Clearance of circulating desialylated thyroglobulins in the rat

Extrathyroidal release of thyroid hormones from thyroglobulin by J774 mouse macrophages

Thyroglobulin appears in the circulation of vertebrates at species-specific concentrations. We have observed that the clearance of thyroglobulin from the circulation occurs in the liver by macrophages. Here we show that the thyroid hormones T3 and T4 were released by incubation of mouse macrophages (J774) with thyroglobulin. Thyroid hormone release was a fast process, with an initial rate of approximately 20 pmol T4/mg per min and approximately 0.6 pmol T3/mg per min, indicating that macrophages preferentially release T4. The bulk of released thyroid hormones appeared after 5 min of incubation of macrophages with thyroglobulin, whereas degradation of the protein was detectable only after several hours. During internalization of thyroglobulin, endocytic vesicles and endosomes were reached at 5 min and lysosomes at 60 min. T4 release started extracellularly by secreted proteases and continued along the endocytic pathway of thyroglobulin, whereas T3 release occurred mainly intracellularly when thyroglobulin had reached the lysosomes. This shows that the release of both hormones occurred at distinct cellular sites. Our in vitro observations suggest that macrophages in situ represent an extrathyroidal source for thyroid hormones from circulating thyroglobulin.

Iodination of thyroglobulin molecules depends on their diffusion velocity in follicular colloid

We have studied in vitro the effects of altered physicochemical properties of thyroglobulin molecules in solution and of the solution itself on iodination kinetics and hormone synthesis. Any change in hydrodynamic properties had a far greater effect in compartmentalized systems, obtained by coating the test tubes with peroxidase, than in conventional homogeneously mixed systems. Increasing thyroglobulin concentration in a range still far below that existing in vivo greatly retarded iodination and hormone synthesis. In contrast, a number of physiological and non-physiological changes of thyroglobulin structure, such as desialylation, preiodination, oxidation and denaturation, strikingly accelerated iodination. The highly variable physical-chemical state of thyroglobulin molecules appears to be a main determinant of protein diffusion within the colloid and, thereby, of iodination kinetics and rate of hormone synthesis. Moreover, alterations of the physicochemical state of thyroglobulin molecules may explain some hitherto ill-understood diffusion phenomena in live follicles.

Disappearance rate of endogenously radioiodinated thyroglobulin and thyroxine after radioiodine treatment

Endogenously radioiodinated thyroglobulin (tg) and thyroxine (T4), in the circulation following, therapeutic doses of radioiodine were studied in five patients with differentiated thyroid carcinoma. Radioactive serum was fractionated on a Sephadex G-200 and the various radioactive peaks thus obtained were analyzed for their biochemical and immunologic characteristics using precipitation with trichloroacetic acid (TCA) and antithyroglobulin antibody extraction with acidified n-butanol and electrophoresis on agar gel. The disappearance rates of endogenously labelled tg and T4 were 3.12 +/- 0.396 days and 10.14 +/- 1.81 days, respectively.