A spin label study of the thyroid hormone-binding sites in human plasma thyroxine transport proteins.

Abstract. In order to verify the hypothesis of the presence of IgM (or an IgM-like substance) capable of inhibiting thyroid hormone binding to serum proteins and, therefore, capable of enhancing serum free thyroid fractions in non-thyroid illnesses (NTI), we measured TBG and TBPA maximum binding capacities and TBG concentration by an immunoradiometric system in normal pooled sera (NPS) before and after enrichment with purified immunoglobulins (IgM and IgG) from euthyroid NTI sera (free T4, free T3 and TSH levels were normal). A known amount of TBG was diluted 1:2–1:6 with deionized water or with IgM from NPS or from each of 6 NTI sera; the measured values were not different from these expected on theoretical grounds. Likewise, IgM or IgG from normal or from each of 7 NTI sera failed to modify both TBG and TBPA capacities of different NPS, and NTI immunoglobulins showed no binding activity directed to [125I]T4, [125I]T3 or [125I]TBG. In addition, no inhibitory influence of TBG and TBPA capacities was observed when the whole euglobulin fraction obtained by precipitating the same 7 NTI sera by PEG was mixed with NPS. On the other hand, a significant IgM inhibitory effect on the binding of labelled T4 to TBG was found, only when IgM concentrations were experimentally rendered 41 times greater than that requested in the working mixtures. We conclude that no immunoglobulin inhibitor of thyroid hormone binding to transport proteins was evident in the NTI sera investigated.

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Thyroid Hormone Binding to Isolated Human Apolipoproteins A-II, C-I, C-II, and C-III: Homology in Thyroxine Binding Sites

Thyroid ◽

10.1089/thy.1994.4.261 ◽

1994 ◽

Vol 4 (3) ◽

pp. 261-267 ◽

Cited By ~ 10

Author(s):

SALVATORE BENVENGA ◽

HANS J. CAHNMANN ◽

DAN RADER ◽

MARIE KINDT ◽

ANTONIO FACCHIANO ◽

...

Keyword(s):

Thyroid Hormone ◽

Binding Sites ◽

Hormone Binding

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Multiple thyroid hormone binding sites on rat liver nuclear envelopes

Biochemical and Biophysical Research Communications ◽

10.1016/0006-291x(85)90984-2 ◽

1985 ◽

Vol 132 (1) ◽

pp. 35-41 ◽

Cited By ~ 12

Author(s):

Jaya T. Venkatraman ◽

Yvonne A. Lefebvre

Keyword(s):

Thyroid Hormone ◽

Rat Liver ◽

Binding Sites ◽

Hormone Binding

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THYROID HORMONE-BINDING INTERACTIONS IN CYTOSOL OF RAT ANTERIOR PITUITARY

Acta Endocrinologica ◽

10.1530/acta.0.0850256 ◽

1977 ◽

Vol 85 (2) ◽

pp. 256-266 ◽

Cited By ~ 3

Author(s):

Valerie Anne Galton

Keyword(s):

Thyroid Hormone ◽

Binding Sites ◽

Anterior Pituitary ◽

Binding Capacity ◽

Relative Effectiveness ◽

Binding Activity ◽

Rat Tissues ◽

Hormone Binding ◽

Cytosol Fraction ◽

Binding Interactions

ABSTRACT Thyroxine (T4) and triiodothyronine (T3)-binding interactions in preparations of rat anterior pituitary gland have been studied. T4 is bound primarily to extranuclear binding sites located in the cytosol fraction of the cell. These sites have a medium affinity for T4: Ka = 2.5 × 108 1/mol and a maximum binding capacity (MBC) of 1.15 pmol/mg tissue (wet weight). Binding of T3 to these sites is minimal. The extent of binding of T4 is influenced by the pH of the system and the temperature of incubation. The relative effectiveness of T4 analogues in displacing bound T4 is tetrac > T4 > triac > D-T4 > T3. Similar T4-binding sites are present in other rat tissues, but in all except serum, binding activity is lower than in the pituitary. T4-binding by serum contaminating the pituitary preparations contributed only partially to the total activity observed. Concomitant assessment of T4-binding activity and T4 metabolism in pituitary homogenates prepared at different pH values indicated an inverse relationship between the two processes. The possible role of thyroid hormone binding in cytosol in influencing the intracellular distribution of thyroid hormones is discussed.

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