Biochemistry: Cholesterol and Steroid Metabolism PDF

BIOCHEMISTRY Cholesterol and Steroid Metabolism BRENDO V. JANDOC, M.D. 1A OUTLINE PLASMA LIPOPROTEINS A. Composit...

BIOCHEMISTRY Cholesterol and Steroid Metabolism BRENDO V. JANDOC, M.D. 1A OUTLINE PLASMA LIPOPROTEINS A. Composition of Plasma Lipoproteins B. Structure of Plasma Lipoproteins C. Digestion of Lipids CM METABOLISM VLDL METABOLISM LDL METABOLISM HDL METABOLISM ROLE OF LIPOPROTEIN (A) IN HEART DISEASE DISORDERS OF LIPOPROTEIN METABOLISM lipid composition, protein composition, size, Differ in density PLASMA LIPOPROTEINS keep lipids soluble as they transport them to spherical macromolecular complexes of lipids plasma Description and specific proteins (apolipoproteins or provide an less perfect in humans apoproteins) efficient than in animals > gradual Bond noncovalent Function mechanism for deposition of lipids hydrophobic interior containing delivering their (especially cholesterol) in triacylglycerols and cholesteryl esters Composition lipid content to tissues > plaque formation encased in an amphiphilic outer layer of the tissues > narrowing of blood protein, phospholipid, and cholesterol vessels (atherosclerosis) Synthesis, degradation, removal from the Constant rate of plasma Principal Lipids TAG, cholesterol (free, esterified) Include Carried Site of Plasma Function formation Lipoproteins Carry dietary triacylglycerol, cholesterol, Chylomicrons Intestines fat-soluble vitamins, and (CM) cholesteryl esters to the peripheral tissues Very Low Transports triglyceride Density from liver to adipose Liver Lipoproteins tissue (VLDL) Transports triglyceride, Low Density cholesterol, and Plasma Lipoproteins phospholipids from liver to Neutral Lipid contains TAG or (LDL) tissues and organs like Core cholesteryl esters or both heart -apolipoproteins Transports triglyceride, (apoproteins) High Density cholesterol, and -phospholipid Liver Lipoproteins phospholipids from tissues Composition Surrounding -nonesterified cholesterol (HDL) and organs like heart back Shell (of -polar portions are to liver Amphipathic) exposed on the surface Intermediate Transports triglyceride, of the lipoproteins > Density cholesterol, and soluble in aqueous Plasma phospholipids from liver to Lipoproteins solution (IDL) peripheral tissues constantly interchange lipids and apolipoproteins with each other > Lipoprotein Particles variable actual apolipoprotein and lipid content of each class of particles TAG and Cholesterol - from the diet (exogenous) Carried - de novo synthesis (endogenous) Trans 6 |ABACCO, ASSISTIN, ALDERITE, BALANZA, BAYAS, BIANG 1 of 12  BIOCHEMISTRY Fatty Acid and Triacylglycerol Metabolism A. Composition of Plasma Lipoproteins - activators or coenzymes for 1. Size and Density (by - determined by floatation rate on enzymes involved in lipoprotein Ultracentrifugation) of centrifuge in 1.063 d NaCl solution metabolism Lipoprotein Particles - the densities of lipoprotein classes - functions of all of the increase as the quantity of lipid apolipoproteins are not yet known contained in the core b. Classification by most classes having subclasses decreases Structure and Function B. Structure of Plasma Lipoproteins 5 Fractions a. CM - lowest in density - largest in size - highest percentage of lipid - smallest percentage of protein b. VLDL, LDL - successively more dense - higher content of protein - lower content of lipid c. HDL - most dense d. IDL 1. Sphere - with the protein plus the amphipathic lipid (phosphatidylcholine and unesterified cholesterol) forming an outer shell - different classes have characteristic lipid : protein ratios - different lipid classes are associated with apoproteins a. Apolar Segments directed inward b. Polar Segments face water outside c. Nonpolar Lipids - form the inside of the sphere - include triglyceride and esterified cholesterol C. Digestion of Lipids 2. Electrophoretic Mobility plasma lipoproteins migrating with 1. Triacylglycerol major dietary lipid of nutritional α-and β-globulins of plasma value 2. Emulsification a. Acid Chyme - from the stomach - enters the duodenum - stimulates secretion of enteric hormones (small peptides) by the duodenal mucosa b. Bile Salts and amphipathic structures > act as Phosphatidylcholine detergents in the duodenum > aid in the formation of mixed micelles 3. Apoproteins may be transferred freely between (large surface area) from fat lipoproteins globules (small surface area) a. Diverse Functions - structural components of the c. Micellar Associations of substrates for hydrolyzing enzymes particles Lipids - provide recognition site for cell- 3. TAG Hydrolysis by 3 Lipid-Specific Enzymes surface receptors Trans 2 |ABACCO, ALDERITE, ASSISTIN, BALANZA, BAYAS, BIANG 2 of 12  BIOCHEMISTRY Fatty Acid and Triacylglycerol Metabolism a. Pancreatic Lipase cleaves TAG > 2-MAG + 2 free fatty 100 > nonsense codon > translation acids of only 48% of the mRNA > apo B-48 i. Lipoprotein Lipase - negatively charged extracellular enzyme - resides on the capillary walls of most tissues - found predominantly in the capillaries of the adipose tissue and cardiac and skeletal muscles - activated by apoC-II on circulating lipoprotein particles - hydrolyzes the TAG contained on the circulating lipoprotein particles ii. Lipoprotein Lipase or dramatic accumulation of TAG-rich ApoC-II Deficiency lipoproteins in the plasma ex: type I hyperlipidemia (familial hypercholesterolemia) 2. CM Assembly - apolipoprotein synthesis begins in b. Cholesterol Esterase hydrolyze cholesterol esters > the rough ER cholesterol + fatty acid - enzymes involved in TAG, c. Phospholipase A2 hydrolyze phospholipid cholesterol and phospholipid synthesis are located in the smooth ER - assembly of apolipoproteins and lipid into CM occurs during transition from the ER to the Golgi apparatus > packaged in secretory vesicles > exocytosis > lymphatic system > blood - requires microsomal TAG transfer protein - loads apo B-48 with lipid 3. Modification of - functionally incomplete Nascent CM Particles - particles released by intestinal mucosal cells CM METABOLISM - contains apolipoprotein B-48 (apoB- assembled in intestinal mucosal cells 48) are TAGs given a coat - protein - when reaches the plasma > composed of - phospholipids modified by receiving apoE (in - cholesterol esters conjunction with apoB-48, is diameter 0.2 - 1. um recognized by hepatic receptors) and carry to the peripheral - dietary TAG C apolipoproteins (including apoC-II, tissues - cholesterol necessary for the activation of - cholesteryl esters lipoprotein lipase, the enzyme that - fat soluble vitamins degrades the TAG contained in the 1. Synthesis of Apolipoproteins CM) a. Apolipoprotein B-48 - synthesis begins on the rough ER i. HDL - source of the apolipoproteins (Apo B-48) - glycosylated as it moves from the ER and Golgi - unique to chylomicrons - constitutes the N-terminal 48% of the protein coded for by the apo B gene b. Apolipoprotein B-100 - synthesized by the liver - found in VLDL & LDL - represents the entire protein coded for by the apo B gene - post-transcriptional editing of a cytosine to uracil in intestinal apo B- Trans 2 |ABACCO, ALDERITE, ASSISTIN, BALANZA, BAYAS, BIANG 3 of 12  BIOCHEMISTRY Fatty Acid and Triacylglycerol Metabolism - allows heart continuing access to circulating fuel even when plasma lipoprotein concentrations are low 6. Formation of CM CM in membrane-bound vesicles > Remnants lateral cell membranes of mucosal cells > exocytosis > extracellular space > plasma a. As the CM Circulates - TAG (lipids) in its core is degraded by lipoprotein lipase (delipidation) > decreased size, increased density - C apolipoproteins return to HDLs - remaining particle is a “remnant” b. In Humans CM remnants are removed by the 4. TAG Degradation by Lipoprotein Lipase liver a. Lipoprotein Lipase - extracellular enzyme c. Lipoprotein Receptor - in hepatocyte membranes - anchored by heparan sulfate to - recognize the combination of capillary walls of most tissues apolipoproteins B-48 and E predominantly in adipose tissue, - bind CM remnants > taken up by cardiac muscle, skeletal muscle hepatocytes by endocytosis > fuse - not present in adult liver with lysosome > hydrolysis of - activated by apo C-II on circulating apolipoproteins, cholesteryl esters, lipoprotein particles > hydrolyze TAG other components of the remnants > in the particles > fatty acids + release of amino acids, free glycerol cholesterol, fatty acids ai. Fatty Acids - stored by the adipose or - cholesterol released (> decreases - used for energy by the muscle cell content of HMG-CoA reductase - if not taken up by a cell > (also allosterically inhibit the enzyme) transported by albumin > regulates aii. Glycerol - used by the liver for lipid synthesis, the rate of de novo cholesterol synthesis in the liver) > bile glycolysis, gluconeogenesis acid synthesis Type I Hyperlipoproteinemia - fatty acid released > phospholipid (Familial Lipoprotein Lipase Deficiency) biosynthesis deficiency of - lipoprotein lipase or - recycling of receptor - apo C-II d. Physiologic Role of deliver dietary fat to the liver chylomicron accumulation (1000 mg/dl or greater) in the plasma CM Remnants (hypertriacylglycerolemia) even in the fasted state b. Hepatic Lipase - on the surface of hepatic endothelial cells - does not significantly attack chylomicrons or VLDL TAG - assists with HDL metabolism 5. Lipoprotein Lipase Activity Regulation a. Insulin - signifying fed state - stimulates lipoprotein lipase synthesis and transfer of lipoprotein lipase to the luminal surface of the capillary b. Isomers of different Kms for TAG Lipoprotein Lipase i. Adipose Enzyme - large Km - allows removal of fatty acids from circulating lipoprotein particles > stored as TAG when plasma lipoprotein concentrations are elevated ii. Heart Muscle - small Km Lipoprotein Lipase Trans 2 |ABACCO, ALDERITE, ASSISTIN, BALANZA, BAYAS, BIANG 4 of 12  BIOCHEMISTRY Fatty Acid and Triacylglycerol Metabolism VLDL METABOLISM - observed during the transition a. VLDLs - produced in the liver from VLDL to LDL in the plasma - composed predominantly of TAG (initial - carry TAG from the liver to the product of VLDL degradation in the peripheral tissues > degraded by plasma) lipoprotein lipase - can also be taken up by cells b. Fatty Liver (Hepatic imbalance causes through receptor-mediated Steatosis) between - hepatitis endocytosis that uses hepatic - obesity apo E as the ligand TAG - uncontrolled DM b. Apo E synthesis and - chronic ethanol 3 Isoforms VLDL ingestion - bind poorly to receptors secretion - homozygosis to apo E2 > 1. VLDL Release - released from the liver as deficiency in the clearance of nascent VLDL particles containing chylomicron remnants and IDLs > E2 apolipoprotein B-100 and A-I familial type III - must obtain apoC-II (required for hyperlipoproteinemia (familial activation of lipoprotein lipase) and dysbetalipoproteinemia or broad apoE from circulating beta disease) HDL E3 a. Abetalipoproteinemia - rare hypolipoproteinemia - confers increased susceptibility to - defect in microsomal TAG E4 late-onset Alzheimer transfer protein > inability to load disease apo B with lipid > no chylomicrons or VLDLs formed > TAGs accumulate in the liver and intestines 2. Circulating VLDL - VLDL > circulation > altered Modification structure - TAG is removed by lipoprotein lipase > decreased size, more dense - surface components (C and E apolipoproteins) are transferred to HDL by exchange reaction (TAG or phospholipid from VLDL to HDL) - accomplished by cholesteryl ester transfer protein - the particles retain apo B-100 LDL METABOLISM a. LDL - retain apoB-100 - lose their other apolipoproteins to HDL - contain much less TAG than VLDL - have high concentration of cholesterol and cholesteryl esters - transport cholesterol esters throughout the body 3. LDL Production from after the VLDL modifications > LDL VLDL in the Plasma a. IDL (VLDL Remnant) - intermediate (density) sized particle Trans 2 |ABACCO, ALDERITE, ASSISTIN, BALANZA, BAYAS, BIANG 5 of 12  BIOCHEMISTRY Fatty Acid and Triacylglycerol Metabolism endosome (LDL stay free in the vesicular lumen) > formation of a new structure called CURL (Compartment for Uncoupling of Receptor and Ligand) Receptors Recycled i. Number of Receptors - availability of these lipoprotein for Lipoproteins Varies particles According to the - needs of the cell ii. Lipoprotein transferred to lysosomes > degradation Remnants in the Vesicle by lysosomal (hydrolytic) enzymes > release of free cholesterol, amino acids, fatty acids, and phospholipids > reutilization by the cell iii. “Down-Regulation” if there is a large amount of particular circulating plasma lipoprotein > number of cell-surface receptors for it decreases iv. “Up-Regulation” if cells are starved with cholesterol > increase the number of cell-surface receptors v. Wolman disease - rare; autosomal recessive - inability to hydrolyze lysosomal cholesteryl esters vi. Niemann-Pick - rare; autosomal recessive disease type C - inability to transport unesterified cholesterol out of the lysosome 2. Effect of CM remnant-, IDL-, and LDL-derived Endocytosed cholesterol affects cellular cholesterol 1. Receptor-Mediated Endocytosis Cholesterol on content through a. Primary Function of bind to receptors on cell-surface Cellular Cholesterol LDL Particles membranes that recognize Homeostasis apolipoprotein B-100 > deposit free a. Increased inhibition of HMG-CoA reductase cholesterol on the membranes of cells Cholesterol Content activity > decreased de novo > provide cholesterol to peripheral cholesterol tissues synthesis LDL Receptors - negatively charged glycoprotein b. If Cholesterol is Not esterified by acyl CoA : cholesterol molecules Required Immediately acyltransferase (ACAT) - clustered in pits on cell membranes (for Some Structural or i. Clathrin - protein coating the intracellular side Synthetic of the pit Purpose) - stabilizes the shape of the pit i. ACAT - transfers a fatty acid from a fatty acyl ii. Deficiency of - significant elevation of plasma LDL CoA derivative to cholesterol > Functional LDL (and therefore plasma cholesterol) cholesteryl ester formation > stored in Receptor - plasma TAG remains normal the cell - greatly accelerate the progress of - activity is enhanced in the presence of atherosclerosis increased intracellular - ex: type II hyperlipidemia (familial cholesterol hypercholesterolemia) iii. Triiodothyronine (T3) have positive effect on the binding of LDL to its receptor iiia. Hypothyroidism hypercholesterolemia LDL Internalization as intact particles by endocytosis Vesicle Containing rapidly losses clathrin coat > fuses with LDL other vesicles > endosome (larger vesicle) c. Decreased LDL Gene - lowered new LDL receptor protein Proton-Pumping pH endosome contents decrease > Transcription synthesis > further entry of LDL Activity of Endosomal separation of LDL from its receptor > cholesterol limited ATPase receptor migrates to 1 side of the Trans 2 |ABACCO, ALDERITE, ASSISTIN, BALANZA, BAYAS, BIANG 6 of 12  BIOCHEMISTRY Fatty Acid and Triacylglycerol Metabolism - regulation of LDL gene receptor - secreted into blood involves a hormone response element - accounts for about 70% of the (HRE) apoproteins in HDL 3. Chemically Modified LDL Uptake by Macrophage Functions Scavenger a. Circulating reservoir - apolipoprotein that is transferred to a. LDL Receptors - have broad ligand-binding specificity of apoC-II VLDL and chylomicrons - can mediate the endocytosis of - activator of lipoprotein lipase chemically modified LDL b. Remove free transferring cholesteryl esters to VLDL b. Chemical - convert circulating LDL into ligands (unesterified) and LDL in exchange for Modifications that can be recognized by the receptors cholesterol from TAG acetylation and apolipoprotein B extrahepatic tissues oxidation and esterifying it using - initiating step involves the the plasma enzyme peroxidation of polyunsaturated fatty phosphatidylcholine: acids in the LDL lipids cholesterol - inhibited by antioxidants (vitamin E) acyltransferase (PCAT c. Macrophages possess high levels of scavenger or LCAT) receptor activity c. Carry cholesteryl esters to the liver d. Scavenger Receptor - bind to broad range of ligands 1. HDL as a Reservoir of Apolipoproteins Class A (SR-A) - mediate the endocytosis of chemically a. Apo C-II - lipoprotein transferred to VLDL and modified LDL > oxidation of the - lipid chylomicrons components > apo B - activator of lipoprotein lipase - not down-regulated in response to b. Apo E apolipoprotein required for receptor- increased intracellular cholesterol mediated endocytosis of IDLs and e. Modified LDL Taken - does not regulate intracellular chylomicron remnants Up by Macrophages cholesterol levels > cholesterol 2. HDL Uptake of Free Cholesterol accumulation a. Newly Secreted - disc-shaped - excessive uptake > “foam cells” > (Nascent) HDL - contains participate in atherosclerotic plaque -primarily phospholipid (largely formation phosphatidylcholine) o apolipoproteins (apoE, apoA, apoC) o unesterified cholesterol o rapidly converted to spherical particles as they accumulate cholesterol - high phospholipid concentration (important cholesterol solubilizer) > excellent acceptors of unesterified cholesterol from o surface of cell membranes o other circulating lipoproteins 3. Free Cholesterol Esterification a. Cholesterol Uptake esterification by PCAT by HDL b. Phosphatidylcholine: - activated by apo A-I of the HDL Cholesterol - plasma levels of apo A-I are increased Acyltransferase (PCAT) by modest alcohol intake or Lecithin:Cholesterol - synthesized by the liver Acyltransferase (LCAT) - binds to nascent HDLs - transfers the fatty acid from carbon 2 HDL METABOLISM of phosphatidylcholine to cholesterol > HDL Particles - released into the blood stream by production of exocytosis from liver and intestines - formed in blood by the addition of lipid to apo A-1 Apo A-1 - apolipoprotein - made by the liver and intestine Trans 2 |ABACCO, ALDERITE, ASSISTIN, BALANZA, BAYAS, BIANG 7 of 12  BIOCHEMISTRY Fatty Acid and Triacylglycerol Metabolism Hydrophobic Lysophosphatidyl Cholesterol Ester choline - sequestered in - binds to the core of HDL > albumin can no longer be transferred to a membrane e. 2/3 of Plasma esterified with fatty acid Cholesterol f. Liver Disease decreased concentrations of plasma cholesteryl esters may be due to o phosphatidylcholine production deficiency o PCAT/LCAT deficiency 4. Fate of HDLs spherical HDL > taken up by liver by receptor-mediated endocytosis > cholesteryl ester degradation cholesterol release > o repackaged in lipoproteins o converted into bile acids o secreted into the bile 5. Reverse Cholesterol Transport a. Key Components of Cholesterol Homeostasis from peripheral HDLs cells to from HDLs to liver for Deficiencies hypercatabolism of lipid-poor HDLs > - bile acid markedly increased HDL synthesis Familial LCAT complete LCAT - disposal via the Deficiency deficiency bile selective cholesterol Fish Eye Disease partial LCAT steroidogenic transfer deficiency cells for - hormone c. As Nascent HDL HDL3 > round, micellar-like particle synthesis Accumulates HDL2 Cholesteryl Esters HDL as good cholesterol carrier inverse relationship between plasma d. Cholesteryl Ester removes some cholesteryl esters from HDL concentration and Transfer Protein HDL to VLDL in exchange for atherosclerosis triacylglycerol > ultimately remains in the LDL until the particle is taken up by b. Involves a cell cholesterol efflux from peripheral by ABCA1 (transport cells to HDL protein) cholesterol by PCAT esterification binding of cholesteryl ester-rich HDL (HDL2) to liver and steroidogenic cells selective transfer of cholesteryl esters release of lipid- Hepatic Lipase depleted HDL (HDL3) degrade - triacylglycerols - phospholipids Trans 2 |ABACCO, ALDERITE, ASSISTIN, BALANZA, BAYAS, BIANG 8 of 12  BIOCHEMISTRY Fatty Acid and Triacylglycerol Metabolism participate in the formation of HDL3 c. Mediator cell-surface receptor binds HDL (scavenger receptor class B type 1, SR-B1) ROLE OF LIPOPROTEIN (A) IN HEART DISEASE 1. Lipoprotein (a) - increased plasma concentration > or lp (a) increased risk of coronary heart disease - nearly identical in structure to LDL particle a. Apo (a) - additional lipoprotein molecule 2. Hypolipidemias deficiency of 1 or more of the (distinguishing feature) plasma lipoproteins - covalently linked at a single site to apo a. - rare genetic disorder B-100 Hypobetalipoproteinemia - recessive inheritance - highly homologous to plasminogen (Abetalipoproteinemia) - characterized by neurologic - elevated lp (a) > competes with symptoms including ataxia and plasminogen for the binding of mental retardation plasminogen activators > slowed blood - plasma TAG and cholesterol levels clot breakdown > trigger heart attacks are decreased b. Levels Genetics primarily - complete absence of β-lipoproteins Determined By Diet trans fatty acids increase (no chylomicrons or VLDLs) levels of lp (a) - greatly reduced fat absorption Estrogen decreases - no effective treatment for the - LDL prevention of the neurologic - lp (a) symptoms b. HDL Deficiency - rare familial disorder (Tangier Disease) - very rare deficiency of ABCA1 - low plasma cholesterol levels DISORDERS OF LIPOPROTEIN METABOLISM - normal or increased plasma TAG 1. Hyperlipidemias abnormally high level of one or more - virtual absence of HDL particles of the plasma lipoproteins due to degradation of lipid-poor apo a. Classification on the basis of electrophoretic A-1 pattern - no known treatment Characteristics - recurrent polyneuropathy - lymphadenopathy - tonsillar hyperplasia - hepatosplenomegaly (from cholesterol storage in reticuloendothelial cells) 3. Case: Hypercholesterolemia a. Presentation 30-year-old male Presents with - evidence of coronary heart disease b. Atherosclerosis - arterial disease that causes heart - xanthomas (cholesterol deposits) attacks and strokes in the tendons - most serious consequence of hyperlipidemia is the increased risk Blood Analysis - plasma cholesterol = 420 mg/dL (normal = 150-240) of atherosclerosis Trans 2 |ABACCO, ALDERITE, ASSISTIN, BALANZA, BAYAS, BIANG 9 of 12  BIOCHEMISTRY Fatty Acid and Triacylglycerol Metabolism - plasma TAG = 75 mg/dL (35-160) b. Diagnosis and Treatment i. Hypercholesterolemia - high plasma cholesterol level - normal plasma TAG level ii. Treatment - dietary modification Implemented - restricted cholesterol intake to < 200 mg/day - calories from saturated fats to < 8% of total calories - combined drug therapy - lovastatin 50 mg/day - colestipol 30 g/day - niacin 5 g/day iii. Outcome after 15 months - plasma cholesterol level = 190 mg/dL - no noted drug toxicity or side effects c. Discussion i. Lovastatin - inhibits HMG CoA reductase activity > blocks endogenous cholesterol synthesis ii. Colestipol - insoluble resin > binds bile acids > prevent reabsorption by the intestines iii. Niacin inhibit the synthesis and secretion of VLDL Trans 2 |ABACCO, ALDERITE, ASSISTIN, BALANZA, BAYAS, BIANG 10 of 12  BIOCHEMISTRY Fatty Acid and Triacylglycerol Metabolism Trans 2 |ABACCO, ALDERITE, ASSISTIN, BALANZA, BAYAS, BIANG 11 of 12  BIOCHEMISTRY Fatty Acid and Triacylglycerol Metabolism Trans 2 |ABACCO, ALDERITE, ASSISTIN, BALANZA, BAYAS, BIANG 12 of 12