Binding of reverse T3 to hepatic nuclear protein

The study was undertaken to examine the potential intrinsic biological activity of reverse T3 by comparing its binding to nuclear protein in vitro with that of other iodothyronines. Nuclear protein was extracted from normal pig liver using methods described for rat tissues. At 25 degrees 30% of the added 125I rT3 was specifically bound to the nuclear protein. The order of potency in displacing 125I rT3 was rT3 > 3'5'T2 > T4 > T3 > 3,3'T2 > 3,5T2. This contrasted with the order of potency in displacing 125I T3 which was T3 > T4 > 3,3'T2 > 3,5T2 > rT3 > 3'5'T2. This difference in the hierarchy of iodothyronine binding is consistent with binding of these radioligands to different components of the nuclear protein extract. These experiments demonstrate separate binding components of nuclear protein for T3 and rT3 in a thyroid hormone responsive tissue and may be relevant to the mechanism of action of thyroid hormones.