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 approximately 6.2 S chromatin-receptor complex, T4-bound receptors are released as fragments of lower sedimentation coefficient (approximately 5.4 S) and as free receptors (not bound to DNA) (approximately 3.9 S). 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.