Abstract
The object of the study was to obtain information on how adipose tissue of normal subjects processes thyroid hormones. L-125 I-thyroxine (125I-T4) is bound by the cytosol fraction of normal human female breast adipose tissue with high affinity. Computer analysis of the binding data revealed the presence of two saturable systems with Kd values of 3.9 and 29.1 nM and binding capacities of 1.7 and 8.7 pmol/mg of cytosol protein, respectively; a third binding system was non-saturable. The binding of the iodothyronines to the cytosol fraction indicated that L-triiodo-thyronine (T3) possessed two-fold higher affinity as compared to L-thyroxine (T4) whereas other iodothyronines had relative affinities of less than 3%. Binding of 125I-T4 was optimal at pH 7.0 and was sensitive to the action of pronase and neuraminidase. Affinity chromatography of the cytosol fraction using T3-epoxy-sepharose 6B and con A-sepharose 4B, yielded a 125I-T4 binding component that was purified 150-fold. Isoelectric focusing of the purified fraction yielded six major bands of protein which had pi values comparable to those of human serum thyroxine-binding-globulin (TBG); however, the pattern of separation was different. Incubation of the fraction with 125I-T4 followed by isoelectric focusing and autoradiography revealed that the protein bands bound radioactivity. The microsomal fraction of the adipose tissue deiodinated 125I-T4 to L-125[-triiodothyronine (125I-T3) to an extent of 0–0.8 fmol/(mg of protein x min) which corresponded to about 0.2%; T4 was not deiodinated to L-3,3′,5′-triiodothyronine (r-T3). 125I-T3 was deiodinated to L-125I-3,3′-diiodothyronine; the conversion was 2.5%. These results show that adipose tissue of human breast contains a TBG-like protein or modified TBG which has high avidity for T4 and T3. The tissue is not an active source for the production of T3 or r-T3 from T4.
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Rao, G.S., Rao, M.L., Quednau, H.D. et al. Further evidence for the presence of thyroxine binding globulin-like protein in human breast adipose tissue. Deiodination of thyroxine and triiodothyronine by the microsomal fraction. J Endocrinol Invest 8, 501–506 (1985). https://doi.org/10.1007/BF03348547
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DOI: https://doi.org/10.1007/BF03348547