Digestion and Absorption of Dietary Lipids • Lipids are taken in the diet mainly as triacylglycerol • . In addition, small amounts of phospholipids, cholesterol, carotenoids and fat-soluble vitamins are taken in diet.
I- Digestion of Triacylglycerol • 1- Lingual lipase • 2- Gastric lipase • 3- Pancreatic lipase, is the main digestive lipase. Emulsification of lipids is important for the action of pancreatic lipase. Emulsification involves mixing in the duodenum with bile salts, phospholipids and lysophospholipids in addition to monoacylglycerol. This leads to breaking of lipid droplets into smaller-sized structures, which increases their surface area exposed to enzyme.
I- Digestion of Triacylglycerol • The products of digestion by pancreatic lipase are: • 72% as 2-monoacylglycerol (MAG) • 22% as glycerol and free fatty acids • 6% as 1-monacylglycerol • 4- Intestinal lipase: This intracellular enzyme hydrolyzes 1-monoacylglycerol to glycerol and free fatty acid.
II- Digestion of Phospholipids III-Cholesterol esters • II- Digestion of Phospholipids • Phospholipids may be absorbed without digestion. Also they may be hydrolyzed by pancreatic phospholipase A2 to lysophospholipids. The resulting lysophospholipids act as emulsifying agents. • III- Digestion of cholesteryl ester • Cholesteryl ester is hydrolyzed by pancreatic cholesterol esterase (cholesteryl ester hydrolase) into cholesterol and fatty acid.
Digestion of Dietary Lipids 2- MAG (72%) Isomerase FA1 H2O FA1 OH FA1 FA1 OH H2O H2O FA3 FA1 FA2 FA2 HO HO FA2 Pancreatic lipase Pancreatic lipase Pancreatic lipase OH OH FA3 OH OH 1,2- DAG Glycerol (22%) 1- MAG (6%) TAG FA1 FA1 H2O FA2 FA2 HO Pancreatic Phospholipase A2 P – Base P – Base Lysophospholipids Phospholipids H2O FFA Cholesteryl esters Cholesterol Cholesterol esterase
Absorption of Dietary Lipids • Bile salts together with products of digestion form micelles.These micelles are soluble thus allowing the products of digestion, together with fat-soluble vitamins to be transported to the brush border of the mucosal cells to be taken to the inside of the epithelial cells. Bile salts pass to the ileum, where most are reabsorbed into the enterohepatic circulation.
Diagram for Digestion and Absorption of Lipids Intestinal Lumen TAG Mucosal Cells Lacteals Chylomicrons Pancreatic lipase FFA Phospholipids Free cholesterol Cholesterol-esters ApoA, B-48 DAG TAG 2-MAG Pancreatic lipase FFA Thiokinase Acyl- CoA FFA 2-MAG CoA , ATP Isomerase Glycerol kinase FFA 1-MAG Glycerol 3-P 1-MAG Glycerol ATP Intestinal lipase Pancreatic lipase Glycolysis Glucose DHAP FFA Portal blood Glycerol Glycerol
B-48 A Chylomicron Structure Apo A Free cholesterol Cholesteryl-ester Phospholipids TAG Apo B-48 The formed triacylglycerol with cholesteryl ester (hydrophobic core) are surrounded by a single layer of phospholipid and cholesterol together with proteins (apo A and apo B-48) to form minute particles < 1μm called chylomicrons within the mucosal cellswhich are transported to the lymphatics (lacteals).
Fate of Absorbed Lipids • I- Uptake by tissues • About 90% of triacylglycerolin the chylomicrons are hydrolyzed by the enzyme lipoprotein lipaseThis enzyme is present in the endothelial cells of extrahepatic tissue (muscle and adipose tissue). Its synthesis is increased by insulin. • Heparin stimulates the release of lipoprotein lipase thus clearing plasma turbidity due to the absorbed chylomicrons, thus heparin is called a clearing factor. • Hydrolysis of the triacylglycerol yields free fatty acids (FFAs) and glycerol. Most of the FFAs are taken by the extrahepatic tissue. • The released glycerol is taken by the liver and the kidney where it can be utilized by the glycerol kinase in these tissues.
Fate of Absorbed Lipids • II-Utilization by tissues • 1- Oxidation: Fatty acids are mostly oxidized by β-oxidation. Glycerol is oxidized by joining the glycolysis pathway. • 2- Conversion to glucose: glycerol (10% of fats) can be converted to glucose. The last 3 carbons of odd chain fatty acids (which are rare in natural fat) may also be converted to glucose. • 3- Formation of tissue fat. • III-Storage • This occurs mainly in the adipose tissue as triacylglycerol(DEPOT FAT). • IV-Excretion • Fats may be secreted in sebaceous glands and also by mammary gland in milk.
Compare between: • Tissue fat: • Constant element (not affected by diet) • Consists mainly of : Phospholipids, glycolipids. • Site: cell membrane, mitochondrial membrane and nervous tissue. • Functions: • 1- membrane permeability • 2- ETC • 3- Nerve impulse transmission • 4- tissue support and protection • Depot fat: • Variable element (affected by diet) • Consists mainly of TAG • Site: Adipose tissue • Function: Source of energy
Oxidation of Fatty Acids • Free fatty acids are taken by most of the tissues. Oxidation of the fatty acids is principally by β- oxidation in the mitochondrialmatrix (adjacent to the TCA cycle and the respiratory chain). • In the mitochondria, fatty acids are oxidized to acetyl-CoA. Acetyl-CoA may be further oxidized completely in the TCA cycle.
Acyl- CoA synthetase R – CO ~ S – CoA R – COOH FFA Acyl- CoA PPi + AMP CoA-SH ATP 2Pi Activation of the fatty acid • is catalyzed by acyl-CoAsynthetase (key enzyme). This step requires ATP and so it is irreversible
Transport of Acyl-CoA into Mitochondria (Carnitine Shuttle) FFA Acyl-CoA CoA-SH CoA-SH Cytosol ATP Outer membrane Acyl-CoA synthetase Carnitine-palmitoyl transferase I Carnitine Acyl-carnitine Inter-membrane space Inner membrane Carnitine Acyl-carnitine translocase Carnitine-palmitoyl transferase II Mitochondrial matrix Carnitine Acyl-carnitine CoA-SH Acyl-CoA Transport of acylCoA into the mitochondria • Long chain fatty acyl-CoA: They are transported to the mitochondrial matrix by carnitine shuttle
-oxidation of Acyl-CoA 2 ATP H2O + FAD Acyl- CoA dehydrogenase ETC O 2 ADP + 2 Pi [O] + FADH2 R – CH2 CH2 – C ~ S – Co A Acyl-CoA ( Cn ) Repeat The Cycle O O R – CH CH – C ~ S – Co A 2– Trans- enoyl-CoA H2O 3 ATP H2O + NAD+ 2– Trans-enoyl-CoA hydratase L,3- Hydroxyacyl-CoA dehydrogenase ETC OH 3 ADP + 3 Pi [O] + NADH,H+ R – CH CH2 – C ~ S – Co A L,3- Hydroxyacyl-CoA CoA-SH -Ketothiolase O O R – C ~ CH2 – C ~ S – Co A 3- Ketoacyl-CoA 2CO2 + 12 ATP O O Citric Acid Cycle & ETC R– C ~ S – CoA CH3– C ~ S – CoA Acetyl- CoA Acyl- CoA (Cn-2 )
Energy yield • Energy yield: Oxidation of palmitic acid (C16) results in the formation of 8 molecules of acetyl-CoA through passing through 7 cycles. • Each cycle yields one molecule of NADH and one molecule of FADH2.These reduced coenzymes will yield 5 ATPs via the respiratory chain. So in 7 cycles: 7 X5 =35 ATPs are produced. The 8 molecules of acetyl CoA will produce 8 X 12=96 ATPs by oxidation in Krebs’ cycle and ETC. • Thus a total of 35 + 96 = 131 ATPs are produced from complete oxidation of one molecule of palmitic acid. Since 2 high energy phosphates are used in the activation of the fatty acid, so the net gain is 131-2= 129 ATPs
ATP Produced by Oxidation of Palmitic Acid CH3(CH2)14-CO ~ S –CoA Palmitoyl-CoA 7 Cycles of β-oxidation 7 FADH2 + 7 NADH,H+ + 8 Active acetate TCA cycle & ETC ETC ETC 16 CO2 14 ATP + 21 ATP + 96 ATP = 131 - 2 for activation = 129 ATP N.B : Oxidation of odd chain fatty acids leaves the last 3 carbons as propionyl-CoA. Propionyl-CoA is carboxylated to methylmalonyl-CoA which is isomerized to succinyl-CoA which enters Krebs’ cycle.
Regulation of β-Oxidation • It depends upon the availability of fatty acids and the consumption of ATP. • 1-Availability of fatty acids: Carbohydrate feeding leads to release of insulin. Insulin stimulates lipogenesis and inhibits lipolysis thus decreasing FFA. • Also during carbohydrate feeding, synthesis of fatty acids is stimulated and so, excess malonyl-CoA is formed. Malonyl-CoAinhibits CPT-I thus inhibiting the uptake and oxidation of the fatty acids.Thus the 2 processes, synthesis and oxidation do not go together. • 2- β-oxidation in a cell depends upon its consumption of ATP. High ATP (and low ADP& Pi) inhibits the respiratory chain, thus β- oxidation becomes inhibited.
Regulation of Fatty Acid β-Oxidation Fatty acids ATP/ADP Fatty acyl-CoA Electron transport chain Acetyl-CoA CPT I Malonyl-CoA Acetyl-CoA Carboxylase Fatty acyl-carnitine + β-Oxidation NADH & FADH2 _ _ _ Insulin Acetyl-CoA
Compound Lipids Phospholipids Sphingolipids Sphingophospholipids - Sphingomyelin Glycerophospholipids - Phosphatidic acid - Phosphatidylcholine (lecithin) - Phsphatudylethanolamine • Phosphatidylinositol Glycolipids - Cerebrosides - Sulfatides - Globosides - Gangliosides
Metabolism of Ketone Bodies • Ketogenesis • Ketone bodies are acetoacetate, β-hydroxybutyrate and acetone. Synthesis of ketone bodies occurs in the mitochondria of theliver. This is because of the presence of HMG-CoAsynthase and HMG-CoAlyase chiefly in the liver. The building unit of ketone bodies is acetyl-CoA derived mainly from oxidation of fatty acids.
Diagram for Ketogenesis Ketogenic Amino acids CH3- CO ~ S – CoA Acetyl-CoA Acyl- CoA -Oxidation Ketothiolase CoA-SH -Oxidation Last C4 Acetoacetyl-CoA Acetyl-CoA HMG-CoA Synthase H2O (Liver mitochondria) 3-Hydroxy-3-methyl glutaryl-CoA (HMG-CoA) CoA-SH CH3- CO ~ S – CoA Acetyl-CoA HMG-CoA Lyase (Liver mitochondria) Acetoacetate NADH,H+ Spontaneous (Lungs & Kidneys) 3- hydroxybutyrate Dehydrogenase CO2 NAD+ Acetone (Expired air & Urine) 3- Hydroxybutyrate
Importance of Ketogenesis • Ketogenesis is of great importance during starvation when fats represents the main source of energy. Although most tissues can utilize fatty acids, they can utilize ketone bodies more easily. During prolonged fasting, the brain adapt to utilize ketone bodies as it cannot utilize fatty acids (fatty acids are bound to plasma albumin and cannot pass the blood brain barrier). Thus ketogenesis is a preparatory step by the liver to facilitate the oxidation of fatty acids during starvation and to provide energy for extrahepatic tissues including the brain.
Ketolysis • It is the complete oxidation of ketone bodies in the mitochondria of extrahepatic tissue. This is due to the high activity of thiophorase (succinyl-CoA:acetoacetateCoAtransferase) in the extrahepatic tissue and its deficiency in the liver. β–hydroxybutyrate is converted to acetoacetate, which gives two molecules of acetyl-CoA. Acetyl-CoA is utilized by Krebs’ cycle for complete oxidation. • Ketolysis is dependent on activity of citric acid cycle as succinyl-CoA needed for the thiophorase reaction is supplied from citric acid cycle and acetyl-CoA enters the cycle for complete oxidation.
Diagram for Ketolysis CH3-CHOH-CH2-COOH 3- hydroxybutyrate NAD+ 3- hydroxybutyrate Dehydrogenase NADH,H+ Succinyl-CoA CH3-CO~CH2-COOH Acetoacetate CoA Transferase Mitochondria of extrahepatic tissues Citric acid cycle Acetoacetyl-CoA CH3-CO~CH2-CO ~ S - CoA Citrate Succinate Ketothiolase CoA-SH Oxaloacetate 2 CH3-CO ~ S - CoA Acetyl-CoA
Ketosis • This is a condition characterized by increased ketone bodies in the blood (ketonemia) and in the urine (ketonuria). • Normally ketone bodies in blood ranges from 0.5-3mg/dL. In urine, it is less than 15mg/day.
Causes of Ketosis • Ketosis occurs in conditions where the rate of ketogenesis exceeds the rate of ketolysis i.e. in conditions where there is marked stimulation of ketogenesis, as in the following: • -Starvation, low carbohydrates and high fat in diet • -Severe diabetes mellitus • -Prolonged administration of anti-insulin hormones • -Prolonged and severe muscular exercise
Effects of ketosis • The increased production and loss of β-hydroxybutyrate and acetoacetate leads to excessive loss of buffer cations Na+, K+ and NH4+ in urine associated with decreased bicarbonate in the blood, which causes acidosis and may lead to coma and death. • Ketogenic substances include fatty acids, ketogenic amino acids and anti-insulin hormones • Anti-ketogenic substances include carbohydrates, glucogenic amino acids, glycerol and insulin.
Diagram for Metabolic Changes During Ketosis Adipose Tissue TAG Increased of Antiinsulin / Insulin ratio in Blood Activation of Lipolysis Brain Oxidation for energy production Glycerol FFA Glucose Ketone bodies Glucose Glucose Oxidation for energy production BLOOD Acetyl-CoA ketolysis Ketone bodies Ketone bodies In Urine (ketonuria) Extrahepatic Tissues (muscles) Glycerol FFA Gluconeogenesis Ketogenesis - Pyruvate - Lactate - Oxaloacetate - Glucogenic amino acids Ketone bodies Liver Ketogenic Amino acids
Metabolism of Cholesterol • Biosynthesis of Cholesterol • It is synthesized in the cytosoland endoplasmic reticulum in all nucleated cells. Plasma cholesterol is made in the liver and intestine. Acetyl-CoA is the source of all cholesterol carbons. • Cholesterol synthesis starts by the formation of HMG-CoA in the same way as in ketogenesis except that it is in the cytosol (ketogenesis occurs in mitochondria). • HMG-CoAis reduced to mevalonate in a reaction catalyzed by HMG-CoAreductase (Key enzyme)and requiring two NADPH.
Importance of Cholesterol • 1-Formation of lipoproteins: Cholesterol regulates membrane fluidity. • 2-Synthesis of vitamin D3: Cholesterol is dehydrogenated, forming 7-dehydrocholesterol. The latter is converted into vitamin D3 under the skin by ultra-violet rays. • 3-Formation of steroid hormones: Cholesterol is the precursor of all steroid hormones, androgens, estrogens, progesterone, and corticoids.
Importance of Cholesterol • 4-Formation of bile acids and salts: Primary bile acids, are synthesized from cholesterol in the liver. Subsequently; these acids are conjugated with glycine or taurine to form (bile salts) • Bile salts are important for emulsification of fats, thus important for digestion and absorption of fats. • Intestinal bacteria deconjugate and dehydroxylate the primary bile acids converting them into secondary bile acid.
Excretion of Cholesterol • About 50% of cholesterol is excreted in feces as bile acids; the remaining 50% are excreted also in stools as neutral sterol (coprostanol).
Plasma Cholesterol • The total plasma cholesterol level ranges from 120-240 mg/dL (recommended level is less than 200 mg/dL). About two thirds are present as cholesteryl ester and one third is present as free cholesterol.
Hypercholesterolemia • Hypercholesterolemia means a plasma cholesterol level higher than 240 mg/dL. There are some factors which may lead to it, these include: • 1-Dietary causes: Diet rich in saturated fat, carbohydrates and cholesterol. • 2- Obesity • 3-Diabetes mellitus • 4-Hypothyroidism: as thyroid hormone stimulates the oxidation of cholesterol and its conversion to bile acids. • 5-Obstructive jaundice: This blocks the pathway for excretion of cholesterol and bile acids. • 6-Nephrosis: • 7-Familial hyperliopoproteinemias
Plasma Lipids and Lipoproteins • Lipids are transported as lipoproteins with special arrangement, so that the most hydrophobic one is present in the core (triacylglycerol and cholesterol-esters).These are surrounded by the amphipathic lipids (phospholipids and cholesterol) and apolipoproteins.
Types of plasma lipoproteins: • 1- Chylomicrons: Synthesized in intestinal cells and consist mainly of TAG. • 2- Very Low Density Lipoprotein (VLDL): synthesized in liver and consist mainly of TAG. • 3- Low Density Lipoprotein (LDL): synthesized in liver and consist mainly of cholesterol. LDLs are important source of cholesterol to extrahepatic tissues. High levels of LDL-cholesterol increase the risk of atherosclerosis. • 4- High Density Lipoprotein (HDL): synthesized in extrahepatic tissues and consist mainly of phospholipids HDLs are important for removal of cholesterol from the tissues to the liver (reverse cholesterol transport) and high levels of HDL protect against atherosclerosis.