Bilirubin UDP-glucuronosyltransferase 1 is the only relevant bilirubin glucuronidating isoform in man

Crigler-Najjar syndrome type I (CN-I) is caused by an inherited absence of UDP-glucuronosyltransferase activity toward bilirubin (B-UGT), resulting in severe nonhemolytic unconjugated hyperbilirubinemia. Based on the expression of cDNAs in COS cells, two UGT isoforms in human liver, B-UGT₁ and B-UGT₂, have been reported to catalyze bilirubin glucuronidation. These isoforms, which are derived from a single gene, ugt1, have identical carboxyl-terminal domains that are encoded by four consecutive exons shared by both isoforms. A critical lesion in any of these common exons should inactivate both B-UGT isoforms, giving rise to CN-I. The amino-terminal domains of the B-UGT isoforms are unique, each being encoded by a different 5' exon. If both B- UGT isoforms contribute significantly to bilirubin glucuronidation, a mutation in one of these unique 5' exons should affect a single isoform, while the other isoforms should provide residual B-UGT activity. However, in two patients with CN-I, we found a mutation only in the unique exon of B- UGT₁, the other exons being normal. To clarify this apparent paradox, we expressed the cDNA for each B-UGT isoform in COS cells and determined the specific B-UGT activity. These studies show that only B-UGT₁ has quantitatively significant catalytic activity. Furthermore, we show that the mutation in B-UGT₁ observed in each of the two CN-I patients inactivates B- UGT₁. Together, the results indicate that B-UGT₁ is the only physiologically relevant isoform in bilirubin glucuronidation.