Differences in Pattern of Release of Triiodothyronine (T₃) and Tetraiodothyronine (T₄) Associated Receptors from Chromatin by Micrococcal Nuclease

Our study aimed to determine whether tetraiodothyronine (T₄) binds to chromatin associated receptors and to compare such binding with that of triiodothyronine (T₃). We found that the in vitro binding profile of both hormones to receptor-containing fragments released from chromatin by micrococcal nuclease was similar, with the exception of the well known lower T₄ affinity. In contrast, the in vivo experiments revealed differences in the sedimentation profile of T₃ and T₄ bound receptors. More specifically, while T₃-bound receptors are excised by micrococcal nuclease as an abundant ∼6.2 S chromatin-receptor complex, T₄-bound receptors are released as fragments of lower sedimentation coefficient (∼5.45) and as free receptors (not bound to DNA) (∼3.95). These data indicate that, in vivo, T₄ binds with receptors which are differently organized in chromatin than those of T₃ and could have a special, as yet undefined, intrinsic nuclear activity.

Key words

T₃ - T₄ - Receptors - Chromatin