Metabolism of Dietary Lipids Definition: Lipids are heterogeneous gp of water-insoluble (hydrophobic) organic molecules stored as triacylglycerol in adipose tissues. Major source of energy for body

Importance Lipids are major components of cell membranes, and are responsible for most of the permeability filter functions of membranes. Membranes act as barriers to separate compartments within eukaryotic cells, and to separate all cells from their surroundings.

Importance Lipids are also used as metabolic fuels. Adipose tissue fat stores contain ~9 kcal/gram (= ~37 kJ/g) of dry weight lipid. This compares to ~4 kcal/g for protein or carbohydrate. Lipids include the vitamins A, D, E, and K, and include some non-vitamin enzyme cofactors.

Importance Lipids include a number of hormones, of which the most important are steroids and prostaglandins Lipids act as insulation, and play a role in physical appearance of animals. Finally, lipids play roles in a variety of pathological conditions, including heart disease and some genetic disorders related to lipid metabolism.

Chemistry of Lipid Lipids are a variety of organic molecules grouped together on the basis of their solubility in nonpolar solvents. Their varied biological functions include 1-energy source 2-energy storage 3-cell membrane structural components, hormones, vitamins, vitamin adsorption, protection, and insulation.

Chemistry of Lipid The four main groups of lipids include: 1. Fatty acids (saturated and unsaturated) 2. Glycerides (glycerol-containing lipids) 3. Nonglyceride lipids (sphingolipids, steroids, waxes) 4. Complex lipids (lipoproteins, glycolipids) Lipids can also be broadly subdivided as either saponifiable or nonsaponifiable.

Classes of Lipids All biological lipids are amphipathic Fatty acids Triacylglycerols Glycerophospholipids Sphingolipids Waxes Cholesterol Prostaglandins &Leukotrienes (eicosanoids) Fat soluble vitamins (A D E K)

Triacylglycerol (TAG) FA on C1 is saturated FA on C2 is unsat. FA on C3 is either Slightly soluble Stored in adipocytes

Digestion Digestion begins in the stomach Acid-stable lipase (lingual lipase) cleaves triacylglycerols (TAG) with short or medium chain FA (less than 12 C) found in milk fat Gastric lipase secreted by stomach

Digestion Emulsification of dietary lipid in small intestine (duodenum) to increase surface area of lipids by bile salts (made in liver & stored in gallbladder) Pancreatic enzymes ( lipase and colipase) which are hormonally controlled

Digestion

Hormones that regulate the utilization of nutrients Gut hormones: A- Gastrin from the gastric antrum and duodenum stimulates gastric acid and pepsin secretion. B- Cholecytokinine from duodenum and jejunum stimulates contraction of gallbladder and secretion of pancreatic enzymes C- Secretin from duodenum and jejunum stimulates of secretion of biocarbonate by the pancreas

Control of Lipid Digestion Cholecystokinin Secretin

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Absorption of Lipids by Intestinal cells Pancreatic enzymes ( lipase and colipase) digests the TAG to 2-monoacylglycerol and free fatty acids, which are packaged into micelles The micelles, which are micro droplets emulsified by bile salts, also contain other dietary lipids such as cholesterol ad fat-soluble vitamins.

Absorption of Lipids by Intestinal cells The micelles travel to the microvilli of the intestinal epithelial cells, which absorb 2- monoacylglycerol and free fatty acids and other dietary lipid The bile salts are reabsorbed, recycled by the liver and secreted into the gut during subsequent digestive cycle.

Absorption of Lipids by Intestinal cells (enterocytes) Free FA, cholesterol, 2-monoacylglycerol & bile salts form mixed Micelles to be absorbed by brusher border mem- -brane  Short & medium FA Do not require micelles & Go to liver via serum albumin

Synthesis of chylomicrons In the intestinal epithelial cells, the fatty acids from micelles are activated by fatty-acyl CoA sythetase (thiokinase) to form fatty acyl CoA Fatty acyl CoA reacts with a 2-mono-acylglycerol. Then form triacylglycerol. The triacylglycerol pass into the lymph packaged in nascent (newborn) chylomicrons, which enter the blood.

Resynthesis of TAG & cholesterol esters

Use of Dietary Lipids by Tissues TAG in chylomicrons is broken to FFA + glycerol by lipoprotein lipase in adipocytes, muscle cells & peripheral tissues. Fate of FFA  adipocytes + muscle cells associated with albumin for energy production or stored as TAG Fate of glycerol  liver  glycerol-3-P  glycolysis or gluconeogenesis Fate of chylomicron remnants  liver