RESUMO
A reassessment of the binding characteristics of [125I]rT3 to putative receptors in nuclear protein extracts of rat and pig liver revealed that significant deiodination of radioligand occurred during incubation. When previously reported separation procedures are used, released radioiodine is included in the protein-bound [125I]rT3 fraction during separation of protein bound from free hormone by Sephadex G-25 chromatography. This misclassification produces artefacts in binding curves and Scatchard plots used to calculate binding affinity and capacity. Previously reported affinities and capacities derived by this methodology are therefore erroneous. Deiodination of rT3 in the nuclear protein extracts appears to be mediated by outer ring deiodinase. Whereas dithiothreitol markedly enhanced radioiodine generation, the enzyme inhibitors ipodate and salicylate reduced iodine production. These effects produced dramatic changes in apparent binding curves for the radioreceptor assay. When [125I]T3 was incubated with nuclear protein extract, no significant deiodination was detected. Whereas it is likely that the deiodinase is a microsomal contaminant of the nuclear preparation, as suggested by the presence of glucose-6-phosphatase in the nuclear protein preparation, the possibility of an intrinsic nuclear-linked deiodinase cannot be overlooked.