In vitro binding of triiodothyronine to rat liver mitochondria

Saturable high affinity T3 binding sites were detected in a mitochondrial fraction enriched in internal membranes and partly solubilized by Triton X-100. Specific T3 binding to the solubilized sites, only detected at low T3 concentrations, was optimal at pH 8.0 and not dependent upon the presence of divalent cations or reducing agents; it was destroyed by heat and proteolytic enzymes. The solubilized T3 binding sites were distributed, after Sephadex G-200 gel filtration, between two peaks of similar affinity for T3 (Ka congruent to 5 x 10(10)l/mol) and similar binding characteristics. T3 was bound with a high stereospecificity, while some analogues of biological importance (L-T4; 3,5,3'-triiodothyroacetic acid; 3,3';-diiodo-L-thyronine) competed with L-T3 in the same range of low concentrations. This suggests that the high affinity mitochondrial T3 binding sites could be of biological relevance in the mitochondrial metabolism.