Potential mechanisms of thyroid disruption in humans: interaction of organochlorine compounds with thyroid receptor, transthyretin, and thyroid-binding globulin.

Organochlorine compounds, particularly polychlorinated biphenyls (PCBs), alter serum thyroid hormone levels in humans. Hydroxylated organochlorines have relatively high affinities for the serum transport protein transthyretin, but the ability of these compounds to interact with the human thyroid receptor is unknown. Using a baculovirus expression system in insect cells (Sf9 cells), we produced recombinant human thyroid receptor ss (hTRss). In competitive binding experiments, the recombinant receptor had the expected relative affinity for thyroid hormones and their analogs. In competitive inhibition experiments with PCBs, hydroxylated PCBs (OH-PCBs), DDT and its metabolites, and several organochlorine herbicides, only the OH-PCBs competed for binding. The affinity of hTRss for OH-PCBs was 10,000-fold lower (Ki = 20-50 microM) than its affinity for thyroid hormone (3,3',5-triiodothyronine, T3; Ki = 10 nM). Because their relative affinity for the receptor was low, we tested the ability of OH-PCBs to interact with the serum transport proteins--transthyretin and thyroid-binding globulin (TBG). With the exception of one compound, the OH-PCBs had the same affinity (Ki = 10-80 nM) for transthyretin as thyroid hormone (thyroxine; T4). Only two of the OH-PCBs bound TBG (Ki = 3-7 microM), but with a 100-fold lower affinity than T4. Hydroxylated PCBs have relatively low affinities for the human thyroid receptor in vitro, but they have a thyroid hormonelike affinity for the serum transport protein transthyretin. Based on these results, OH-PCBs in vivo are more likely to compete for binding to serum transport proteins than for binding to the thyroid receptor.