GLUCURONIDATION OF THYROID-HORMONE BY HUMAN BILIRUBIN AND PHENOL UDP-GLUCURONOSYLTRANSFERASE ISOENZYMES

The glucuronidation of thyroid hormone by UDP-glucuronyltransferases ( UGTs) stably transfected in Chinese hamster V79 lung fibroblasts was i nvestigated. Human bilirubin UGT (HP3) and phenol UGT (HP4) both catal ysed the glucuronidation of T4 and rT3, whereas glucuronidation of T3 was not significant. rT3 was the preferred substrate for both isoenzym es, glucuronidation rates being 1.6- and 6.4-times higher than conjuga tion of T4 by HP3 and HP4 clones, respectively. This is the first iden tification of thyroid hormone as potential alternative endogenous subs trate for bilirubin UGT.