NUTR 470 1st Edition Lecture 13Nutrient Metabolism:Triglycerides and Lipoproteins• Source of Triglycerides• Dietary source— chylomicrons• Endogenous source— VLDL• Significance of Triglycerides• Triglycerides play an important role inmetabolism as fuel sources andtransporters of dietary fat.• Triglycerides contain more than twiceas much energy (9 kcal/g) ascarbohydrates andproteins.• Enzymes for Hydrolysis ofTriglycerides• Lipoprotein lipase• Triglycerides of CM or VLDL are hydrolyzedby lipoprotein lipase, which produces freefatty acids and glycerol.• Free Fatty acids are converted back totriglycerides and stored in fat tissue. • Hormone-sensitive lipase• HSL, found in adipocytes, hydrolyzestriglycerides, which in turn provides freefatty acids for the heart and skeletal muscleas fuel sources.• Glucagon increases the activity of HSL.• Enzymes for Hydrolysis ofTriglycerides• Lipid digestion – intestine lumen• Pancreatic lipase hydrolyzes triglycerides inthe form of micelles. Free fatty acids enterinto the enterocytes by simple diffusion.• Lipid transport – circulation • Lipoprotein lipase hydrolyzes triglyceridesin forms of CM and VLDL.• Lipolysis – fat cells• Lipoproteins• Chylomicrons• CMs are formed in enterocytes.• VLDLs are formed in the liver cells.• Very low density lipoproteins• IDLs are formed as triglycerides areremoved from VLDLs.• Intermediate density lipoproteins• LDLs are formed as by removing moretriglycerides from IDLs.• High density lipoproteins• HDLs are synthesized de novo in the liverand small intestine.• Very Low Density Lipoproteins• Composition • Fatty acids: de novo lipogenesisdietary source (CM)• Glycerol: glycolysis, dietary source (CM)• Cholesterol: dietary source (CM)• Apoprotein: apoB-100, apoE, and others• Very Low Density Lipoproteins• Function • Endogenous source of triglycerides• Providing free fatty acids for energystorage in adipose tissue orutilization by muscle • Source of LDL • VLDLs are Good and also Bad lipoproteins• intermediate DensityLipoproteins• Composition• IDLs are Good and also Bad lipoproteins• Similar to VLDLs, but less triglycerides• Function—Providing free fatty acids and glycerol• Low Density Lipoproteins• Composition• Cholesterol: dietary source (CM)• Triglycerides• Apoprotein: apoB-100• Diets and hormones can modify LDLcomposition.• Function —The predominant form for transportof cholesterol, the cell membranecomponent• LDLs are Bad lipoproteins.• High Density Lipoproteins• Composition• Cholesterol: nearly no cholesteroland cholesteryl esters in newly formed HDL• Apoprotein: apoA-I, apoC-I, apoC-II and apoEHigh Density Lipoproteins• Function• HDLs acquire cholesterol by extractingit from cell surface membranes –lowering the level of intracellularcholesterol.• HDLs act as circulating stores of apoC-I,apoC-II and apoE.• Surface• phospholipids• apo-B proteins• cholesterols• Core• cholesterols arebound into longesteris by fattyacid chains• Composition of LDLs• Component• Diets and hormones modify the compositionof LDLs.• Particle size• Consumption of dietary trans FAs isassociated with a deleterious increase insmall, dense LDLs.• Small Particle LDLs• Small particle LDLs are particularly proneto oxidation.• Small particle LDLs• Smaller, denser LDL particles may bemore atherogenic.• A possible role in infertility?• LDL Receptor• The LDL Receptor is acell-surface receptor.• The LDL Receptorrecognizes the apoB100which is embedded inthe phospholipid outerlayer of LDL particles.• Regulation of Lipoprotein• Dietary effects• Excessive glucose is converted into fattyacids, which are then used for theformation of VLDLs.• Metabolism• High levels of insulin and low levels ofglucagon promote the synthesis oftriglycerides and the secretion of VLDLs.• Summary• Source of triglycerides• Low density lipoprotein• Lipoproteins• LDL and