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The thyroid hormone, triiodothyronine, has been shown to be bound by the intranuclear chromatin protein associated with active DNA, where it is believed to stimulate transcription. Evidence exists that the thyroid hormones have direct action not only on nuclei, but also on mitochondria. Threfore, specific proteins that bind thyroid hormones in the mitochondria should be demonstrable. Mitochondria were isolated from homogenized rat livers by sedimentation through 0.25 M sucrose solution, followed by washing four times to free them of microsomes. Strong binding of thyroid hormones was observed in mitochondrial fractions prepared from both the membranes and the matrix. After incubation in an ice bath with increasing amonts of triiodothyronine with added tracer 125Itriiodothyronine, the matrix infrequently contained specific saturable receptor sites, but usually exhibited strong "nonspecific" interaction...
Sterling et al. (Fri,) studied this question.