CLONING AND CHARACTERIZATION OF CDNAS ENCODING MOUSE UGT1.6 AND RABBIT UGT1.6 - DIFFERENTIAL INDUCTION BY 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN

In this report, cDNAs for mouse liver Ugt1.6 and rabbit liver UGT1.6 h ave been cloned and characterized. The predicted amino acid sequence o f mouse Ugt1.6 is 93% and 78% similar to the rat and human UGT1.6 sequ ences, respectively, while the rabbit UGT1.6 is 79% and 83% similar to the rat and human UGT1.6 sequences, respectively. To examine the subs trate specificities of the proteins encoded by the mouse Ugt1.6 and ra bbit UGT1.6 cDNAs, the recombinants were expressed in monkey kidney CO S-1 cells. Transfection of the mouse and rabbit recombinants allowed f or the expression of the UGT1.6 proteins as determined by immunoprecip itation of newly synthesized protein. The expressed UGTs conjugated sm all planar phenolic molecules such as 4-nitrophenol, 1-naphthol, and 4 -methylumbelliferone. While the bulky phenol 4-hydroxybiphenyl was not a substrate for the enzymes, 2-hydroxybiphenyl was an excellent subst rate. Androgens and estrogens were not conjugated by either mouse Ugt1 .6 or rabbit UGT1.6. In rodents, UGT1.6 mRNA is expressed constitutive ly and induced when the animals are treated with the Ah receptor ligan d 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Using wild-type mouse he patoma cells and the Ah receptor deficient class II cells, it was demo nstrated that induction of mouse Ugt1.6 was dependent upon a functiona l Ah receptor complex. However, when New Zealand white rabbits were tr eated with TCDD and liver mRNA was examined by Northern blot analysis, it was shown that TCDD had no effect on the induction of UGT1.6 mRNA. These results indicate that the Ah receptor is involved in the induct ion of Ugt1.6 mRNA by TCDD in rodents, but there exist species differe nces related to the induction and expression of the UGT16 gene by TCD D.