The conjugation of bilirubin occurs in which organ?

• A. Liver
• B. Kidney
• C. Spleen
• D. Lungs

Answer: (A)

Bilirubin conjugation occurs in the liver. Bilirubin is formed from the breakdown of heme and is initially insoluble in water (unconjugated bilirubin) and bound to albumin in the blood. In hepatocytes, the enzyme UDP-glucuronosyltransferase adds glucuronic acid to bilirubin, producing a water-soluble form called bilirubin diglucuronide. This conjugated bilirubin is then excreted into bile and ultimately reaches the intestine for elimination. The liver is the key site because this glucuronidation step is specific to hepatocytes and is necessary to make bilirubin water-soluble for excretion. The kidneys, spleen, and lungs do not perform this conjugation, though the kidneys handle excretion of conjugated bilirubin in urine and the spleen is involved in RBC turnover, not bilirubin conjugation.