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 non-hemolytic 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.
Bosma
P J
PJ
Division of Gastroenterology, Academic Medical Centre, Amsterdam, The Netherlands.
Seppen
J
J
Goldhoorn
B
B
Bakker
C
C
Oude Elferink
R P
RP
Chowdhury
J R
JR
Chowdhury
N R
NR
Jansen
P L
PL
D000818:Animals; D001483:Base Sequence; D001663:Bilirubin / Q000378:metabolism*; D002384:Catalysis; D002460:Cell Line; D003001:Cloning, Molecular; D003414:Crigler-Najjar Syndrome / Q000201:enzymology / Q000235:genetics; D005091:Exons; D014453:Glucuronosyltransferase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007527:Isoenzymes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008099:Liver / Q000201:enzymology; D009154:Mutation; D011994:Recombinant Proteins
D007527:Isoenzymes; D011994:Recombinant Proteins; C019787:bilirubin glucuronoside glucuronosyltransferase; D014453:Glucuronosyltransferase; D001663:Bilirubin