Cholesterol's history includes its discovery, the elucidation of its structure, the understanding of its function, and the recognition of its role in disease.

Early Identification and Naming

• In 1784, compounds were isolated from bile, including cholesterol.

• In 1816, Chevreul coined the term "cholesterine" for an alcohol-soluble substance found in gallstones, from the Greek words chole (bile) and stereos (solid).

• In 1843, Vogel identified cholesterol in normal animal tissues as well as in atheromatous lesions.

• By the mid-1800s, the idea that bile acids were responsible for solubilizing cholesterol in bile emerged.

Structural Determination

• Heinrich Wieland deduced the structure of cholesterol and bile acids, for which he was awarded the Nobel Prize in Chemistry in 1926.

• The structure of cholesterol was finally solved by Wieland and Windaus around 1932.

Biosynthesis and Metabolism

• Investigations into the cholesterol biosynthetic pathway required the development of isotopic tracer methods in the 1930s.

• Konrad Bloch and David Rittenberg showed that the ring structure and side chain of cholesterol were derived from acetate and identified intermediates in the pathway.

• Subsequent work by Bloch, John Cornforth, and George Popjak established the biosynthetic origin of all 27 carbons of cholesterol.

• The cholesterol biosynthesis pathway involves enzymes that are in the cytoplasm, microsomes (ER), and peroxisomes.

• The liver is the only mammalian organ that can excrete cholesterol from the body.

• The synthesis and utilization of cholesterol must be tightly regulated to prevent over-accumulation and abnormal deposition within the body.

• A key regulatory step in cholesterol biosynthesis is the reduction of hydroxymethylglutaryl-CoA (HMG-CoA), which is regulated at the transcriptional level and by post-transcriptional methods.

• The rate of incorporation of tritiated water into cholesterol in vivo provides a reliable gauge for assessing the impact of specific dietary, pharmacological, and genetic manipulations on the rate of sterol synthesis in individual organs and the body.

Cholesterol's Role in Health and Disease

• In 1929, the nutritional importance of specific lipid molecules was revealed when rats on a fat-free diet showed retarded growth, scaly skin, tail necrosis and eventual death. These conditions were reversed by feeding specific fats. Linoleic acid was recognized as the active agent, and the term 'essential fatty acid' was coined.

• Around the 1930s, it was observed that high levels of serum cholesterol were associated with heart disease (angina pectoris) and likely a contributor to myocardial infarction.

• The Framingham Heart Study was key to elevating serum cholesterol to the status of a coronary artery disease (CAD) risk factor.

• The "lipid hypothesis," that elevated serum LDL-cholesterol concentrations play a direct role in coronary heart disease (CHD), was a major finding related to the connection between serum cholesterol concentrations and CHD risk.

• There is a strong positive relationship between serum cholesterol concentration and incidence of CHD, as well as between the consumption of saturated fat and serum cholesterol.

• Atherosclerosis, the abnormal deposition of cholesterol and cholesterol-rich lipoproteins in the coronary arteries, is a leading factor in diseases of the coronary arteries.

• Atherosclerosis is characterized by the build-up of fatty deposits in the arterial wall, with the first stage being the fatty streak.

• The primary event in atherogenesis is cholesterol deposition in the arterial wall, originating from circulating plasma lipoproteins.

• Oxysterols, which are oxidized derivatives of cholesterol, have been identified as regulators of the metabolism and homeostasis of cholesterol and sterols.

• Cholesterol is transported in the blood through lipoproteins, which have a hydrophobic lipid core surrounded by apolipoproteins and phospholipids.

• The liver is the only mammalian organ that can excrete cholesterol from the body.

• The liver is the only mammalian organ that can excrete cholesterol from the body.

• Bile acids are synthesized from cholesterol.

• Cholesterol is essential for the growth and maintenance of all eukaryotic cells.

• ABCA1 is required for apolipoprotein-mediated cholesterol and phospholipid release, which generates nascent HDL particles.

Cholesterol as a therapeutic target

• Statins are a group of drugs widely used for reducing membrane cholesterol. They act as competitive inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase.

• Plant sterols and stanols are used as hypocholesterolemic agents.

• Ezetimibe is a lipid-lowering drug that selectively inhibits the intestinal absorption of cholesterol.

• There has been considerable research on cholesterol transport, and it has been found that cholesterol is transported in combination with other substances as lipoproteins.

• The liver is the only mammalian organ that can excrete cholesterol from the body.

• Bile acids are synthesized from cholesterol.

Several inherited disorders are associated with cholesterol metabolism.

Smith-Lemli-Opitz Syndrome (SLOS):

• This relatively common autosomal recessive disorder is characterized by elevated levels of plasma 7-dehydrocholesterol and low plasma cholesterol levels.

• It is due to a deficiency in 7-dehydrocholesterol Δ7-reductase activity.

• Mutations in the DHCR7 gene, located on chromosome 11q13.4, cause the deficiency in DHCR7.

• SLOS is characterized by multiple developmental malformations and behavioral problems.

Desmosterolosis:

• This autosomal recessive disorder is mimicked by triparanol, an inhibitor of cholesterol biosynthesis, which acts on 24-dehydrocholesterol reductase (24-DHCR) and results in accumulation of desmosterol.

• Desmosterolosis is caused by mutations in the 3β-hydroxysterol Δ24-reductase gene.

Rhizomelic chondrodysplasia punctata: This condition exhibits defective sterol synthesis due to the lack of key peroxisomal enzymes of cholesterol biosynthesis.

Other inborn errors of sterol biosynthesis:

• These include Mevalonic aciduria, CDP X-linked dominant (CDPX2), CHILD syndrome (congenital hemidysplasia with ichthyosis and limb defects), and Greenberg skeletal dysplasia.

• These disorders involve defects in various enzymes such as mevalonate kinase, sterol Δ8,Δ7-isomerase, sterol C-4 demethylase, and sterol Δ14-reductase.

Niemann-Pick Type C (NPC) disease:

• This lysosomal storage disease is characterized by striking cholesterol accumulation.

• The molecular defects are in either of two proteins, NPC1 or NPC2.

• NPC fibroblasts show retarded cholesterol transport from the lysosomal compartment to the plasma membrane.

Cerebrotendinous xanthomatosis: This is associated with biochemical abnormalities.

Tangier disease:

• It is a rare disease caused by loss-of-function mutations in the ABCA1 gene, leading to severe deficiency in plasma HDL level.

• Patients display a higher risk for premature cardiovascular heart disease.

Cholesterol is a crucial biological molecule with roles in membrane structure and as a precursor for synthesizing steroid hormones, bile acids, and vitamin D.

Steroid Hormones: Cholesterol serves as the precursor for the synthesis of steroid hormones. Steroid hormones include:

• Glucocorticoids, mineralocorticoids, estrogens, progesterones, and androgens.

• These hormones are produced via modifications of cholesterol in the endoplasmic reticulum and mitochondria.

Bile Acids: Cholesterol is a precursor to bile acids, which are essential for digestion and absorption of fats and fat-soluble molecules in the intestine.

Key points about bile acid synthesis:

• The synthesis occurs in various subcellular compartments, including the cytosol, endoplasmic reticulum, mitochondria, and peroxisomes.

• The process involves at least 18 distinct reactions to transform cholesterol into bile acids.

• Bile acids are C24-sterol derivatives formed by trimming the cholesterol side chain in mammals.

• 7α-hydroxycholesterol is the primary bile acid.

• Oxysterols formed from cholesterol are involved in the activation of bile acid synthesis.

Vitamin D: Cholesterol is a precursor to vitamin D. Specifically, 7-dehydrocholesterol, derived from cholesterol, is converted to vitamin D.

Oxysterols: Cholesterol can be metabolized to oxysterols, which have regulatory roles in cholesterol metabolism.

Key roles of oxysterols:

• Regulation of cholesterol synthesis: Oxysterols can inhibit cholesterol synthesis by blocking SCAP-facilitated proteolysis of SREBP.

• Activation of bile acid synthesis: They activate bile acid synthesis.

• Cholesterol esterification: Oxysterols promote cholesterol esterification.

• Activation of nuclear transcription factors: Certain oxysterols activate LXRs and FXR, which then heterodimerize with RXR to induce genes involved in reverse cholesterol transport.

Other Isoprenoids: Isoprenoid intermediates of cholesterol biosynthesis can be diverted to synthesize other molecules.

Examples include:

• Dolichol, used in synthesizing N-linked glycoproteins.

• Coenzyme Q, a component of the oxidative phosphorylation pathway.

• The side chain of heme a.

• Lipid modification of some proteins.

The regulation of cholesterol synthesis and its conversion to other compounds is carefully controlled in the body.

Cholesterol synthesis can be inhibited at various steps, using a variety of compounds.

Statins:

• Statins, such as atorvastatin, fluvastatin, pravastatin and simvastatin, are competitive inhibitors of HMG-CoA reductase, a key rate-limiting enzyme in cholesterol biosynthesis. By inhibiting HMG-CoA reductase, statins decrease hepatic cholesterol content, leading to an up-regulation of LDL receptors on the liver cell surface and increased clearance of LDL-C from the blood.

• Statins have benefits beyond cholesterol-lowering, including the regulation of S-nitrosylated COX-2 production.

• Inhibition of HMG-CoA reductase by statins reduces hepatocyte unesterified cholesterol concentrations, up-regulating hepatic LDL-receptor expression and removing more LDL particles from circulation.

• Statins, by inhibiting isoprenoid synthesis, can interfere with the synthesis of other important isoprenoid compounds besides cholesterol, posing a risk of toxicity with long-term use.

• When cultured cells are acutely incubated with statins, sterols have no effect on HMG-CoA reductase mRNA translation; however, mevalonate reduces the HMG-CoA mRNA translation.

Distal Inhibitors of Cholesterol Biosynthesis:

• AY9944 and BM15766 inhibit 7-dehydrocholesterol reductase (7-DHCR), leading to the accumulation of 7-dehydrocholesterol (7-DHC).

• Triparanol inhibits 24-dehydrocholesterol reductase (24-DHCR), causing desmosterol accumulation.

Other inhibitors:

• Bempedoic acid, a pro-drug converted to bempedoyl-CoA, inhibits ATP citrate lyase (ACL), suppressing fatty acid and cholesterol synthesis and stimulating mitochondrial fatty acid β-oxidation.

• 5-tetradecyloxy-2-furoic acid (TOFA), inhibits acetyl-CoA carboxylase (ACC).

• Ro 48-8071, inhibits 2,3-oxidosqualene:lanosterol cyclase and selectively suppresses intestinal cholesterol synthesis.

• Compactin and mevinolin, natural antibiotics from Penicillium spp. and Aspergillus terreus, inhibit HMG-CoA reductase.

Regulation by Oxysterols:

• Oxysterols inhibit cholesterol synthesis by blocking SREBP activation.

• 24(S),25-epoxycholesterol can also repress 3-hydroxy-3-methyl-glutaryl coenzyme A reductase activity.

In addition, several other compounds impact cholesterol synthesis or related processes:

• Cyclodextrins: Methyl-β-cyclodextrin (MβCD) and (2-hydroxypropyl)-β-cyclodextrin (HPCD) deplete cellular cholesterol.

• Mipomersen: This antisense oligonucleotide reduces apoB-100 protein synthesis, thus reducing VLDL secretion by the liver.

• Farnesyl-PP: Conditions that decrease farnesyl-PP levels stabilize HMG-CoA reductase levels, whereas conditions that increase farnesyl-PP levels accelerate HMG-CoA reductase degradation.

• Short chain fatty acids: Mixtures of short chain fatty acids have been shown to suppress cholesterol synthesis in the rat liver and intestine.

Cholesterol synthesis is tightly regulated to maintain a steady level in the body and prevent abnormal deposition. It involves multiple enzymes in the cytoplasm, endoplasmic reticulum (ER), and peroxisomes.

The key regulatory mechanisms include:

• HMGR Regulation: The primary means for controlling cholesterol biosynthesis is the regulation of HMG-CoA reductase (HMGR) activity.
  
  

  • Feedback Inhibition: Cholesterol acts as a feedback inhibitor of pre-existing HMGR and induces rapid degradation of the enzyme. This is facilitated by the sterol sensing domain (SSD) of HMGR.

  • Gene Expression: Cholesterol excess reduces HMGR mRNA levels by decreasing gene expression.

  • Enzyme Degradation: Cholesterol induces polyubiquitylation of HMGR, leading to its degradation in the proteasome. HMGR stability is regulated based on the flux through the mevalonate synthesis pathway; high flux increases HMGR degradation, while low flux decreases it. Lanosterol interacts with Insig proteins, inducing their binding to the SSD of HMGR and recruitment of ubiquitin ligases RNF139 and AMFR.

  • Phosphorylation-Dephosphorylation: HMGR activity is also controlled by phosphorylation.

• Regulation of Intracellular Free Cholesterol: Excess intracellular free cholesterol is regulated by sterol O-acyltransferases SOAT1 and SOAT2, with SOAT2 being predominant in the liver.
  
  

• LDL Receptor-Mediated Uptake and HDL-Mediated Reverse Transport: Plasma cholesterol levels are regulated via LDL receptor-mediated uptake and HDL-mediated reverse transport.
  
  

• SREBP Activation: The intracellular sterol content is altered through the regulation of key sterol synthetic enzymes and cell-surface LDL receptors, which is mediated by sterol-regulated transcription of rate-limiting enzymes. Activation of transcriptional control occurs through the regulated cleavage of the membrane-bound transcription factor sterol regulatory element binding protein (SREBP). Sterol control of transcription affects genes involved in cholesterol, triglyceride, phospholipid, and fatty acid biosynthesis, requiring an octamer sequence called the sterol regulatory element (SRE-1).
  
  

• Role of GPR146: Dietary cholesterol is absorbed in the small intestines via carboxyl ester lipase (CEL) and Niemann-Pick C1-like-1 (NPC1L1). NPC1L1-mediated cholesterol uptake stimulates the production of cholesin, a gut hormone that regulates hepatic cholesterol synthesis.
  
  

• LXR Regulation: The nuclear Liver X Receptor (LXR) is activated by sterol ligands and upregulates genes including those encoding ABCA1 and ABCG1, which export cholesterol from the cell.
  
  

• Dietary Impact:
  
  

  • Dietary cholesterol has a relatively weak effect on serum cholesterol concentration due to powerful regulatory systems.

  • Saturated fatty acids with 12-16 carbons raise cholesterol, while stearic acid (18:0) may slightly reduce it.

  • Short chain fatty acids can suppress cholesterol synthesis in the liver and intestine.

• Hormonal and Nutritional Regulation:
  
  

  • Thyroid hormones, glucocorticoids, and insulin can modulate bile acid synthesis.

  • Dietary fats modulate the regulatory potential of dietary cholesterol on cholesterol 7α-hydroxylase gene expression.

• Other Factors:
  
  

  • Isoprenoid synthesis is regulated by the sterol end-product of the biosynthetic pathway, by non-sterol intermediates, and also by physiological factors.

  • Diurnal rhythm, insulin and glucagon, thyroid hormone, glucocorticoids, estrogen, and bile acids influence cholesterol synthesis through transcriptional, translational, and post-translational mechanisms.

• Inhibitors of Cholesterol Synthesis:
  
  

  • Statins inhibit HMG-CoA reductase, reducing hepatic cholesterol content and up-regulating LDL receptors.

  • Distal inhibitors like AY9944 and BM15766 inhibit 7-dehydrocholesterol reductase, while triparanol inhibits 24-dehydrocholesterol reductase.

  • Oxysterols are potent suppressors of cholesterol synthesis.

Normal Cholesterol Levels:

• Historically, the normal range for cholesterol was determined statistically from the general population. This meant the "normal range" extended to 300 mg/dL (7.76 mmol/L), with only the top 2.5% of the population considered as having dangerously high cholesterol. Practitioners sometimes increased this range for older patients.

• Contemporary guidelines place blood cholesterol levels within the context of a global heart disease risk profile. A risk score is computed referencing the probability of a heart attack within 10 years, with recurring assessment at five-year intervals beginning in young adulthood.

Desirable Cholesterol Levels (mg/dL):

• Total Cholesterol: Less than 200 is considered desirable.

• LDL-C: Less than 100 is considered optimal.

• HDL-C: Greater than 60 is considered high. HDL-C less than 40 mg/dl is considered low for men, and HDL-C of 50 mg/dl is considered low for women.

• Triglycerides: Less than 150 is considered normal.

Methods of Cholesterol Estimation:

• Colorimetric enzymatic assay: This method relies on the production of hydrogen peroxide and cholestanone from cholesterol by the enzyme cholesterol oxidase. This method may detect precursor sterols that can act as substrates for cholesterol oxidase.

• Enzymatic Procedures: Cholesterol concentrations may be determined using enzymatic procedures.

• INTEGRA analyzer: Lipid fractions such as HDL-cholesterol and triacylglycerols (TG) in the plasma can be measured enzymatically with INTEGRA analyser.

• Gas Chromatography-Mass Spectrometry (GC-MS): This method can be used to identify sterol molecules. This method allows for both qualitative and quantitative characterization of sterol molecular lipid species. Lipids are isolated from matrix or matrices by homogenization and solvent extraction, and converted into species amenable for ionization either by derivatization or adduct formation.

• TLC: Sterols can be analyzed by Thin Layer Chromatography.

• Other methods Electrophoretic or immunological methods

• Isotope methods: Methods using GC/MS can measure activity of cholesterol synthetic enzymes, using labeled sterol intermediates and measuring substrate conversion levels to determine relative enzymatic activity.

• Direct Injection Mass Spectrometry (DI-MS/MS): This method can be used for quantification of oxysterols by comparison of specific SRM transitions with their heavy isotopes.

Cholesterol Basics

1. Question: What is the role of cholesterol in the body? Answer: Cholesterol is a major component of cell membranes and a precursor for steroid hormones, bile acids, and vitamin D.

2. Question: Where can pure cholesterol crystals be found in the body? Answer: The sources do not specify where pure cholesterol crystals are found in the body.

3. Question: What is the structure of cholesterol? Answer: Cholesterol is a steroid with four fused rings.

4. Question: How many carbons are there in cholesterol, and what is their source? Answer: There are 27 carbons in cholesterol, all derived from acetyl-CoA.

5. Question: What are the three compounds based on isoprenoid units that play a role in cholesterol biosynthesis? Answer: The sources do not explicitly list three isoprenoid compounds involved in cholesterol synthesis.

6. Question: What is the rate-limiting enzyme in cholesterol biosynthesis? Answer: HMG-CoA reductase is the key and rate-limiting enzyme in cholesterol biosynthesis.

7. Question: What is the starting material for cholesterol biosynthesis? Answer: Acetyl-CoA is the starting material for cholesterol biosynthesis.

8. Question: Where is the enzyme system for cholesterol biosynthesis located? Answer: The enzyme system of cholesterol biosynthesis is associated with cytoplasmic particles (microsomes) and the soluble fraction of the cytosol.

9. Question: What is the key intermediate in cholesterol synthesis that is produced from HMG-CoA? Answer: Mevalonate is produced from HMG-CoA.

10. Question: How is HMG-CoA reductase regulated? Answer: HMG-CoA reductase is regulated at the transcriptional level, and by dephosphorylation which is mediated through Cyclic AMP dependent protein kinase.

11. Question: How does increased cAMP affect cholesterol synthesis? Answer: Increased cAMP inhibits cholesterol synthesis by converting HMG-CoA reductase to its inactive form.

12. Question: What is the role of squalene in cholesterol biosynthesis? Answer: The sources do not directly explain the role of squalene in cholesterol synthesis.

13. Question: What is the significance of the conversion of lanosterol to cholesterol? Answer: The sources do not detail the significance of lanosterol conversion to cholesterol.

14. Question: Which enzyme converts cholesterol to 7-α-hydroxycholesterol? Answer: Cholesterol 7-alpha-hydroxylase converts cholesterol to 7-alpha-hydroxycholesterol.

15. Question: What are the two main types of cholesterol found in the body? Answer: Free cholesterol and cholesterol ester.

16. Question: Which form of cholesterol is in greater metabolic demand? Answer: Free cholesterol is in greater metabolic demand because it is converted to steroid hormones and bile acids.

17. Question: How is ester cholesterol utilized? Answer: Ester cholesterol must first be hydrolyzed by the enzyme cholesterol ester hydrolase before it can be utilized.

Cholesterol Transport

18. Question: How are lipids transported in the blood? Answer: Lipids are transported in the blood via lipoproteins.

19. Question: What are lipoproteins? Answer: Lipoproteins are complexes of lipids and proteins that transport lipids in the blood.

20. Question: What are the major classes of lipoproteins? Answer: Major classes of lipoproteins include chylomicrons, VLDL, LDL, and HDL.

21. Question: Which lipoproteins are primarily involved in triacylglycerol transport? Answer: Chylomicrons and VLDL are primarily involved in triacylglycerol transport.

22. Question: Which lipoprotein is largest? Answer: Chylomicrons are the largest lipoproteins.

23. Question: Which lipoprotein is the densest? Answer: HDL is the densest lipoprotein.

24. Question: Which lipoprotein is considered “bad” cholesterol? Answer: LDL is considered “bad” cholesterol.

25. Question: Which lipoprotein is considered “good” cholesterol? Answer: HDL is considered "good" cholesterol.

26. Question: Which lipoprotein is instrumental in removing cholesterol from plaques on arteries? Answer: HDL is instrumental in removing cholesterol from plaques on arteries.

27. Question: How does LDL deliver cholesterol to cells? Answer: The sources do not explicitly detail the mechanism of cholesterol delivery by LDL.

28. Question: What makes LDL soluble in blood plasma? Answer: The sources do not detail what makes LDL soluble in the blood plasma.

29. Question: What is the role of apolipoproteins in lipoprotein function? Answer: Apolipoproteins are the protein components of lipoproteins.

30. Question: What are the functions of apolipoproteins A1, B100, C, and E in the processing of lipoproteins? Answer: The specific functions of these apolipoproteins are not detailed in the provided sources.

31. Question: What is the function of lipoprotein lipase? Answer: Lipoprotein lipase hydrolyzes triglycerides in chylomicrons.

32. Question: What is the role of LCAT (lecithin-cholesterol acyltransferase)? Answer: LCAT helps in the conversion of cholesterol to cholesterol ester.

33. Question: What is the role of ACAT? Answer: ACAT esterifies cholesterol by taking cholesterol from Acyl CoA.

34. Question: What is reverse cholesterol transport? Answer: The sources do not explicitly define reverse cholesterol transport.

35. Question: What are chylomicrons? Answer: Chylomicrons transport dietary lipids from the intestine.

36. Question: What are VLDLs? Answer: VLDLs transport endogenously synthesized triglycerides.

37. Question: What are IDLs? Answer: The sources do not detail IDLs in lipoprotein metabolism.

38. Question: What is the metabolic fate of LDL? Answer: The sources indicate that LDL interacts with cell membranes via LDL receptors to increase the concentration of cellular cholesterol.

39. Question: How does nascent intestinal HDL differ from nascent hepatic HDL? Answer: The source does not describe the differences between nascent intestinal and hepatic HDL.

40. Question: What is the scavenging action of HDL? Answer: The sources refer to the "scavenging" action of HDL, but do not provide specifics.

41. Question: How are dietary lipids transported from the intestine? Answer: Dietary lipids are transported from the intestine in chylomicrons.

42. Question: What is the main transporter of cholesterol to peripheral tissues? Answer: LDL is the main transporter of cholesterol to peripheral tissues.

43. Question: Which lipoprotein has the maximum amount of apoproteins? Answer: HDL has the maximum amount of apoproteins.

Cholesterol Regulation and Disorders

44. Question: What is hypercholesterolemia? Answer: Hypercholesterolemia is an increase in serum cholesterol level above normal.

45. Question: What conditions cause hypercholesterolemia? Answer: Hypercholesterolemia can occur in diabetes mellitus, nephrotic syndrome, obstructive jaundice, myxedema, xanthomatous biliary cirrhosis and idiopathic hypercholesterolemia.

46. Question: What is hypocholesterolemia? Answer: Hypocholesterolemia is a decrease in blood cholesterol levels below normal.

47. Question: What are some conditions that cause hypocholesterolemia? Answer: The sources do not provide conditions associated with hypocholesterolemia.

48. Question: What is familial hypercholesterolemia? Answer: Familial hypercholesterolemia is due to impaired clearance of LDL from the plasma.

49. Question: How does a defect in LDL receptors affect blood cholesterol levels? Answer: Defective LDL receptors leads to high blood cholesterol levels.

50. Question: What is abetalipoproteinemia? Answer: Abetalipoproteinemia is a disorder associated with lipoprotein metabolism.

51. Question: What are the effects of low carnitine levels on metabolism? Answer: The sources do not detail the effects of low carnitine on metabolism.

52. Question: What is the link between cholesterol, HDL, and myeloproliferative diseases? Answer: The sources do not specify the link between cholesterol, HDL, and myeloproliferative diseases.

53. Question: What is the effect of dietary saturated fatty acids on blood cholesterol? Answer: Dietary fats containing high saturated fatty acids increase blood cholesterol levels.

54. Question: What is the effect of dietary polyunsaturated fatty acids on blood cholesterol? Answer: Consumption of polyunsaturated fatty acids decreases blood cholesterol levels.

55. Question: What are statins? Answer: Statins are a class of pharmaceuticals.

56. Question: How do statins affect cholesterol synthesis? Answer: The sources indicate that statins are pharmaceuticals that affect cholesterol synthesis.

57. Question: How do bile salts aid in lipid emulsification? Answer: The amphipathic nature of bile salts aids in dietary lipid emulsification.

58. Question: What role does the liver play in fat metabolism? Answer: The liver plays a significant role in fat metabolism.

59. Question: How do cells adjust the fluidity of their membranes? Answer: The sources do not explain how cells adjust membrane fluidity.

60. Question: What is the primary method of transporting free fatty acids in the blood? Answer: The sources do not state the method of transporting free fatty acids in the blood.

Metabolic Pathways and Enzymes

61. Question: What is the function of HMG-CoA synthase? Answer: The sources do not mention HMG-CoA synthase directly.

62. Question: What are the key features of cholesterol synthesis? Answer: The sources detail cholesterol synthesis by highlighting the role of HMG-CoA reductase and its regulation.

63. Question: What is the role of squalene monooxygenase in cholesterol biosynthesis? Answer: The sources do not detail the function of squalene monooxygenase.

64. Question: What is the role of cholesterol esterase in digestion? Answer: Cholesterol esterase can catalyze both the esterification and hydrolysis of cholesterol.

65. Question: What is the role of the enzyme 7-alpha hydroxylase? Answer: The 7-alpha hydroxylase is the rate limiting enzyme in bile acid synthesis.

66. Question: What is the significance of the enterohepatic circulation of bile salts? Answer: Interfering with the enterohepatic circulation of bile salts is useful in hypercholesterolemia treatment.

67. Question: What is the function of squalene epoxidase? Answer: The sources do not discuss the function of squalene epoxidase.

Clinical Aspects 

68. Question: What are the clinical disorders associated with lipoprotein metabolism? Answer: The sources refer to clinical disorders associated with lipoprotein metabolism, but do not specify them. 

69. Question: What is Tangier's disease? Answer: In Tangier's disease the cellular protein ABCl, is lacking which is required for cholesterol uptake by HDL. 

70. Question: What is the total cholesterol if LDL-C = 136 mg/dl, HDL-C = 45 mg/dl, and TAGs = 150 mg/dl? Answer: The provided sources do not describe the formula used to calculate total cholesterol. 

71. Question: What is the relationship between dietary fat and atherosclerosis? Answer: The sources indicate that dietary lipids influence the incidence of atherosclerosis. 

72. Question: What is the significance of a broad beta band on lipoprotein electrophoresis? Answer: A broad beta band on electrophoresis is associated with type II hyperlipoproteinemia. 

73. Question: What are some risk factors for coronary artery disease? Answer: The sources mention risk factors for coronary artery disease, but do not specify them. 

74. Question: What is the metabolic fate of cholesterol within cells? Answer: Cholesterol within cells can be converted to steroid hormones and bile acids. 

75. Question: How does a carbohydrate-rich diet affect lipoprotein levels? Answer: A person on a fat-free, carbohydrate-rich diet can continue to grow obese, due to increased lipoproteins.

Key recent advances in knowledge on cholesterol metabolism include:

• Understanding the role of lipids in diseases such as heart disease, diabetes, obesity, stroke, cancer, and neurological diseases.

• Realization that lipids participate widely in signaling pathways that impact basic biological processes.

• Elucidation of the details of the enzymes of fatty acid biosynthesis.

• Identification of proteins that interact with Ino4p, indicating there is still much to learn about transcriptional regulation of phospholipid biosynthetic genes in yeast.

• Recognition of sphingosine as a potent inhibitor of protein kinase C.

• Insight that cellular functions of sphingolipids may reside not only in the complex species, but also in the lipid backbones.

• New ideas about how sphingolipids are involved in disease etiology, and how naturally occurring and/or synthetic sphingolipids may be useful in disease prevention and treatment.

• Discovery that bile acids are active regulators of cellular processes, such as signal transduction, by influencing the activity of proteins involved in signaling cascades, and gene expression by influencing the turnover of specific mRNA species as well as by serving as the natural ligand for the nuclear receptor FXR.

• The finding that cholesterol can be an alternative form of lipid storage in mycobacteria.

• Identification of ABCA1 and SR-BI.

• The recognition that the efflux of cholesterol from cells to plasma lipoproteins is highly regulated by cell membrane proteins.

• The finding that certain oxysterols are activators of nuclear transcription factors.

• The finding that the lysosomal protein synaptotagmin VII (Syt7) and the peroxisomal lipid PI (4, 5)P2 mediated transient membrane contacts between lysosome and peroxisome, allowing cholesterol to move from one organelle to another.

• Identification of novel aspects of the cholesterol synthetic pathway, and interacting pathways, by measuring the activity of the terminal enzymes DHCR7 and DHCR24 via gas chromatography/mass spectrometry (GC/MS).

• Recognition that ABCA1 prefers to use sterols newly synthesized endogenously, not cholesterol derived from LDL, or cholesterol being recycled within the cells, as its preferred substrates.

• Identification of five possible probiotic mechanisms including assimilation of cholesterol during growth, binding of cholesterol to cellular surface, disruption of cholesterol micelle, deconjugation of bile salt and bile salt hydrolase (BSH) activity.

• Use of omics technology, which provides new insight into the mechanisms of lipid metabolism influenced by probiotics.

• The knowledge that the luminal diameter of coronary epicardial and resistance vessels and major peripheral arteries is highly dynamic in response to flow-mediated (shear stress) and agonist-mediated (nitric oxide and endothelin-1) factors has greatly advanced the understanding of atherosclerosis.

• The finding that diet affects new lipid and lipoprotein risk factors for CVD such as LDL and HDL size, lipoprotein (a), and postprandial lipids and lipoproteins.

• Increased awareness of the need to study other lipids for their roles in the process of mitochondrial morphodynamics.

• The use of mass spectrometric analysis to monitor lipids at the level of molecular species.

Cholesterol metabolism is a complex process involving synthesis, transport, and degradation, critical for various bodily functions.

Cholesterol Synthesis:

• Cholesterol is synthesized de novo from acetyl-CoA, primarily in the liver and intestines, but also in other tissues.

• The synthesis pathway is endergonic and requires NADPH and ATP.

• The rate-limiting step is the conversion of HMG-CoA to mevalonate, catalyzed by HMG-CoA reductase, which is targeted by statin drugs.

• The pathway involves multiple steps including the formation of mevalonate from HMG-CoA, the synthesis of isoprene units, squalene formation, and cyclization to form cholesterol.

• Cholesterol synthesis is regulated by intracellular cholesterol concentration, hormones like insulin and glucagon, and gene expression of HMG-CoA reductase.

• When dietary cholesterol is sufficient, synthesis is suppressed.

• HMG-CoA reductase is inhibited by cholesterol, bile salts and mevalonate, and induced when blood insulin levels are elevated.

• Insulin promotes cholesterol synthesis, while glucagon inhibits it.

• The sterol regulatory element-binding protein (SREBP) pathway regulates the transcription of genes involved in cholesterol uptake and metabolism.

• The cholesterol synthesis occurs in the cytoplasm and smooth endoplasmic reticulum (SER).

Cholesterol Transport:

• Cholesterol is transported in the blood via lipoproteins, including chylomicrons, VLDL, LDL, and HDL.

• Chylomicrons transport dietary cholesterol from the intestines to the liver.

• VLDL and LDL transport cholesterol from the liver to peripheral tissues (forward cholesterol transport).

• HDL transports cholesterol from peripheral tissues back to the liver (reverse cholesterol transport).

• The liver is a major site for cholesterol metabolism, including synthesis, uptake, and excretion.

• Cholesterol efflux from cells to HDL is facilitated by the cholesterol efflux regulatory protein (CERP), an ATP-binding protein transporter.

• Cholesterol is transported across cell membranes through specific transporters and via endocytosis of lipoproteins.

• Cholesterol ester transfer protein (CETP) mediates the exchange of cholesterol esters between lipoproteins.

• Free cholesterol is in greater metabolic demand than esterified cholesterol as it is converted to steroid hormones and bile acids.

• The enzyme lecithin-cholesterol acyl transferase (LCAT) present in plasma, esterifies cholesterol for transport.

Cholesterol Degradation and Excretion:

• The ring structure of cholesterol cannot be degraded in humans, instead it is eliminated from the body as bile salts, steroid hormones, or through secretion into the bile.

• The liver converts cholesterol to bile acids, which are then excreted.

• Cholesterol is also excreted directly into the bile.

• The rate-limiting step in bile acid synthesis is catalyzed by cholesterol-7-α-hydroxylase, which is inhibited by bile acids.

• Biliary excretion is the primary route for cholesterol elimination.

• The liver regulates cholesterol levels through mechanisms that synchronize cholesterol synthesis, cholesterol intake and LDL receptors.

Regulation of Cholesterol Levels

• Cellular cholesterol content is regulated by de novo synthesis and external supply.

• Factors that inhibit the activity of HMG-CoA reductase include high membrane cholesterol concentration and a decrease in the concentration of HMG-CoA.

• Cholesterol regulates its own degradation by stimulating the transcription of cholesterol 7a-hydroxylase.

• Disturbances in cholesterol metabolism may cause diseases such as familial hypercholesterolemia where cholesterol uptake via LDL receptors is diminished.

• High serum cholesterol is associated with heart disease.

• Statins lower plasma cholesterol by inhibiting HMG-CoA reductase.

Key Molecules and Processes

• HMG-CoA Reductase: The enzyme catalyzing the rate-limiting step in cholesterol synthesis.

• Acyl-CoA-cholesterol acyltransferase (ACAT): An enzyme involved in cholesterol esterification within cells.

• Lecithin-cholesterol acyl transferase (LCAT): An enzyme that produces cholesterol esters in plasma.

• Cholesterol ester transfer protein (CETP): Mediates the transfer of cholesterol esters between lipoproteins.

• Sterol regulatory element-binding protein (SREBP): Transcription factor regulating cholesterol metabolism.

Clinical Significance

• Elevated plasma cholesterol levels are a risk factor for atherosclerosis and coronary heart disease.

• Dyslipidemia refers to conditions like familial hypercholesterolemia and familial dyslipidemia which are associated with abnormal lipoprotein metabolism.

• Statins and PCSK9 inhibitors are used to lower cholesterol.

HMG-CoA reductase is a key enzyme in the synthesis of cholesterol. It catalyzes the reduction of HMG-CoA to mevalonate, which is the rate-limiting and committed step in cholesterol synthesis. The enzyme is found in the smooth endoplasmic reticulum (SER) membrane with its catalytic domain projecting into the cytosol.

• Role in Cholesterol Synthesis: HMG-CoA reductase facilitates the conversion of HMG-CoA to mevalonate, a crucial six-carbon molecule that serves as a precursor for cholesterol. This reaction is irreversible and requires two molecules of NADPH as a reducing agent, releasing CoA in the process.

• Regulation: The activity of HMG-CoA reductase is tightly regulated at multiple levels:

  • Feedback Inhibition: Cholesterol, bile salts, and other sterols can inhibit HMG-CoA reductase activity. High levels of cholesterol reduce the transcription of the HMG-CoA reductase gene and accelerate the degradation of the enzyme.

  • Hormonal Control: Insulin increases HMG-CoA reductase activity by promoting its dephosphorylation, while glucagon and epinephrine have the opposite effect. Insulin stimulates cholesterol synthesis, and glucagon stimulates the inhibition of HMG-CoA reductase, shutting down the synthetic pathway.

  • Phosphorylation/Dephosphorylation: The enzyme's activity is controlled by phosphorylation and dephosphorylation. The phosphorylated form of HMG-CoA reductase is inactive, while the dephosphorylated form is active. This is regulated by AMP-activated protein kinase (AMPK), which phosphorylates and inactivates HMG-CoA reductase. When ATP levels are low, AMPK is activated, decreasing cholesterol synthesis.

• Sterol-accelerated degradation: When sterol levels are high, HMG-CoA reductase binds to INSIG proteins, leading to ubiquitination and degradation of the reductase.

• Transcriptional Control: When intracellular cholesterol levels are low, SREBP (sterol regulatory element-binding protein) moves to the Golgi, where it is cleaved and the active fragment enters the nucleus to increase the expression of HMG-CoA reductase.

• Inhibition by Drugs: Statins are a class of drugs that act as competitive inhibitors of HMG-CoA reductase. They bind to the active site of the enzyme, preventing HMG-CoA from binding and thus reducing cholesterol synthesis.

• Location and Structure: HMG-CoA reductase is an integral membrane protein of the SER with its catalytic domain in the cytosol. The enzyme consists of two HMGR dimers that associate to form a tetramer. Each dimer has an active site at the interface between the two dimers.

• Role in Ketogenesis: HMG-CoA is also an intermediate in ketone body synthesis, however, the HMG-CoA used in ketogenesis is produced in the mitochondria whereas the HMG-CoA used for cholesterol synthesis is produced in the cytosol. HMG-CoA lyase, which converts HMG-CoA to acetoacetate and acetyl-CoA, is present in the mitochondria.

• Relationship to other pathways: HMG-CoA can be converted to mevalonate in the cytosol and to acetyl-CoA and acetoacetate in the mitochondria. A decrease in HMG-CoA reductase can lead to an accumulation of HMG-CoA which is then converted into acetoacetate by HMG-CoA lyase.