Bilirubiinien

Bilirubiinien, commonly referred to as bilirubins, are a group of bile pigments produced during the breakdown of heme, a component of hemoglobin. They contribute to the color of bile, urine, and feces and play a key role in the elimination of heme-derived waste products from the body.

Bilirubiinien exist mainly in two forms in the blood: unconjugated (indirect) bilirubin, which is lipophilic and transported bound to albumin to the liver; and conjugated (direct) bilirubin, formed in the liver when bilirubin is conjugated with glucuronic acid by the enzyme UDP-glucuronosyltransferase. A smaller form, delta bilirubin, represents bilirubin covalently bound to albumin. Most unconjugated bilirubin is derived from the degradation of heme to biliverdin and then to bilirubin.

In the liver, conjugation renders bilirubin water-soluble, allowing it to be secreted into bile and stored in the gallbladder or released into the intestine. In the gut, conjugated bilirubin is deconjugated by bacteria and converted to urobilinogen; some is reabsorbed into the circulation (enterohepatic circulation), while the rest is converted to stercobilin and urobilin, giving stool its color and urine its yellow-brown hue.

Clinical relevance centers on hyperbilirubinemia, which can be prehepatic (increased unconjugated bilirubin due to hemolysis), hepatic (impaired conjugation or hepatocellular function), or posthepatic (bile duct obstruction leading to elevated direct bilirubin). Neonatal jaundice is common due to immature conjugation capacity. Measurement distinguishes total bilirubin and its direct and indirect fractions, aiding diagnosis and monitoring.