IN-VITRO BINDING OF 3,5-DI-IODO-L-THYRONINE TO RAT-LIVER MITOCHONDRIA

In this study we have demonstrated that specific binding sites for 3,5-di-iodo-L-thyronine (3,5-T-2) can be detected in rat liver mitochondria. After incubation with the homogenate, liver mitochondria bound only a small portion of [3,5-I-125]T-2. The addition of a 100-fold excess of unlabelled 3,5-T-2 caused the displacement of on average 50-60% of the [3,5-I-125]T-2 bound. Specific binding of 3,5-T-2 to rat liver mitochondria occurred rapidly; a maximum was achieved after 5 min. Maximal binding was obtained at 37 degrees C, while at 0 degrees C and 20 degrees C the values were only slightly lower. Binding was maximal at pH 7.0; mean (+/-S.E.M.) values for the apparent association constant and the binding capacity (calculated at pH 7.0, 0 degrees C and after 30 min of incubation) were 0.5+/-0.04 x 10(8) 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-T-2>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-T-2 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.