Cholesterol and Bile Salts

• Presentation is titled "Cholesterol & Bile salts", prepared for 1st-grade MBM module, delivered by Dr. Basil O Saleh & Dr. Maysaa J Majeed on April 8, 2025

Objectives

• Define the cholesterol family and its blood forms
• Illustrate cholesterol homeostasis
• List dietary cholesterol sources
• Briefly describe cholesterol biosynthesis precursors (endogenous pathway)
• Define the regulatory enzyme in cholesterol biosynthesis
• Define the action of cholesterol-lowering drugs from a biochemical perspective
• Group vital compounds derived from cholesterol
• Determine the types & functions of bile salts

Core Clinical Problems (CCPs) 21: Chest Pain

• Mr. Ahmed, 57, has been experiencing chest pain for a couple of months
• He also complains of tightness in his chest
• He is under pressure at work due to an approaching project deadline
• His mother had "bad cholesterol" and died at 65 due to a "heart problem"
• Physical examination reveals that he is slightly pale
• His BP is 140/80 mm Hg and his heart rate is 80/min and irregular
• Blood test results shows low HDL-cholesterol and high LDL-cholesterol
• Other blood parameters are normal
• Following additional tests, Mr. Ahmed was diagnosed with Coronary Artery Disease (Angina Pectoris)

Cholesterol Composition and Origin

• Cholesterol is a soft, fatty substance belonging to the sterol group
• It is mainly produced endogenously (85%)
• A small fraction is derived from diet (15%)
• Serum cholesterol measurement does not require fasting
• Dietary ingestion has a negligible effect on the final result

Cholesterol Importance and Forms

• Cholesterol is a major sterol in animal tissues
• Beta-sitosterol found in plants, it is poorly absorbed by humans and blocks animal dietary cholesterol absorption
• Cholesterol is vital for cell and tissue function, not inherently bad
• High cholesterol levels (hypercholesterolemia) causes accumulation in blood vessels which disrupts function leading to bad lipid consequences

Cholesterol Transport

• Two-thirds of plasma cholesterol is esterified
• Free OH at C3 combines with RCO- of fatty acid RCOOH
• Remainder one-third is free cholesterol, which has free OH at C3
• Free cholesterol (Fch) is more polar (charged) than esterified cholesterol (Ech)
• Both cholesterol types need association with specific proteins to form Lipoprotein compounds to be transported in the blood
• Normal serum or plasma total cholesterol is less than 200 mg/dl

Cholesterol Transport and Excretion

• Cholesterol is carried in the blood by low-density lipoprotein-cholesterol (LDL-C; 2/3) and high-density lipoprotein cholesterol (HDL-C, 1/3)
• The body converts ~50% of amounts of cholesterol beyond the needed into bile acids in the liver, and ~50% is excreted as unmetabolized cholesterol via bile and intestines into feces
• Total cholesterol = Fch + Ech = LDL + HDL + very small amount in VLDL-Cholesterol

Cholesterol Sources and Excretion

• Cholesterol sources arise from diet, hepatic de novo synthesis, and synthesis in extrahepatic tissues
• Cholesterol is then secreted via HDL & VLDL (and so LDL), found as free cholesterol in bile, or converted to bile salts/acids

Dietary Cholesterol Sources

• Egg yolks are the highest source of dietary cholesterol
• Oils high in saturated and trans fats (palm and coconut oils) increase blood cholesterol levels
• Meat and poultry are major sources of cholesterol, with organ meats being higher
• Removing skin from poultry helps reduce cholesterol intake
• Dairy products (cheese, ice cream, milk) are primary sources of saturated fat and cholesterol
• A tablespoon of butter contains 10% of the daily cholesterol value
• A cup of whole milk provides 12% of the daily value
• Commercially prepared baked goods and processed foods are typically high in trans fat, raising LDL and decreasing HDL
• A single doughnut contains 8% of the daily cholesterol value

Dietary Cholesterol Facts

• Animal products like eggs contribute to dietary cholesterol
• About 50% of dietary cholesterol gets absorbed
• Increased cholesterol intake is typically lead to decreased absorption
• On average, 1g/day is excreted as bile acids

Functions of Cholesterol

• It is a membrane component (Fch) of the cell wall and membranes enclosing organelles like the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus
• Cholesterol is a precursor to bile acids/salts
• It is a precursor to Vitamin D3
• It is a precursor to steroid hormones, including male & female sex hormones (testosterone, androgens, Estrogens) and adrenal cortex hormones (glucocorticoids like cortisol & mineralocorticoids like aldosterone)

Cholesterol Synthesis

• Occurs in the cytoplasm
• Requires NADPH & ATP
• Highly regulated, HMG-CoA reductase is the regulatory enzyme
• Synthesis occurs 80% in the liver, ~10% in the intestine, and 10% in the brain, skin, adrenal cortex & reproductive tissues
• Precursors are carbohydrates & fatty acids
• HMG-CoA reductase; Hydroxy Methyl Glutaryl-CoA reductase

Cholesterol Synthesis Pathway

• Glucose is converted to Citrate
• Acetyl-CoA is then produced along with Acetate
• Fatty acids are produced together
• Acetoacetate-CoA is produced
• HMG-CoA is the result
• HMG-CoA reductase+NADPH act as a cofactor, turning it into Mevalonate
• Which is then converted to Sequalene
• Final step is Cholesterol creation

HMG-CoA Reductase Regulation

• The regulatory step is at HMG-CoA reductase
• The enzyme and cholesterol synthesis is stimulated post CHO diet & in the presence of insulin
• The opposite occurs in prolonged fasting & presence of glucagon
• Cholesterol synthesis is highly regularized and responsive to changes in cholesterol concentration
• Regulatory mechanism (step of HMG-CoA reductase) balances cholesterol synthesis and excretion

Cholesterol Imbalance

• Imbalance leads to elevated plasma or serum cholesterol (Hypercholesterolemia), associated with coronary artery disease (CAD)
• Patients with hypercholesterolemia are treated with statins (atorvastatin, pravastatin, simvastatin)
• These drugs regulate blood cholesterol levels by inhibiting HMG-CoA reductase, thus stopping the endogenous pathway and controlling blood and body cholesterol

Bile Acids and Salts

• Organic compounds consisting of primary bile acids (cholic and chenodeoxycholic acids), synthesized from cholesterol in the liver
• Cholesterol-7-alpha-hydroxylase is the regulatory enzyme of this pathway
• Cholesterol stimulates it while cholic acid inhibits it

Bile Acid Conjugation and Formation

• Bile acids conjugate with glycine and taurine (derived from cysteine) to form bile salts (more polar, water soluble)
• Bile salts are either secreted into the small intestine/duodenum or stored in the gall bladder
• Intestinal flora in the small intestine converts primary bile acids into secondary bile acids: cholic acid gets turned into deoxycholic acid
• Chenodeoxycholic acid gets turned into lithocholic acid

Bile Salts Function and Action

• Bile salts produced are synthesized in the liver from cholesterol
• Primary bile acids include Cholic acid and Chenodeoxy cholic acid
• Bile acids get conjugated in liver with Taurine & choline
• Cholic Acid + Taurine = Taurocholic Acid
• Cholic Acid + Glycine = Glycocholic acid
• Bile salts aid in fat digestion and absorption
• Intestinal flora removes glycine and taurine from bile salts, regenerating bile acids
• Primary bile acids can be converted into secondary bile acids by removing a hydroxyl group, producing deoxycholic acid from cholic acid and lithocholic acid from chenodeoxycholic acid

Enterohepatic Circulation of Bile Salts

• 90-95% of bile salts that enter the duodenum are reabsorbed back into the portal vein
• The bile salts will return to the liver
• This process is known as Entero Hepatic Circulation of Bile salts
• Remaining 5-10% enters the colon, acted upon by colon bacteria to form Deoxy cholic acid & lithocholic acid

Bile Salt Composition

• Bile salts are the most important components of bile mixture and bile duct (involved in excretion of many substances from the liver like bilirubin & drugs)
• Bile consists of 85% water, 10% bile salts (Cholic, chenodeoxycholic, deoxycholic, and lithocholic acid), mucus, 3% pigments (bilirubin glucuronide), 1% fats (Phospholipids (lecithin), cholesterol) and 0.7% inorganic salts
• It has a daily secretion of 500-1000ml
• It is a transparent alkaline fluid and light yellow in colour
• Its pH: 7.8 – 8.6

Bile Salts Importance and Digestion

• Bile salts are crucial for digestion and absorption of fat (lipid) and fat-soluble vitamins (D, K, A, and E)
• They act as emulsifying agents in the intestine, aiding lipid handling and exposure to pancreatic enzymes
• Bile acids and salts are mechanism for excess cholesterol excretion via the bile duct, small intestine, and ultimately feces

Summary of Cholesterol and Bile Salts

• Cholesterol is a major lipid component, existing as free (hydrophilic) and esterified (hydrophobic) forms
• Mainly produced endogenously, with a small fraction from diet; primarily found in LDL and HDL lipoproteins
• Synthesized mainly in the liver, intestine, and gonads, with HMG-CoA reductase as the regulatory enzyme under feedback control
• Homeostasis involves intestinal excretion and hepatic primary bile acid synthesis
• Bile acids and bile salts are crucial for fat and fat-soluble vitamin digestion and absorption

Case Scenario

• A 45-year-old man with a 5-year history of hypertension presents for biochemical investigations and has serum cholesterol levels of 250 mg/dl
• Discussion points:
  • Discuss the physiological importance of cholesterol
  • Illustrate the basic points of cholesterol biosynthesis
  • Define the types of bile acids and their functions
  • Discuss the subtypes of cholesterol in blood