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

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 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. Clearance of lipid from circulation is mediated by adipose,muscle and liver: via the enzyme Lipoprotein Lipase (LPL)

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 Acetyl Co A monogastrics 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 Principal Tissue Site Carbon Source Poultry Human Pig Mouse Sheep Cattle Liver Adipose Glucose 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 capillary Lipoprotein lipase Cell I.e. adipocyte muscle mammary Chylomicrons VLDL 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 adipose muscle starvation down up Well Fed up down insulin up 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 Glycerol  Adenylate Cyclase  cAMP  Hormone Sensitive Lipase Triglyceride  Fatty acids + 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 multi-phosphorylated, 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 re-esterified 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.