GLUCURONIDATION OF THYROID-HORMONE IN RAT-LIVER - EFFECTS OF IN-VIVO TREATMENT WITH MICROSOMAL-ENZYME INDUCERS AND IN-VITRO ASSAY CONDITIONS

We investigated the effects of in vivo treatment with different micros omal enzyme inducers, including clofibrate (CLOF), hexachlorobenzene ( HCB), 3-methylcholanthrene (MC), 3,3',4,4'-tetrachlorobiphenyl (TCB), and 2,3,7,8-tetrachloro-p-dioxin, as well as of in vitro addition of t he detergent Brij 56 on the glucuronidation of T4, T3, and rT3 by UDP- glucuronyltransferase (UGT) activities of rat liver microsomes. The re sults were compared with measurements of UGT activities for bilirubin, p-nitrophenol (PNP), and androsterone. In general, glucuronidation ra tes were 5-fold or more higher with rT3 than with T4 or T3 as substrat e. In liver microsomes from untreated rats, T4 UGT activity was stimul ated by Brij 56 to a maximum of about 2-fold at 0.025% detergent. Trea tment of Wistar rats for 4 days with CLOF (200 mg/kg BW . day) resulte d in significant increases in UGT activities for T4 (to 154%), rT3 (to 155%), and bilirubin (to 194%), in particular if assayed in the prese nce of 0.025% Brij 56, but had little effect on the UGT activities for T3, PNP, and androsterone. The CLOF-induced increases in T, and rT3 U GT activities were not observed in Gunn rats, which have a complete la ck of bilirubin UGT activity and greatly impaired PNP UGT activity. Tr eatment of Wistar rats with a single injection of MC (50 mg/kg BW), TC B (50 mg/kg BW), or 2,3,7,8-tetrachloro-p-dioxin (6.25 mug/kg BW) resu lted, after 4 days, in 6.3- to 7.3-fold increases in T4 UGT activity a nd 15.1- to 16.7-fold increases in rT3 UGT activity if determined in t he absence of Brij 56, whereas T4 UGT activity was only increased by 3 3-68% when assayed in the presence of Brij 56. T3 glucuronidation was not affected (with Brij 56) or was increased by only 33-68% (without B rij 56) after treatment with these MC-type inducers. PNP UGT activity was induced 3.6- to 4.3-fold, whereas bilirubin and androsterone UGT a ctivities were changed little by these treatments. Similar findings re garding T4, rT3, PNP, and bilirubin UGT activities were obtained after chronic treatment of WAG rats with HCB, another MC-type inducer. Howe ver, WAG rats lack androsterone UGT and show low T3 UGT activity, whic h was increased about 2.3-fold by HCB treatment. On the basis of these and previous findings it is concluded that at least three UGT isoenzy mes are involved in the glucuronidation of thyroid hormone. The type I UGT isoenzyme, which is absent in Gunn rats, glucuronidates bilirubin , T4, and rT3 and is stimulated in vivo by CLOF and in vitro by Brij 5 6. The type II UGT isoenzyme, which is also absent in Gunn rats, glucu ronidates bulky phenols, including T4 and rT3, and is stimulated in vi vo by MC-type compounds, but inhibited in vitro by Brij 56. The type I II UGT isoenzyme, which is absent in WAG rats, glucuronidates androste rone and T3 and is influenced little by any of the in vivo and in vitr o treatments.