Solubilization of L-triiodothyronine binding site from human erythrocyte membrane

A thyroid binding peripheral membrane protein(s) has been characterized in human red cell. Two classes of affinity sites for triiodothyronine have been demonstrated. The high affinity, low capacity site showed values for dissociation constant of 2 X 10(-10)M. The binding activity depended on the presence of free -SH group and showed a high stereospecificity for L-triiodothyronine, L-thyroxine was less potent (about 1,000-fold) than L-triiodothyronine in competing for this site. The results are discussed with respect to their cellular significance.