Tri-iodothyronine (T3) and thyroxine (T4) as well as 3,5-di-iodothyronine (T2) stimulated O2 consumption by isolated perfused livers from hypothyroid rats at a concentration as low as 1 pM by about 30% within 90 min. Application of T2 resulted in a faster stimulation than with application of T3 or T4. Inhibition of iodothyronine monodeiodinase by propylthiouracil, thereby blocking the degradation of T4 to T3 and of T3 to T2, demonstrated that only T2 is the active hormone for the rapid stimulation of hepatic O2 consumption: T3 and T4 lost all of their stimulative activity, whereas T2 was as potent as in the absence of propylthiouracil. Perfusion experiments with thyroid-hormone analogues confirmed the specificity of the T2 effect. The nucleus is unlikely to contribute to the rapid T2 effect, as can be deduced from perfusion experiments with cycloheximide and lack of induction of malic enzyme by T2. In conclusion, a new scheme of regulation of mitochondrial activity is proposed: T2 acts rapidly and directly via a mitochondrial pathway, whereas T3 exerts its long-term action indirectly by induction of specific enzymes.

The prohormone thyroxine (T4) is activated by outer ring deiodination (ORD) to 3,3',5_triiodothyronine (T3) and both hormones are degraded by inner ring deiodination (IRD) to 3,3',5'-triiodothyronine (rT3) and 3,3'-diiodothyronine, respectively. Indirect evidence suggests that the type I iodothyronine deiodinase (ID-I) in liver has both ORD and IRD activities, with preference for rT3 and sulfated iodothyronines as substrates. To establish this, we have compared the ORD of rT3 and IRD of T3 and T3 sulfate by homogenates of cells transfected with rat ID-I cDNA and by rat liver microsomes.In both preparations rT3 is the preferred substrate, while deiodination of T3 is markedly accelerated by its sulfation. Kinetic analysis provided similar K,,, and V,,,,, values in cell homogenates and liver microsomes. These data demonstrate unequivocally that ID-I is capable of both activating and inactivating thyroid hormone by ORD and IRD, respectively.

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-Di-iodo-L-thyronine (T2) is a naturally occurring metabolite of thyroxine (T4). Contrary to earlier findings, T2 has recently been shown to have rapid effects in rat liver and in mononuclear blood cells. In the experiments described here, T2 was tested to determine whether it has a TSH suppressive effect in rats in vivo and in rat pituitary fragments in vitro. In experiments over 2 weeks in rats in vivo, low doses of T2 (20-200 micrograms/100 g body weight per day) had no significant influence on body and organ weights, but significantly decreased TSh and T4 serum concentrations. At 200 micrograms/100 g per day, T2 suppressed TSH to 43% and T4 to 29% of control levels. At 1-15 micrograms/100 g per day, 3,5,3'-tri-iodo-L-thyronine (T3), used as a comparison to T2, had significant effects on TSH and T4 levels, and also on body weight. Fifteen micrograms T3/100 g per day decreased TSH to 44%, T4 to 25%, and body weight to 59% of control levels. In experiments over 3 months in rats in vivo, a low dose (25 micrograms/100 g per day) of T2 suppressed TSH to 60% and T4 to 57% of control levels and had no significant influence on other parameters. Conversely, 0.1 microgram/100 g per day T3 had significant effects on body and organ weights as well as pellet intake, but a less pronounced TSH suppressive effect: TSH concentrations were unchanged and T4 concentrations were down to 80% of control values.(ABSTRACT TRUNCATED AT 250 WORDS)

Specific binding sites for 3,3'-T2 can be detected in swollen and osmotically treated mitochondria (OTM) from normal and hypothyroid rat liver. In hypothyroid animals, maximal values of binding were obtained at 0°C and 20°C while values were lower at 37°C and no specific binding could be observed at 60°C. Binding was maximal at pH 6.4 and the mean values for the apparent association constant (Ka) and the binding capacity were on average 0.5 x 108 M -~ and 2.4 pmol/mg mitochondrial protein, respectively. No differences were observed between normal and hypothyroid rats with the exception of the capacity that was higher in normal animals (5.5 pmol/mg mitochondrial protein). The specificity of 3,3'-T2 binding, examined in competition studies, followed this order: 3,3'-T2 > 3,5-T 2 > rT3. The other iodothyronines (3',5'-T2, T3, T4, 3-T~, 3'-T~, 3,5-Diac and 3,3'-Diac) showed only a small competition or none at all.