Metabolism Of Dietary Lipids PDF

Metabolism of Dietary Lipids Ass. Lec. ZainabYasoob Ejam Metabolism of Dietary Lipids Lipids are a heterogeneous group of water-insoluble (hydrophobic) organic molecules that can be extracted from tissues by nonpolar solvents, because of their i...

Metabolism of Dietary Lipids Ass. Lec. ZainabYasoob Ejam Metabolism of Dietary Lipids Lipids are a heterogeneous group of water-insoluble (hydrophobic) organic molecules that can be extracted from tissues by nonpolar solvents, because of their insolubility in aqueous solutions. body lipids are generally found compartmentalized, as in the case of membrane-associated lipids or droplets of triacylglycerol in adipocytes, or transported in plasma in association with protein, as in lipoprotein particles or on albumin. Lipids are a major source of energy for the body, and they provide the hydrophobic barrier. Lipids serve additional functions in the body, for example, some fat-soluble vitamins have regulatory or coenzyme functions, and the prostaglandins and steroid hormones play major roles in the control of the body's homeostasis. Classification of lipids They may be classified based on their structure: A. Simple lipids or Homolipids/ are Simple lipids are the esters of fatty acids with various alcohols. Fats and Oils (triglycerides and triacylglycerol) - These are esters of fatty acids with a trihydroxy alcohol, glycerol. It is found as Simple Triglycerides and Mixed Triglycerides. Waxes are the esters of fatty acids with high molecular weight monohydroxy alcohols. B. Compound lipids or Heterolipids/ are esters of fatty acids with alcohol and possess additional groups also. Metabolism of Dietary Lipids Ass. Lec. ZainabYasoob Ejam Phosphoglycerides these are the major phospholipids found in membranes and contain two fatty acid molecules or esterified with glycerol. Phosphoinositides these have been found to occur in phospholipids of brain tissue and of soybeans and are of considerable importance because of their role in transport processes in cells. Sphingolipids are a class of lipids containing a backbone of sphingoid bases, a set of aliphatic amino alcohols. Glycolipids (Glycosphingosides) are the compounds of fatty acids with carbohydrates and contain nitrogen but no phosphoric acid. C. Derived lipids/ are the substances derived from simple and compound lipids by hydrolysis. These include fatty acids, alcohols, monoglycerides and diglycerides, steroids,terpenes, carotenoids. Steroids are a type of hormone which is usually recognized by their tetracyclic skeleton, consisting of three fused six-membered and one five-membered ring Cholesterol is an important lipid found in the cell membrane. It is a sterol, which means that cholesterol is a combination of steroid and alcohol. The steroid ring structure of cholesterol provides a rigid hydrophobic structure that helps boost the rigidity of the cell membrane. Without cholesterol, the cell membrane would be too fluid. Terpenes in majority are found in plants. Example: Natural rubber. Metabolism of Dietary Lipids Ass. Lec. ZainabYasoob Ejam Digestion, Absorption, Secretion, and Utilization of dietary lipids Classification of lipids A. Processing of dietary lipid in the stomach: The digestion of lipids begins in the stomach, catalyzed by an acid stable Lipase (lingual lipase) that originates from glands at the back of the tongue. TAG molecules, particularly those containing fatty acids of short- or medium-chain length (fewer than 12 carbons, such as are found in milk fat), are the primary target of this enzyme. These same TAGs are also degraded by a separate gastric lipase, secreted by the gastric mucosa. Overview of lipid digestion B. Emulsification of dietary lipid in the small intestine: The critical process of emulsification of dietary lipids occurs in the duodenum. Emulsification increases the surface area of the hydrophobic lipid droplets so that the digestive enzymes, which work at the interface of the droplet and the surrounding aqueous solution, can act effectively. Emulsification is accomplished by two complementary mechanisms, namely, use of the detergent properties of the bile salts, Metabolism of Dietary Lipids Ass. Lec. ZainabYasoob Ejam and mechanical mixing due to peristalsis. Bile salts, made in the liver and stored in the gallbladder, are derivatives of cholesterol. C. Degradation of dietary lipids by pancreatic enzymes: 1. TAG degradation: The pancreatic lipase acts at the ester bond, hydrolyzing the bond and "releasing" the fatty acid. In triglyceride form, lipids cannot be absorbed by the duodenum. Fatty acids, monoglycerides (one glycerol, one fatty acid), and some diglycerides are absorbed by the duodenum. The secretion of lipases and bile, triglycerides are split into monoacylglycerol and free fatty acids in a process called lipolysis. Micelles are Monoglycerides and fatty acids associate with bile salts and phopholipids to form micelles. Micelles are about 200 times smaller than emulsion droplets. 2. Cholesterol ester degradation: Most dietary cholesterol is present in the free (nonesterified) form, with 10–15% present in the esterified form. Cholesterol esters are hydrolyzed by pancreatic cholesterol ester hydrolase (cholesterol esterase), which produces cholesterol plus free fatty acids. Cholesterol ester hydrolase activity is greatly increased in the presence of bile salts. Use of dietary lipids by the tissues The triglycerides are rebuilt in the enterocytes from their fragments and packaged together with cholesterol and proteins to form chylomicrons. These are excreted from the cells and collected by the lymph system and transported to the large vessels before being mixed into the blood. Triacylglycerol contained in chylomicrons is broken down primarily in the capillaries of skeletal muscle and adipose tissues, but also those of the heart, lung, kidney, and liver. Triacylglycerol in chylomicrons is Metabolism of Dietary Lipids Ass. Lec. ZainabYasoob Ejam degraded to free fatty acids and glycerol by lipoprotein lipase. The free fatty acids derived from the hydrolysis of TAG be transported in the blood in association with serum albumin and they are taken up by cells where it oxidize fatty acids to produce energy. - Glycerol that is released from TAG is used almost exclusively by the liver to produce glycerol 3-phosphate, which can enter either glycolysis or gluconeogenesis by oxidation to dihydroxyacetone phosphate.