The molecular mechanisms governing the regulation of the drug-detoxifying enzyme, UDP glucuronosyltransferase (transferase), and the structural differences between members of this family are being investigated in the rat. This animal, as exemplified by the Gunn rat, provides the only known animal model for investigating the defect in the glucuronidation of bilirubin and certain xenobiotics, characteristic of the Crigler-Najjar syndrome in humans. Certain strains of Wistar rat also have an inherited defect in the ability to glucuronidate the steroid hormone, androsterone. The isolation and sequencing of 7 cDNA clones has demonstrated the existence of at least 4 different forms of transferase belonging to two gene subfamilies. One clone pUDPGTr-2 encodes a phenobarbital-inducible form of transferase which glucuronidates 4-methylum-belliferone and the 17-OH position of testosterone and dihydrotestosterone. This was demonstrated by transfection into COS cells of the cDNA under the control of the SV40 promoter. This technique also demonstrated that a second clone, pUDPGTr-4, whose mRNA is not detected by phenobarbital or 3-methylcholanthrene, encodes a form of transferase which glucuronidates the 3-OH position of the androgens, androsterone and etiocholanolone. The substrate specificity of other transferases are currently under investigation. Sequence studies have also shown that the transferase forms encoded by the three cDNAs contain signal peptide and membrane anchoring regions, and potential asparagine-linked glycosylation sites. In vitro translation studies indicate that the signal sequence is cleaved during insertion into the endoplasmic reticulum and subsequent glycosylation occurs. Genomic clones to UDPGTr-4 have been isolated and are being sequenced. Genomic and cDNA clones will be utilized to study the regulation of each form as a function of age, tissue distribution and administration of prototypic inducers.
