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A Novel NADPH-Dependent Cytosolic 3,5,3'-Triiodo-LThyronine- Binding Protein (CTBP; 5.IS) in Rat Liver: A Comparison with 4.7S NADPH-Dependent CTBP

Department of Geriatrics, Endocrinology, and Metabolism, Shinshu University School of Medicine Matsumoto 390, Japan

Abstract

A novel NADPH-dependent cytosolic 3,5,3''-tritodo-L-thyronine (T₃)-binding protein (CTBP) was purified by sequential fractionation of rat liver cytosol on Q-Sepharose, phenyl-Sepharose, red-Sepharose, and polyacrylamide gel electrophoresis under nondenaturing conditions. The CTBP had a sedimentation coefficient of 5.1S, a Stokes’ radius of 35 Å, and a calculated mol wt of 76,000. The apparently homogenous protein consisted of a dimer of a polypeptide chain with a mol wt of 38,000 as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. NADPH increased both the affinity and maximal binding capacity for T₃ in the 5.1S CTBP, The maximal activity to bind T₃ was obtained by 3.0 x 10^-8 M NADPH. The calculated maximal affinity constant was 2.4 x 10⁹ M^-1, and the maximal binding capacity was 21,000 pmol T₃/mg 5.1S CTBP. The order of affinity of iodothyronine analogs to the S.lS CTBP was as follows: D-T₃ > L-T₃ > t,-T₄ > triiodothyroacetic acid. The optimal pH for T3 binding was 7.2- 7.5. Ca²⁺, Mg²⁺, and Mn²⁺ (0.1–10 mM) did not influence T₃ binding to CTBP. Zn²⁺ (1.0 mM), however, inhibited the binding. These results suggested that 5.1S NADPH-dependent CTBP, which is distnct from 4.7S CTBP that had been purified in our laboratory from rat kidney, is present in rat liver. (Endocrinology 129: 1701–1708, 1991)

Footnotes

^* To whom requests for reprints should be addressed.

Received April 18, 1991.

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