UDP-glucuronosyltransferase
Summary
UDP-glucuronosyltransferase (UGT) is a phase II drug-metabolizing enzyme that conjugates glucuronic acid to lipophilic substrates (drugs, bilirubin, hormones) to increase water solubility for excretion. It is located in the smooth endoplasmic reticulum of hepatocytes and other tissues. Deficient or impaired UGT1A1 activity causes hereditary hyperbilirubinemias like Gilbert and Crigler-Najjar syndromes.
Detail
UGTs are a superfamily of enzymes (phase II conjugation reactions) that catalyze glucuronidation—the transfer of glucuronic acid from UDP-glucuronic acid to a nucleophilic functional group (hydroxyl, carboxyl, amine, thiol) on lipophilic substrates, forming water-soluble glucuronides that are readily excreted in bile or urine. This differs from phase I reactions (cytochrome P450-mediated oxidation) in that phase II reactions typically do not require prior modification and directly increase polarity.
Key substrates include bilirubin, steroid hormones, thyroid hormones, bile acids, and many drugs (e.g., acetaminophen, morphine, irinotecan, lamotrigine, NSAIDs). UGT1A1 is the primary isoform responsible for bilirubin conjugation in the liver, converting unconjugated (indirect) bilirubin to conjugated (direct) bilirubin for excretion into bile.
Clinical significance: - Gilbert syndrome: mild reduction in UGT1A1 activity (~30% of normal) due to a promoter polymorphism (TA repeat), causing mild unconjugated hyperbilirubinemia, often triggered by stress, fasting, or illness. Benign, no treatment needed. - Crigler-Najjar syndrome: more severe UGT1A1 deficiency. Type I (complete absence) causes severe unconjugated hyperbilirubinemia, kernicterus risk, requires phototherapy/liver transplant. Type II (partial deficiency) is less severe and responds to phenobarbital (which induces UGT expression). - Neonatal jaundice: physiologic due to immature UGT1A1 activity at birth. - Drug interactions: UGT1A1 also glucuronidates irinotecan's active metabolite (SN-38); patients with reduced UGT1A1 activity (e.g., Gilbert syndrome, UGT1A1*28 polymorphism) are at increased risk of irinotecan toxicity (severe diarrhea, neutropenia). - Enzyme induction: phenobarbital and other inducers upregulate UGT1A1, useful therapeutically in Crigler-Najjar type II and neonatal jaundice.
Understanding UGT is essential for boards regarding bilirubin metabolism, jaundice differentials, and pharmacogenomics of drug metabolism.
Sources
- First Aid for the USMLE Step 1
- Katzung's Basic and Clinical Pharmacology
- Robbins and Cotran Pathologic Basis of Disease
- UpToDate: Gilbert syndrome and Crigler-Najjar syndrome
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