THE METABOLISM OF RADIOACTIVE L-5′ ISOPROPYL-3′.3.5 TRIIODOTHYRONINE AND ITS RELATION TO THYROID HORMONE ACTION

Abstract

ABSTRACT The biological activity of L-3′ isopropyl-5′,3,5 triiodothyronine (ISO-T3) an isopropyl analogue of thyroxine (T4) was determined. The minimal goitre prevention dose of ISO-T3 was found to be 1.05 μg/100 g b. w./day. The molar ratio of activity of T3:ISO-T3:T4 equalled 1:3:8.5. T3 and ISO-T3 had the same effect on growth in thyroidectomized rats when injected in a molar ratio of 1:3. The distribution and disappearance of 125I-labelled ISO-T3 and T3 was studied in the thyroidectomized rats maintained on either of these substances. The disappearance rate of ISO-T3 was T½ = 185 min and of T3 T½ = 126 min. The difference was not statistically significant. The molar ratio of accumulated ISO-T3:T3 was 3:1 in skeletal muscle and 1.3:1 in the pituitary gland and hypothalamus. The relative binding of ISO-T3 to plasma proteins was twice as strong as T3. Chromatographic analysis of tissue indicates that ISO-T3 is not appreciably transformed into T3 or 3′ isopropyl-T2. This model would indicate that the activity of a thyroid hormone analogue depends on a combination of the entry into the target site (muscle) and the sensitivity of the target receptor metabolic system. The entry is not solely dependent on the degree of binding by plasma proteins but may involve a receptor entry mechanism.