Medical Biochemistry Robert F. Waters, PhD

1. Medical BiochemistryRobert F. Waters, PhD Lipid Digestion Absorption Transport

2. Lipoproteins • Chylomicron (Most Positive) • Chylomicron Remnant • VLDL • IDL • LDL • HDL

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

4. Lipoprotein Size • Chylomicron (5000 A) • VLDL (800-500 A) • IDL/LDL (300-250 A) • HDL (80 A) • Arrows point to fat droplets in enterocyte after high lipid meal • G points to Golgi Apparatus

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

6. Lipid Digestion • Emulsification • Mixed Micelle Formation • Deacylation • Transport Across Membrane

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

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

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

10. Disorders of Small Intestine • 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. • Mechanism not known

11. Pancreatic Deficiency and Fat Digestion • 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 • Treat with low fat diets and hydrolysates of protein and starch (due to lack of pancreatic amylase and response to proteolytic hormones)

12. Uptake of Fats affected by Bile Salt Deficiency • Pancreatic deficiency affects TG digestion • 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

13. Abetalipoproteinemia • Once thought to be caused by lack of Apo-B 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

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

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

16. 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 • Once thought not important

17. Lipids and Satiety • 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

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

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

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

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

22. Reverse Cholesterol Transport • Free Peripheral Cholesterol • Picked up by HDL (Nascent) • Apo A • Activates Phosphatidylcholine:Cholesterol acyltransferase • OPCs • Returned to Liver

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

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

25. Absorption of Lipophilic Drugs and Toxic Substances • 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 • 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 • However, when ingested with a lipid meal may become very toxic