In vitro triiodothyronine binding to cytoplasmic proteins from human red blood cells

Abstract In vitro incubations of cytosol proteins from human red blood cells with [ 125 I] labelled L-3,5,3′ triiodothyronine demonstrated the existence of high affinity and limited capacity binding sites for T 3 . At 4°C, the rate constant of association was 3 × 10 7 M −1 h −1 , and the rate constant of dissociation was 9.10 −3 h −1 . The dissociation constant K d was calculated from these data or measured by Scatchard analysis and found to be between 3 and 7.10 −10 M. The maximum binding capacity was 1.4 f moles of L-3,5,3′ triiodothyronine per mg cytosol proteins. A close parallel between the biological pontency of the analogs of L-T 3 was observed.