x Bilirubin is an important biochemical compound primarily associated with the breakdown and excretion of heme, which is a component found within hemoglobin – the oxygen-carrying molecule present in red blood cells. The term “bilirubin” originates from its yellow coloration (bile = yellow) and its association with bile pigments.
There are two main types of bilirubin: unconjugated (indirect) and conjugated (direct). Both forms play crucial roles in the body’s metabolism, but they differ significantly in their chemical properties and functions.
Unconjugated Bilirubin:
Unconjugated bilirubin is a yellow pigment that circulates within the bloodstream bound to serum albumin due to its hydrophobic nature. It results from the breakdown of heme, which occurs when red blood cells are destroyed (hemolysis) or during normal cell turnover in various tissues.
The process begins with the catabolism of heme by enzymes called heme oxygenases, primarily located within macrophages that engulf and digest aged or damaged red blood cells. Heme is converted into biliverdin (green pigment) and free iron. Biliverdin then undergoes a reduction reaction to form unconjugated bilirubin.
Unconjugated bilirubin is not water-soluble, which makes it difficult for the body to excrete efficiently. Therefore, this type of bilirubin must be transported to the liver where it undergoes further processing and elimination.
Conjugated Bilirubin:
Conjugated bilirubin is a water-soluble form that results from the conjugation process in which unconjugated bilirubin binds to glucuronic acid, forming bilirubin diglucuronide. This reaction occurs within hepatocytes (liver cells) and is catalyzed by an enzyme called UDP-glucuronyl transferase.
The conjugation process makes bilirubin more water-soluble, allowing it to be excreted into bile – a fluid produced in the liver that helps with digestion and waste removal. Bile is then transported through the common bile duct and eventually released into the small intestine during digestion. Written by Obsidian Copilot, an AI-powered note-taking assistant for comprehensive understanding of bilirubin.