PARENT SESSION
MA7 Bioassays for Specific Hazards.
9:00 AM to 12:30 PM, Monday, 07 May 2001
Session Chair: Hans Toni Ratte
Room 7

(056) A new in vitro bioassay for thyroid disruption.

Cerrillo, I.¹, Olmos, B.¹, Serna, A.¹, Araque, P.¹, Soutto, M.², Fernandez-Soto, M.³, Molinero, P.², Olea, N.¹, ^{1 2 3}

ABSTRACT- Thyroid endocrine disruptors (EDs) may alter thyroid morphology and hormone function, either by inhibiting hormone synthesis, altering serum transport proteins or increasing hormone catabolism. Despite international interest to establish a reliable battery of tests for thyroid disruption, few assays have been developed so far. Most of them are designed for chemicals that alter the synthesis, storage, transport and catabolism of thyroxine (T₄) and 3, 5, 3'-triyodotironina (T₃), or the ligand binding and activation of the thyroid hormone receptor. Thyroxine 5'-deiodinase is responsible for the conversion of T₄ to T₃. Its type II iso-enzyme (5'DII) is under the control of regulatory factors such as the level of circulating thyroid hormones and plays an important role in maintaining intracellular levels of T₃ and could serve as a defence against thyroid hormone deficiency. We present a new bioassay that investigates the inhibitory effects of chemicals on the activity of 5'DII enzyme. It measures the release of radioiodine from labelled (5'-¹²⁵I) T₄ either in the absence of competitors (Negative control) or in the presence of testing chemicals. The method is specific for 5'DII activity because the substrate contains ¹²⁵ I only in position 5'. The enzyme is collected from brown adipose tissue of normal and hypothyroid rats after treatment with methimazole. The ¹²⁵I measured is directly proportional to the activity of the enzyme and is fully suppressed by iopanoic acid (Positive control of inhibition). We assayed different chemicals: Bisphenol-A and F and halogenated bisphenols, PCBs, DDT and metabolites, metoxychlor, lindane, HCB, aldrin, dieldrín, endosulfans and polybrominated diphenyl ethers. Some of them were effective in inhibiting the release of radioiodine: the inhibitory activity of BPA on 5'D-II was significant at concentrations as low as 10 uM. The assay is easy to perform and may be useful for risk assessment.

Key words: Thyroid, endocrine disruptors, bioassays