Medical Biochemistry Robert F. Waters, PhD Lipid Digestion Absorption Transport

Lipoproteins Chylomicron (Most Positive) Chylomicron (Most Positive) Chylomicron Remnant Chylomicron Remnant VLDL VLDL IDL IDL LDL LDL HDL HDL

Lipoprotein Separation (EM) Chylomicron (Cathode) Chylomicron (Cathode)  Very little mobility LDL (  -Lipoprotein) LDL (  -Lipoprotein) VLDL (Pre-  Lipoprotein) VLDL (Pre-  Lipoprotein) HDL (Anode) HDL (Anode)   Lipoprotein  Most electronegative

Lipoprotein Size Chylomicron (5000 A) Chylomicron (5000 A) VLDL ( A) VLDL ( A) IDL/LDL ( A) IDL/LDL ( A) HDL (80 A) HDL (80 A)  Arrows point to fat droplets in enterocyte after high lipid meal  G points to Golgi Apparatus

Lipoprotein Composition Chylomicron Chylomicron  90% TG  10% Protein, PL, C and CE  Formed in intestine VLDL VLDL  60% TG  Remainder Protein, PL, C and CE  Mainly liver but some (limited) TG rich VLDLs formed in intestine LDL LDL  8% TG HDL HDL  5% TG  High Percentage of C and CE

Lipid Digestion Emulsification Emulsification Mixed Micelle Formation Mixed Micelle Formation Deacylation Deacylation Transport Across Membrane Transport Across Membrane

Role of Intestinal Luminal Phosphatidyl Choline in Chylomicron Formation Used in the proper coating of chylomicrons and VLDLs Used in the proper coating of chylomicrons and VLDLs Involved in chylomicron metabolism in blood Involved in chylomicron metabolism in blood Involved in integrity of subcellular organelle membranes in enterocytes Involved in integrity of subcellular organelle membranes in enterocytes  Membrane transport and association of enzymes to membranes

Hormones and Lipid Absorption Little is known Little is known The hormone neurotensin seems to enhance lymph lipid transport The hormone neurotensin seems to enhance lymph lipid transport

Diagnosing Lipid Absorption Disorders Fat quantification in stool Fat quantification in stool Normal is excretion of about 6g of fat per 24 hr period (less than 5% of fat intake) Normal is excretion of about 6g of fat per 24 hr period (less than 5% of fat intake) 95% of fat is normally absorbed 95% of fat is normally absorbed

Disorders of Small Intestine Malabsorption of fat by celiac sprue Malabsorption of fat by celiac sprue  Lesions in mucosa lining  Caused generally by gluten Protein rich in proline and glutamine found in wheat, oats, barley, rye, etc.Protein rich in proline and glutamine found in wheat, oats, barley, rye, etc. Mechanism not knownMechanism not known

Pancreatic Deficiency and Fat Digestion Symptoms are abdominal pain and steatorrhea (large, pale, frothy stools) caused by undigested fat Symptoms are abdominal pain and steatorrhea (large, pale, frothy stools) caused by undigested fat Lower pancreatic digestive enzymes (e.g., pancreatic lipases, response to protease hormones) due to pancreatic deterioration or pancreatectomy Lower pancreatic digestive enzymes (e.g., pancreatic lipases, response to protease hormones) due to pancreatic deterioration or pancreatectomy Treat with low fat diets and hydrolysates of protein and starch (due to lack of pancreatic amylase and response to proteolytic hormones) Treat with low fat diets and hydrolysates of protein and starch (due to lack of pancreatic amylase and response to proteolytic hormones)

Uptake of Fats affected by Bile Salt Deficiency Pancreatic deficiency affects TG digestion Pancreatic deficiency affects TG digestion Bile salt deficiency causes poor micelle formation and solubilization of micelles Bile salt deficiency causes poor micelle formation and solubilization of micelles May be caused by liver disease or blockage of bile duct by stones or other structural blockage May be caused by liver disease or blockage of bile duct by stones or other structural blockage

Abetalipoproteinemia Once thought to be caused by lack of Apo- B production Once thought to be caused by lack of Apo- B production Now known to NOT be lack of Apo-B 48 gene production Now known to NOT be lack of Apo-B 48 gene production Caused by the defect in the association of lipids with the Apo-B48 protein and therefore causes malformation of chylomicron Caused by the defect in the association of lipids with the Apo-B48 protein and therefore causes malformation of chylomicron

Chylomicron Retention Disorder Probably a defective packaging system relating to Golgi Apparatus malfunction Probably a defective packaging system relating to Golgi Apparatus malfunction  Intracellular transport defect Considered later sequential defect than the “so-called” abetalipoproteinemia Considered later sequential defect than the “so-called” abetalipoproteinemia

Lipid Absorption and Mucosa Injury Long chain fatty acids are injurious to mucosa lining especially in neonates and children Long chain fatty acids are injurious to mucosa lining especially in neonates and children

Direct Transport of Fatty Acids into Blood When lack of chylomicrons the transport of fatty acids (even long chain fatty acids) directly into blood increases dramatically When lack of chylomicrons the transport of fatty acids (even long chain fatty acids) directly into blood increases dramatically Once thought not important Once thought not important

Lipids and Satiety Less satiety with Intravenous Injected fatty acids than ingested fatty acids Less satiety with Intravenous Injected fatty acids than ingested fatty acids  Indicates that probably chylomicrons are the stimulus for satiety rather than individual lipid components (e.g., FA, TG, etc.) Possibly a chylomicron apolipoprotein (Apo A- IV) my affect satiety response on the CNS Possibly a chylomicron apolipoprotein (Apo A- IV) my affect satiety response on the CNS

Lipids in Enterocyte Reacylation Reacylation  Fatty Acyl-CoA Synthetase  Activation of Fatty Acids  Acyltransferase B-48 Production B-48 Production Chylomicron Formation Chylomicron Formation  Least Dense Chyle Chyle Fatty Acid Absorption Fatty Acid Absorption  Short Chain

Chylomicron Activation Addition of Apo CII and Apo E Addition of Apo CII and Apo E  From HDLs Apo CII activation of lipoprotein lipase Apo CII activation of lipoprotein lipase  Chylomicron Remnant Formation Apo E and B-48 Apo E and B-48  Activates receptor-mediated endocytosis by the liver Apo CII returned to HDLs Apo CII returned to HDLs Final Degradation of Chylomicron in liver Final Degradation of Chylomicron in liver

Liver VLDL Production Pre-  Lipoprotein Pre-  Lipoprotein Apo B-100 Apo B-100 Add CII and E Add CII and E  From HDL Apo CII activates extracellular lipoprotein lipase Apo CII activates extracellular lipoprotein lipase Becomes IDL then LDL Becomes IDL then LDL

Receptor Mediated Endocytosis (IDL) Apo B-100 Activates Endocytosis Apo B-100 Activates Endocytosis CII and E are returned to HDL CII and E are returned to HDL LDL Receptor Formation (Genetic Control) LDL Receptor Formation (Genetic Control) LDL Endocytosis with C and CE LDL Endocytosis with C and CE  Addition of Clathrin  Formation of Endosome Deacylation (Free Cholesterol and FA) Deacylation (Free Cholesterol and FA) Apo B-100 yields free FA (LDL Decomp.) Apo B-100 yields free FA (LDL Decomp.) Control of HMG CoA Reductase Control of HMG CoA Reductase Cholesterol Stored as CE (Reacylation) Cholesterol Stored as CE (Reacylation)  ACAT (Lecithin:Cholesterol Acyl Transferase)

Reverse Cholesterol Transport Free Peripheral Cholesterol Free Peripheral Cholesterol  Picked up by HDL (Nascent)  Apo A Activates Phosphatidylcholine:Cholesterol acyltransferaseActivates Phosphatidylcholine:Cholesterol acyltransferase OPCsOPCs  Returned to Liver

Foam Cell Formation High LDL Concentration High LDL Concentration  Become Oxidized to LDLox  Superoxides  Nitric Oxide  Hydrogen Peroxide  Antioxidants  Vitamin E  Ascorbic Acid   -Carotene, etc.

Foam Cells Continued: Overview LDLox engulfed by macrophages LDLox engulfed by macrophages  Form large “foam cells” Foam Cells Foam Cells  Associated with endothelial cells  Release Growth Factors  Stimulate Smooth Muscle  Calcification of Plaque

Absorption of Lipophilic Drugs and Toxic Substances Fat soluble vitamins trapped in chylomicrons and transported in lymph and transported to peripheral cells by lipoproteins Fat soluble vitamins trapped in chylomicrons and transported in lymph and transported to peripheral cells by lipoproteins Some substances that are lipophilic are absorbed better with a lipid rich meal Some substances that are lipophilic are absorbed better with a lipid rich meal Toxic DDT (1,1-bis(p-chlorophenyl)-2,2,2- trichloroethane) :banned pesticide in U.S. Toxic DDT (1,1-bis(p-chlorophenyl)-2,2,2- trichloroethane) :banned pesticide in U.S.  Toxicity enhanced with a fat meal Tetrachloroethylene used in hookworm treatment not normally absorbed in the intestine Tetrachloroethylene used in hookworm treatment not normally absorbed in the intestine  However, when ingested with a lipid meal may become very toxic