3,5,3-Triiodothyronine biological activity

Triiodothyronine (3, 5,3-L-triiodothyronine, T3) is a thyroid hormone. It is producedby outer ring deiodination of thyroxine (T4) in peripheral tissues. The biologic activity of T3 is 3-8 times higher than that of T4. T3 is 99.7% protein-bound and is effective in its free non-protein-bound form. The half-life of triiodothyronine is about 19 h. The daily tur nover of T3 is 75%. Triiodothyronine acts via nuclear receptor binding with subsequent induction of protein synthesis. Effects of thyroid hormones are apparent in almost all organ systems. They include effects on the basal metabolic rate and the metabolisms of proteins, lipids and carbohydrates. 

Traube purine synthesis, 423 Triamcinolone, 201 Triamcinolone acetonide, 201 Triamterine, 427 Triazines, synthesis, 280 Triazolam, 368 Trichlomethiazide, 359 Trichomonas, 238 Tricyclic antidepressants, 149 Trifluoperidol, 306 Trifluoromethyl group, potentiation of biologic activity, 380 Triflupromazine, 380, 381 Trihexyphenidil, 47 Triiodothyronine, 95 Trimeprazine, 378 Trimethadione, 232 Trimethobenzamide, 110 Trimethoprim, 262 Trioxsalen, 334 Tripelenamine, 51 Triprolidine, 78 Tropinone, 5 Tropocaine, 7 Truth serum, 274... 

The structural formulas of thyroxine and triiodothyronine as well as reverse triiodothyronine (rT3) are shown in Figure 38-2. All of these naturally occurring molecules are levo (L) isomers. The synthetic dextro (D) isomer of thyroxine, dextrothyroxine, has approximately 4% of the biologic activity of the L isomer as evidenced by its lesser ability to suppress TSH secretion and correct hypothyroidism. 

This skewed conformation also imparts further stereospecific characteristics to the hormone T3 which contains only a single outer ring iodine. Because of the restricted rotation about the two diphenyl ether bonds, the chemically equivalent 3 and 5 -iodines are conformationally distinct, giving rise to a distal (away) or proximal (near) conformation (Figure 3). To verify the importance of this conformational feature, numerous structural analogues of triiodothyronine were synthesized (4) in an effort to determine the biologically active conformer of T. ... 

A comparison of the affinity of the thyroxine-binding globulin for thyroxine and that of the biologically most effective of the triiodothyronines suggests that the specificity of the binding protein is directed toward the biologically active compound [24]. 

Type-1 iodothyronine deiodinase (ID-1) is an enzyme containing a selenocysteine residue in its active site that catalyzes the conversion of a human thyroid prohormone (thyroxine T4) to a biologically active hormone (3,5,3 -triiodothyronine T3) through 5 -deiodination (Schone 11.29). ... 

In thyroxine (//.//) and the more biologically active triiodothyronine, the iodine atoms in the 3- and 4- positions force the two rings into a conformation in which they are perpendicular to one another. Further work, with analogues, indicates that this arrangement is essential for thyroid function (Dietrich et al. 1977). 

Addendum 1 (Section 2.3, p. Ill) Recently Chopra (C7a) demonstrated that 3,3, 5 -triiodothyronine or reverse Tj (rTa) is present in normal serum in a concentration of approximately 40 ng/100 ml and that there are large increases in rTa concentration in the serum of the newborn when serum Ta concentration is very low. As rTa has no physiological activity, he postulated that deiodination of T may switch from T, to rTa as regulatory mechanism controlling the biological action and metabolism of Ta. 

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