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In this study we have demonstrated that specific binding sites for 3,5-di-iodo-L-thyronine (3,5-T2) can be detected in rat liver mitochondria. After incubation with the homogenate, liver mitochondria bound only a small portion of [3,5-125I]T2. The addition of a 100-fold excess of unlabelled 3,5-T2 caused the displacement of on average 50-60% of the [3,5-125I]T2 bound. Specific binding of 3,5-T2 to rat liver mitochondria occurred rapidly; a maximum was achieved after 5 min. Maximal binding was obtained at 37 °C, while at 0 °C and 20 °C the values were only slightly lower. Binding was maximal at pH 70; mean (±S.E.M.) values for the apparent association constant and the binding capacity (calculated at pH 7·0, 0 °C and after 30min of incubation) were 0·5±0·04x108 M-1 and 0·4±0·04 pmol/mg mitochondrial protein respectively. The specificity of binding, examined in competition studies, followed the order: 3,5-T2>3,3'-di-iodo-L-thyronine>3',3,5-tri-iodo-L-thyronine>thyroxine. Other iodothyronines (3',5'-di-iodo-L-thyronine, 3,5-di-iodo-D-thyronine, 3,3', 5'-tri-iodo-L-thyronine, 3-iodo-L-thyronine and 3,5-di-iodothyroacetic acid) showed little or no competition. This suggests that the specific 3,5-T2 binding sites could be of biological relevance with respect to the understanding of the mechanism of physiological action of thyroid hormones at the cellular level.
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