In vitro binding of 3,5-di-iodo-L-thyronine to rat liver mitochondria

Abstract

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 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-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.

3,5-Diiodo- l -thyronine and 3,5,3′-triiodo- l -thyronine both improve the cold tolerance of hypothyroid rats, but possibly via different mechanisms

“…Possibly, the effect of 3,5-T2 may be consequent upon a direct interaction of 3,5-T2 with mitochondria. Our previous studies showing the presence of specific binding sites for 3,5-T2 in rat liver mitochondria [9] and direct interaction of 3,5-T2 with isolated CO complexes [10] are consistent with this notion. In addition, O' Reilly and Murphy [23] have reported that the stimulatory effect exerted by 3,5-T2 on rat liver mitochondrial respiration is independent of the protein synthesis.…”

Growth Hormone Internalization in Mitochondria Decreases Respiratory Chain Activity

“…We speculated that the mitochondrial respiratory complexes II and IV could be direct targets for the short-term actions of GH, by analogy to what has been previously demonstrated for 3′,5′di-iodothyronine (T2). Although the direct mitochondrial effect obtained with T2 may result from a different molecular mechanism owing to its chemical structure, it has been reported in addition to the detection of T2-binding sites in the organelle [3,37,38] that T2 can bind to a subunit of COX, leading to a conformational change of the enzyme. Moreover, D’Alecy et al [39] have shown that a peptide hormone, the glucagon, that stimulated respiration via its membrane receptor by activation of adenylate cyclase, could directly act on isolated mitochondria and specifically alter oxidative metabolism.…”

Translating Pharmacological Findings from Hypothyroid Rodents to Euthyroid Humans: Is There a Functional Role of Endogenous 3,5-T2?

“…Recent pharmacological applications of 3,5-T2 mainly in hypothyroid rodent models (3,4) challenged this hypothesis because they revealed rapid and chronic metabolic changes, partly independent of the classical T3 receptors (TR). 3,5-T2 rapidly enhanced hepatic oxygen consumption (5,6), and hepatic mitochondria were identified as specific targets (7,8). Moreover, 3,5-T2 was suggested to influence glucose metabolism by stimulating glucose-6-phosphate dehydrogenase (G6PD) activity (9) or by modulating gluconeogenesis via sirtuin 1 (10,11).…”