CHAPTER 5
 CLONING OF UDP-GLUCURONOSYLTRANSFERASES FROM CHANNEL
 CATFISH LIVER AND INTESTINE

 Piscine UGT Gene Structure and Isoforms

 Fish are the most ancient vertebrate phylum, and account for over 40% of all living

vertebrate species (Clarke et al. 1992a). Clarke and co-workers (1992b) compared the

hepatic glucuronidation of several xenobiotics and endobiotics in plaice (Pleuronectes

platessa) and rat (Rattus norvegicus), species that are separated by more than 350 million

years of evolutionary divergence. Despite the fact that the plaice showed reduced

glucuronidation activity towards substrates such as morphine, bilirubin and steroids,

weak immunological cross-reactivity was obtained when anti-rat UGT antibodies were

used, indicating the presence of conserved common structural motifs between the two

vertebrates.

 Characterization of plaice UGTIB 1 (Accession number (AN): X741 16), an isoform

which conjugates planar phenols and is inducible by polyaromatic hydrocarbons (PAH),

confirmed the strong degree of conservation in gross exon structure and amino acid

character (signal peptide, membrane insertion, and stop sequences) between fish and

mammals. The greatest degree of similarity in amino acid sequence was found with

UGT1 rather than UGT2 (Clarke et al., 1992b, George et al., 1998). Allelic variations in

this UGTIB1 gene are presumed to be functionally silent (George and Leaver 2002).

While there is strong evidence for other distinct isoforms conjugating bilirubin, estrogen

and androgens, to date these have not been characterized. At least six distinct UGTs