The selenoprotein glutathione peroxidase 3 is attached to thyroglobulin stored in the human thyroid follicular lumen

The biosynthesis of thyroid hormone is a complex process implying various regulated steps. In particular H₂O₂ is indispensable for iodination of tyrosyl residues and subsequent coupling of iodotyrosyls of thyroglobulin (Tg), but on the other hand it may have strong cytotoxic effects. Therefore, the thyrocyte is dependent on antioxidative defence mechanisms against this lifelong exposure to high H₂O₂ concentrations. We hypothesize this protective function could be performed by GPx3, a selenoenzyme highly expressed in the epithelial cells of the thyroid gland. GPx3 might be released into the lumen of the follicles and catalyze the detoxification of excess H₂O₂. To test this hypothesis, we purified human Tg from post mortem thyroid tissues. Isolated Tg globules were incubated in PBS with 0.5% SDS to extract proteins loosely adherent to the surface of the globules. After centrifugation, the pellet was incubated in PBS with 50 mM DTT to solubilize proteins bound by covalent disulfide crosslinks. By Western blot analysis we detected GPx3 in the SDS soluble fraction while no GPx3 was observed in the DTT fraction. Contamination of Tg with cellular proteins was excluded as incubation with an antibody against the intracellular membrane protein 5'deiodinase type I did not produce any signal in neither of the two fractions. Our data demonstrate that GPx3 is indeed present in the follicular lumen and attached to the surface of Tg globules. Here, GPx3 might (1) influence the rate of thyroid hormone production by controlling the availability of H₂O₂, (2) participate in crosslinking of Tg, by using excess H₂O₂ released into the follicular lumen or (3) might even be incorporated itself into the highly polymerized storage form of Tg. So far nothing is known about the activity state of a GPx3 linked to Tg, but possibly this is a further mechanism to fastly provide an enzyme which can interfere with the local H₂O₂ availability.

Supported by DFG Priority Programme.