Studies were carried out to compare the 5' deiodination reactions of thyroxine (T
4) and 3, 3', 5'-triiodothyronine (rT
3) in rat liver and kidney homogenates. The 5'-deiodinase activity was assayed by the 3, 5, 3'-triiodothyronine (T
3) produced from T4 or by the
125I-iodide released from
125I-rT
3. The two 5' deiodination reactions had similar ranges of optimal pH, incubation temperature, and apparent Km, T4 1.1 and rT
3 1.3μM. However, the apparent Vmax values for T
4 and rT
3 deiodination reactions were 0.9 and 220 pmol/mg protein/min, respectively. Both reactions were stimulated by thiol reagent but only rT
3 deiodination showed complete thiol dependence. The inhibitory effect of 6-propy1-2-thiouracil (PTU) on the 5' deiodination of rT
3 was 50 times as great as that of T
4. Only the 5' deiodination of rT
3 was inhibited by low concentrations of calcium and magnesium. The 5' deiodination reactions in the liver and kidney tissues showed very similar substrate specificity. However, only the hepatic deiodinase activity was reduced to 60-65% of the control value after fasting, whereas the renal 5'-deiodinase activity was unaffected or even enhanced by fasting up to 72 hours. The results showed the existence of a diverse and complex 5' deiodination system in the rat tissues which is comprised of multiple similar but distinct 5'-deiodinase enzymes with respect to their substrate specificity, tissue specificity and regulation.
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