Bile Synthesis Lecture Notes

Introduction

• Focus: Bile acid and bile salt synthesis.
• Location: Takes place in the liver, specifically in hepatocytes.

Anatomy Overview

• Liver: Site of bile synthesis.
• Gallbladder: Stores and concentrates bile, secretes it into small intestines.
• Small Intestines: Consist of duodenum, jejunum, ileum.
• Portal Vein/System: Absorbs vitamins, electrolytes, nutrients for transport back to the liver.

Composition of Bile

• Water: More than 95% of bile.
• Bile Acids and Salts: Second most prominent components.
  • Emulsification: Helps break down fats for enzyme action.
  • Absorption: Important for fat-soluble substances (triglycerides, cholesterol, phospholipids, vitamins D, E, K, A).
• Other Components:
  • Bilirubin
  • Cholesterol
  • Fatty acids
  • Proteins, amino acids
  • Ions like sodium and bicarbonate

Bile Acid and Salt Synthesis

• Cholesterol Source: From diet or synthesized from Acetyl-CoA.
• Conversion: Cholesterol is converted into colic acid and chenodeoxycholic acid (primary bile acids).
  • Enzyme: CYP7A1 (rate-limiting step from cytochrome P450 family).
• Primary Bile Acids: Colic acid, Chenodeoxycholic acid.

Secondary Bile Acids

• Formation: Bacteria in intestines convert primary bile acids to secondary ones.
  • Colic Acid → Deoxycholic Acid
  • Chenodeoxycholic Acid → Lithocholic Acid
• Fate: 5% excreted in feces; 95% recirculated back to liver via portal vein.

Conjugation in Liver

• Amino Acids: Glycine and Taurine conjugate with bile acids to form bile salts.
  • Glycine: Forms glycolithocolic acid, glycolic salt.
  • Taurine: Forms taurocholic acid, taurochenodeoxycholic acid.
• Properties: Negatively charged, water-loving, aiding in emulsification.

Enterohepatic Circulation

• Process: Bile acids and salts recirculate between liver and intestines.
• Unconjugation: Bacteria can remove conjugation, restarting cycle.

Function in Fat Digestion

• Ingestion: Fats stimulate enteroendocrine cells in duodenum.
• Hormone Release: Cholecystokinin (CCK) causes gallbladder to contract, releasing bile.
• Bile Function: Bile acids bind fats, amino acids remain water-exposed for emulsification.
• Lipase Action: Pancreas releases enzymes to chop fats.
• Absorption: Fatty acids and lipophilic substances absorbed into lymphatic system as chylomicrons.

Conclusion

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These notes summarize the core elements of bile synthesis and its physiological role in fat digestion and absorption.