Which enzyme conjugates bilirubin with glucuronic acid in the liver?

• A. Lactase.
• B. Amylase.
• C. Lipase.
• D. UDP-glucuronyl transferase.

Answer: (D)

Bilirubin is made water-insoluble in its usual form, so it needs a modification in the liver to be excreted. The enzyme UDP-glucuronyl transferase attaches glucuronic acid from UDP-glucuronic acid to bilirubin, producing bilirubin diglucuronide. This conjugation increases its water solubility, allowing it to be excreted into bile and then out of the body. This step is a classic example of phase II metabolism, where a compound is conjugated to a water-soluble group to enable elimination. The other listed enzymes—lactase, amylase, and lipase—are digestive enzymes that break down lactose, starch, and fats, respectively, and they do not modify bilirubin.