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JANINE TORRESANI, LESLIE J. DEGROOT, Triiodothyronine Binding to Liver Nuclear Solubilized Proteins in Vitro, Endocrinology, Volume 96, Issue 5, 1 May 1975, Pages 1201–1209, https://doi.org/10.1210/endo-96-5-1201
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Nuclear proteins extracted from purifiednuclei with 0.4M KC1 at pH 7.4 or 8.5 are ableto bind L-triiodothyronine (T3) giving rise to nuclearthyroid hormone binding protein-T3 (NTBP-T3)complexes. Binding is maximum in 3 h at 20 C. It isthermolabile even at 36 C, inhibited by phydroxymercuribenzoateand markedly enhancedby dithiothreitol. Optimum pH is between 7.8 and8.5. Divalent cations are not necessary. TheNTBP-T3 complex exhibits similar anodal electrophoreticmigration in polyacrylamide gel at pH8.5, whether formed in vivo or in vitro. Scatchardplots obtained with various amounts of T3 from 0.15nM to 0.15 μM and either unlabeled nuclearproteins or in vivo formed NTBP-[125I]-T3 complexes,give apparent association constants Ka of0.2 × 1010 M-1 at pH 7.4 and 0.8 × 1010 M-1 at pH8.5. Capacity is about 0.5 pmol T3 per mg protein or800 pg/g liver. The presence of dithiothreitol markedlyenhances the Ka. The nuclear binding sitesare not highly specific for L-T3 since they are able tobind D-T3 with almost equal affinity and triiodothyroaceticacid with a higher affinity. L-thyroxine(T4) can also displace L-T3 but with about 10-foldlesser effectiveness.
Nuclear binding proteins of low capacity and highaffinity have been demonstrated in vitro. TheNTBP-T3 complexes formed in vivo, with wholenuclei, or in vitro are indistinguishable. (Endocrinology96: 1201, 1975)
