Binding of Thyroid Hormone by Human Erythrocyte Cytosol Proteins

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Abstract
Gel filtration (G-100, 0.01 M Tris, pH 7.4) of post-100,000 × g supernatant from lysate of washed human erythrocytes (RBC) revealed 3 fractions (R-1, R-2, R-3) which bound labeled T3 and T4. Major peak R-2 emerged with the hemoglobin fraction (A560 nm) and binding by this fraction was partially dissociable; the dissociable site bound D-T4, but not tetraiodothyroacetic acid or reverse T3. Non-dissociable binding characterized peaks R-1 and R-3. R-1, R-2 and R-3 were pronase-digestible and R-1 binding was acid-unstable (pH 6.3 vs. 7.4). Evidence developed herein and elsewhere indicates that hemoglobin, itself, accounts for the binding within fraction R-2. Intact RBCs maintained for 72 hr at 4C in buffer enriched with T3 or T4 showed progressive incorporation with time of iodothyronines into the hemoglobin fraction.