Biochemistry Tutorial 7: Lipids Digestion, Absorption & Transport PDF

Biochemistry – Winter 2024/1 Tutorial 7: Lipids Digestion, Absorption & Transport Nourhan Ihab Elfar, PhD Nihal Moustafa Mansour, PhD Assistant Professor of Biochemistry Assistant Professor of Biochemistry [email protected]...

Biochemistry – Winter 2024/1 Tutorial 7: Lipids Digestion, Absorption & Transport Nourhan Ihab Elfar, PhD Nihal Moustafa Mansour, PhD Assistant Professor of Biochemistry Assistant Professor of Biochemistry [email protected] [email protected] Mohamed Mohamed Naguib, PhD Assistant Professor of Biochemistry [email protected] Tutorial 7 November 6, 2024  Tutorial 7 Contents I. Dietary Sources of Lipids : ✓ Animal & Vegetable Sources. II. Types of Dietary Lipids Undergoing Digestion. III. Digestion of Lipids (3 Stages) : ✓ Mouth, Stomach & Small Intestine (Pancreatic Juice). IV. Absorption of Lipids : ✓ Assembly of Chylomicrons. V. Clinical Significance : Defect in Digestion & Absorption of Lipids ✓ Lipid Malabsorption: Steatorrhea. VI. Transportation of Lipids : Lipoproteins ✓ Definition, Importance & Structure of Lipoproteins. ✓ Exogenous & Endogenous Pathways: Classes of Lipoproteins & Their Fates. 2  I. Dietary Sources of Lipids * Animal sources (mainly as fats): ✓ e.g. Dairy products like milk, butter, ghee, meat, fish, etc. * Vegetable sources (mainly as oils): ✓ e.g. Various cooking oils from various seeds namely sunflower oil, groundnut oil, cottonseed oil, mustard oil, etc. 3 II. Types of Dietary Lipids Undergoing Digestion ❑ Different forms of Dietary Lipids are driven to digestion & then absorption processes including 1.Neutral fats or Oils (Triacylglycerol; TAGs). 2. Phospholipids (PLs). 3. Cholesterol Esters (CEs; fatty acid + cholesterol). 4. Free Fatty acids (F.F.As) and Glycerol. **Important Note: ✓ Free Fatty acids and glycerol do not require any digestion because they are absorbed as such. 4  III. Digestion of Lipids Journey From Mouth to Intestinal juice… * Stage I: Digestion of Lipids in Mouth * Lingual lipase ✓ Secreted by dorsal surface of Tongue. ✓ But, this enzyme is Not significant in Adult humans. ✓ Little or No digestion of lipids occur in mouth. ✓ Action of Lingual lipase, if present (in Infants), starts in stomach as it is an acid-stable enzyme. 5  III. Digestion of Lipids * Stage II: Digestion of Lipids in Stomach * Gastric lipase ✓ Secreted by Stomach. ✓ Lipids requires Slightly Acidic pH (pH 6) for digestion. ✓ This lipase hydrolyzes TAG molecules, particularly those are found in Milk fat. Consequently, it particularly plays an important role in lipid digestion in Infants for whom Milk fat is the Primary source of Calories. ** Note: Stomach pH in Adults is (1.5-2.5), While, it is (4-6) in Infants. 6  III. Digestion of Lipids ** Stage III: In the Small Intestine Acidic Chyme containing dietary fat passes from stomach to duodenum, that inturn stimulate the secretion of pancreatic juice. Generally, Digestion of lipids in Upper Small Intestine (duodenum) proceeds by 2 main steps I. Emulsification II. Hydrolysis Bile salts acts as an Pancreatic juice contains a effective emulsifying agent number of lipolytic enzymes (lipases) of different lipids. 7 for fats.  III. Digestion of Lipids Follow… ** Stage III: In the Small Intestine I. Emulsification Bile Salt Role of Bile Salts in Lipid Digestion Bile salts are required for proper functioning of pancreatic lipase enzyme 1. Bile salts help in combination of lipase with co–lipase (small protein) in the intestinal lumen. 2. Bile salts help in the emulsification of fats as follows. As water–soluble lipases can act only on the surface of water–insoluble fat droplets so, bile salts help in lipase activity by emulsifying fats into fine droplets by lowering the surface tension and thus increasing the surface area 8 of these droplets for pancreatic lipase activity.  III. Digestion of Lipids Follow… ** Stage III: In the Small Intestine II. Hydrolysis ✓ Action of Pancreatic enzymes (Pancreatic juice) on Dietary lipids in Small intestine Pancreatic lipase TAGs 2-Monoacylglycerol (MAG) + 2 F.F.As. Phospholipase A2 Phospholipids Lysophospholipids Cholesterolesterase Cholesterol Ester Cholesterol + F.F.A 9  III. Digestion of Lipids (Turning from Digestion to Absoption) 1. Primary Link.. Products of Fat Digestion 2. Bile Salts/Acids Aid in Micelle Formation 3.Absorption of Micelles 10  IV. Absorption of Lipids (Formation of Chylomicrons) “Mechanism of Lipid Absorption” (3 Stages) 1. Passive Diffusion of Micelles * Micelles reaches bruch border of intestinal mucosal cells. * Lipid compounds from micelles pass passively through mucosal cells and are absorbed. 2. Re-Estrification & Assembly of Chylomicron 3. Exocytosis to Lymph then to Blood 11  IV. Absorption of Lipids Main Stage: Re-estrification & Assembly of Chylomicrons ✓ Resynthesis of triglycerides now occurs, by the utilization of partial glycerides and the liberated free acids. ✓ The resynthesized triglycerides then pass into the lymphatics of the abdominal cavity and through the thoracic duct the fats appear in blood as Lipoprotein particles called Chylomicrons. ✓ Structurally the Chylomicrons consist mainly of: A Triglyceride Core covered with Phospholipids and Protein Outer Layer. 12  Schematic Representation: Summery of Digestion & Absorption of Lipids BIOC 302 – Vitamins and Inorganic Metabolism Lecture 13 1: Homeostasis  V.Clinical Significance: Defect in Digestion & Absorption of Lipids “Steatorrhea” (Lipid Malabsorption) ✓ A clinical condition manifested by increased fat content of faeces (daily excretion of fat in faeces exceeds 6 gm/day). ✓ Causes Defective digestion & absorption of fat. 14  VI. Lipids Transport : Lipoproteins Definition, Importance & Structure of Lipoproteins ✓ Definition: Lipoproteins are large water soluble spherical complexes. ✓ Importance: Lipoproteins transport lipids from various tissues for storage & utilization. ✓ Lipoproteins Structure: - hydrophobic Core- TAGs & Cholesterol Esters. - hydrophilic Surface- Phospholipids, Unesterified cholesterol & Apoproteins. 15  VI. Lipids Transport : Lipoproteins Exogenous & Endogenous Pathways of Lipid Transport ✓ All lipids in plasma are transported in HDL the form of lipoproteins. Transport process is generally divided into 2 stages (pathways) A. Exogenous B. Endogenous Pathway Pathway Transport Transport lipids dietary lipids from Liver to from intestine to peripheral tissues. Liver. VLDL (Very Low Density Chylomicrons 16 Lipoproteins)  VI. Lipids Transport : Lipoproteins Transport Function of Lipoproteins & Their Fates Lipoprotein Source Transported lipids Fate Chylomicrons (CM) Small Transport Dietary Chylomicron remnants containing Intestine Exogenous TAG from the cholesterol and remaining fat are intestine to adipose tissue. removed from the blood by the liver. Very Low Density Liver Transport Endogenously Lipoprotein (VLDL) synthesized TAG in liver to Circulate deposited fats in all peripheral tissues e.g. tissues and become LDL. adipose tissue for storage. Low Density Derived from Transport of Cholesterol Eventually removed from the blood Lipoprotein (LDL: VLDL in synthesized in the liver to by the liver and other tissues in a Bad Cholesterol) circulation peripheral tissues. lesser degree. High Density Liver Transport of Excess tissue Eventually removed from the blood Lipoprotein (HDL: Cholesterol to liver for primarily by the liver. Good Cholesterol) disposal in feces. 17 VI. Lipids Transport : Lipoproteins Clinical Significance Elevated levels of cholesterol in the body can be associated with some pathogenic conditions as; a) Heart Diseases. b) Atherosclerosis. c) Gallstones. 18  VII.Lipid profile ✓ Lipid profile is a blood test looks at the levels of cholesterol and other fats as TAG in blood. ✓ You might need this test if your doctor wants to assess your risk of heart disease and stroke. ✓ You will usually need to fast for 8 to 12 hours before having this test. A lipid profile measures the following fats: Total cholesterol, LDL ("bad") cholesterol, and HDL ("good") cholesterol. TAG. The atherogenic index of plasma (AIP) that may be an important predictor of atherosclerosis and cardiovascular disease; if its value exceeds 1. Atherogenic index can be calculated from the following formulas: 1. Atherogenic index = LDL-cholesterol / HDL-cholesterol 2. Atherogenic index = TAG concentration / HDL-cholesterol 19 THE END