – CH2CH2N(CH3)3
   O O^- R2

   => CH2 – O – C – R1 CH – OH + CH2 – O – P – O – CH2CH2N(CH3)3
   O O^-

10. Mature HDL carries cholesterol esters (CE) back to the liver.

Lipid Movement & Enzymes Involved

O = C = O

R2

• CETP (cholesterol ester transport protein) is involved in the exchange of lipids between lipoproteins.

Major Lipid Sources and Routes in/out of the Liver

• Major sources of lipids in the liver:
  • Chylomicron remnants
  • Cholesterol from extrahepatic tissues
  • De novo synthesis in the liver from carbs, lipids & proteins
• Major routes by which lipids leave the liver:
  • Secretion of VLDL
  • Bile

Lipoprotein Functions

Reactive Oxygen Species (ROS)

Biological Implications of Oxidation

• Beneficial:
  • Metabolism
  • Cell signaling
  • Kill bacteria
• Harmful:
  • Inactivate hormones
  • Cell death
  • Heart disease
  • Cancer
  • Neurodegeneration
  • Alzheimer’s disease

Formation of ROS

• Exogenous (outside):
  • Cigarettes
  • Ionizing radiation
  • UV light
  • Heavy metals
  • Ozone
  • Pollution
  • Viruses
• Endogenous (inside):
  • Electron transport chain in mitochondria (metabolism)
  • Respiratory burst of white blood cells (immune system)
  • Endoplasmic reticulum
  • Peroxisomes
  • Oxidases (COX, LOX, NADPH oxidase)
  • Cytochrome P450 enzymes

Reactive Species

• Reactive oxygen species (ROS):
  • Radical: Superoxide (O_2 oldsymb{\bullet}), Hydroxyl (\boldsymb{\bullet}OH)
  • Nonradical: Singlet oxygen (^1O2), Hydrogen peroxide (H2O_2), Peroxynitrite (ONOO^-
• Reactive nitrogen species (RNS):
  • Radical: Nitric oxide (\boldsymb{\bullet}NO)
  • Nonradical:

Free Radical Chemistry

• Atom or molecule that has 1 or more unpaired electrons
• Denoted with •
• L• = lipid radical
• R• = non-lipid radical
• Oxygen (O) has 6 valence e- , 2 pairs & 2 unpaired - making it very reactive!

Electron Transport Chain

• The electron transport chain can lead to the formation of superoxide (O_2 oldsymb{\bullet}^−).

Respiratory Burst in WBC

• NADPH + O2 --> NADP + Cl^- --> H2O2 --> Fe^{2+} -> Fe^{3+} + H2O2 --> HOCl + OH\boldsym{\bullet} + ^1O2

Hydrogen Peroxide/Hydroxyl Radical

• In the presence of free iron, copper, chromium, or cobalt, a hydroxyl radical can form.

(1) Fe^{2+} + H2O2 -> Fe^{3+} + \boldsym{\bullet}OH + OH^-

• Superoxide can react with hydrogen peroxide to form the hydroxyl radical.

(2) H2O2 + O2 \boldsym{\bullet}^- -> O2 + \boldsym{\bullet}OH + OH^-

Cellular Oxidative Damage

• Inductors of ROS -> ROS -> Oxidative Stress -> Lipid peroxidation, Protein damage, DNA damage
• A free radical will steal a hydrogen (H) based on the least energy, with the following values in kcal/mol: sn-1 (75), sn-2 (103) & sn-3 (50).

Antioxidants

• Inductors of ROS -> ROS -> Oxidative Stress -> Lipid peroxidation, Protein damage, DNA damage -> Antioxidants
• AOX enzymes, fruits & veggies, Vitamins ACE

Enzymatic Antioxidants

• Superoxide dismutase (SOD): Converts superoxide to hydrogen peroxide.
• Catalase (CAT): Converts hydrogen peroxide to water.
• Glutathione peroxidase (GPx): Converts hydrogen peroxide to water, using glutathione.
• Glutathione reductase: Regenerates glutathione.

Nonenzymatic Antioxidants

• H donors:
  • Fat-soluble: Vit E, carotenoids, CoQ10
  • Water-soluble: Vit C, polyphenols, glutathione, BHA, BHT
• Metal chelators:
  • Phytate (whole grains), oxalate (spinach, legumes, nuts), EDTA
• Oxygen scavengers: Vit C
• Singlet oxygen quenchers: Carotenoids

Polyphenols

• Found in high concentrations in fruits, veggies, whole grains & legumes.
• Water-soluble.
• Include various categories and compounds like:
  • Acetophenones, Anthraquinones, Benzoquinone, Coumarins, Chromones, Lignans, Lignins
  • Phenolic acids: Benzoic acid, Caffeic acid, Cinnamic acid, Courmaric acid, Ferulic acid, Gallic acid, Genistic acid, Sinapic acid, Salicylic acid
  • Stilbenes, Flavonoids, Xanthones, Flavonoids Anthocyanidins Chalcones Flavones Flavanols Flavonols Flavanones Isoflavones Proanthocyanidin
  • Phytoestrogens Vanillic acid Lignans Stilbenes Isoflavones