Discussion

 Multiplicity of UGT isoforms. At least two different UGT isoforms were

identified from channel catfish liver (livUGTn) and intestine. One isoform (liv/intUGTn)

was sequenced in its entirety and was cloned from both liver and intestine. Another

isoform (I35RC) was sequenced from intestine; amplification of this partial sequence

towards its 5'- and 3'-ends was unsuccessful. However, the partial sequence obtained,

particularly a 45-bp stretch at the known 5'-end of this sequence (which includes part of

the UDPGA binding site) was significantly different from the sequence of the other

isoform when both sequences were aligned.

 The liv/intUGTn sequence appeared to be analogous to mammalian UGTIA1, or

bilirubin UGT, while blastp searches for the predicted partial protein sequence of I3 5RC

resulted in better matches with the higher-numbered UGT isoforms such as UGTIA4,

UGTIA7, UGTIA6. Of course, one cannot conclude anything further since these

sequences are only partial, lacking the substrate-binding site which is responsible for the

distinct specificity of the UGT isoform.

 The presence of different UGT isoforms in catfish liver and intestine are probably

one of the reasons for the disparate glucuronidation kinetics observed in these organs

with substrates such as 3-hydroxybenzo[a]pyrene and polychlorinated biphenylols.

 Characteristics of the predicted protein for livUGTn. As seen from Figure 5-13,

the UGT isoform obtained from catfish shows several strongly conserved regions with

mammals, even though 350 million years of evolution separate the two phyla. These

indicate amino acid residues that are important for the function of the protein.

 By drawing an analogy with mammalian UGTs, which have been extensively

studied, several interesting features regarding the catfish UGT sequence were noted. Two