Post-Absorptive Lipid Metabolism Lipid Metabolism Terms • • • • • • • • Lipogenesis – Making of fat from dietary fat or dietary CHO Lipolysis – Breaking down of fat: GIT, capillary and adipocyte De Novo lipogenesis – Making of fat from CHO (takes place in liver and adipocyte) – Fat exported from liver as VLDL (very low density lipoprotein) Pancreatic lipase – Breaks down TG’s in GIT Lipoprotein Lipase – Breaks down TG’s from chylomicron and VLDL in the capillary -oxidation: – Breaking down of fatty acids into acetyl-CoA Hormone Sensitive Lipase – Breaks down TG’s within the adipocyte NEFA – Non-esterified fatty acids: fatty acids mobilized (exiting) the adipocyte Lipid Metabolism Lipid Absorption glucose -glycerolĞP glucose cholesterol cholesterol fatty acids MAG cholesterol ester FA-CoA TAG Chylomicrons lysolecithin Lymphatics lecithin apoproteins short chain FAs </= C10 FA + albumen Blood Plasma Lipid • Transported in two primary vesicles: – Chlyomicrons • From intestine • Packages dietary lipid – Very Low Density Lipoprotein (VLDL) • From the liver • Packages: – Fatty acids derived from excess carbohydrates – Fatty acids taken up from circulation Plasma Lipid Clearance Unlike glucose and amino acids, most lipids from a meal do not directly enter the bloodstream. Instead, they are packaged into chylomicrons and released into the lymph. The lymph dumps into the aortic arch (near the heart), where it then is transported through the bloodstream to be cleared (taken up) by: adipocytes muscle liver Clearance of lipid from circulation is mediated by adipose,muscle and liver: via the enzyme Lipoprotein Lipase (LPL) Thus, unlike carbohydrates and protein, most lipids do not use the enterohepatic circulatory system. This allows lipids to be cleared by the whole body and avoids overwhelming the liver with lipid. Regulation of Lipid metabolism • Well fed: – Insulin lipogenesis & lipolysis • Starving: – epinephrine/norepinephrine lipolysis – Insulin lipolysis • Very Low CHO, high PTN diet: – No Insulin lipogenesis – No Insulin lipolysis Lipid Synthesis (lipogenesis) Creation of fat is via two primary routes • 1) De novo fatty acid synthesis – Process by which simple non-lipid nutrients are converted to long chain fatty acids and stored as triglycerides, especially in adipose tissue – Monogastrics: glucose is the major source of carbon for fatty acid synthesis – Ruminants: acetate is the major source of carbon for fatty acid synthesis • 2) Preformed uptake: incorporation of dietary fat – Most of human adipose is derived from diet • Both are stimulated by insulin De novo fatty acid Synthesis • Two Key Enzymes: – Acetyl CoA Carboxylase (ACC) • Rate limiting enzyme – Fatty Acid Synthase (FAS) • Animals on a high fat diet experience little if any de novo fatty acid synthesis • Typical western civilization diet is high in fat – agriculture species usually fed a high CHO diet • Fetal animals have large de novo activity De novo Fatty Acid Synthesis glucose Fatty Acids NADPH pyruvate FAS Acetyl Co A ACC TCA Citrate monogastrics Acetyl Co A ruminants Acetate • Why glucose is not a C-source for fatty acid synthesis in ruminants – Limiting enzymes Citrate lyase Malate dehydrogenase – Use of glucose for fat synthesis • Supply NADPH • Synthesis of glycerol Acetyl CoA Carboxylase (ACC) Allosteric modification Activated by: Citrate Inhibited by: LCFA Covalent Modification Activated by: Dephosphorylation Inhibited by: Phosphorylation ACC FAS Fatty Acid Synthase (FAS) • • • • 2nd and final step Multifunctional polypeptide High in the well-fed state Not regulated by either allosteric or covalent modification • Regulated by the amount of [PTN] – High in fed-state – Low in fasting-state • Palmitate is usually the end product PPP NADPH FAS ATP Citrate Lyase Species comparison of fatty acid synthesis Species Poultry Human Pig Mouse Sheep Cattle Principal Tissue Site Liver Liver Adipose Adipose Adipose Adipose Carbon Source Glucose Glucose Glucose Glucose Acetate Acetate Preformed Fatty Acid Uptake • Dietary derived – Dietary TG packaged in chylomicrons • Liver derived – Either repackaged TG from chylomicron remnants or TG synthesized de novo and secreted as VLDL • TG in both are hydrolyzed by lipoprotein lipase (LPL) in capillary bed LPL action on TG rich lipoproteins Glycerol + 3 Fatty acids TG capillary Lipoprotein lipase Cell Chylomicrons VLDL I.e. adipocyte muscle mammary Fatty acids Triglycerides LPL Mediated Fatty Acid Uptake Lipid breakdown (lipolysis) • The breaking down (hydrolysis) of intracellular triglycerides – Can be reesterified or mobilized • Mobilization – Net release of fatty acids from adipocytes • NEFAs are transported in blood bound to albumin • Undergo -oxidation to produce acetyl CoA’s • Oxidized by energy needing cells • Stimulated by epinephrine AND the lack of insulin Triglyceride breakdown • Lipoprotein Lipase: found on endothelial (vessel) walls lining tissues such as adipose and muscle. Releases FFA from TAGs in CM/VLDL for cellular uptake and usage as either energy (muscle) or storage (adipocyte). Thus insulin & glucagon differentially regulate this enzyme on muscle vs. adipose cells. TAG 2-MAG + FFA cell • Hormone-sensitive lipase: Only found INSIDE adipocyte. Releases FFA from adipocyte TAG stores, sends to serum. Incr by glucagon, epinephrine. TAG 2-MAG + FFA serum • Regulation of LPL Activity: factor starvation Well Fed insulin adipose down up up muscle up down down Energy Content of Human Carcass -Complete oxidation of fatty acids yields ~9 kCal/g, where as, proteins and carbohydrates yield ~4 kCal/g. An average 70 kg man: 100,000 kCal in triacylglycerols 25,000 kCal in proteins (muscles) 600 kCal in glycogen 400 kCal in glucose -Triacylglycerols constitute about 11 kg of his total body weight. If this amount were stored in glycogen, his total body weight would be 55 kg greater. -In mammals, the major site of accummulation of triacylglycerols is the cytoplasm of adipose cells (fat cells). Droplets of triacylglycerol coalesce to form a large globule, which may occupy most of the cell volume. - Adipose cells are specialized for the synthesis and storage of triacylglycerols and for their mobilization into fuel molecules that are transported to other tissues by the blood. Lipolysis Overview Epinephrine Fatty Acids Adenylate Cyclase cAMP Hormone Sensitive Lipase Triglyceride Fatty acids + glycerol Glycerol Schematic representation of the activation of lipolysis by lipolytic hormones Under basal conditions, perilipin (Per) is located on the surface of the single triacylglycerol droplet, with HSL in the cytoplasm. Upon lipolytic stimulation, both perilipin and HSL become multiphosphorylated, with perilipin being displaced from the droplet, allowing access for HSL. There is also evidence that fatty acids (FA) are removed from HSL by FABPs, preventing accumulation and resultant product inhibition. Biochemical Journal. www.biochemj.org Biochem. J. (2004) 379, 11-22 Re-esertification vs. Mobilization Adipocytes do not have Glycerol Kinase Glucose metabolism requires insulin to stimulate GLUT-4 translocation and to stimulate glycolytic enzymes Therefore, in order for FA’s to be reesterified there must be glycerol 3-P (generated from glycolysis). In the well-fed state, any FAs liberated by HSL are re-esterified In the fasting state fatty acids liberated by HSL are all mobilized.