Höffken B, Ködding R, Köhrle J, Hesch R D
Clin Chim Acta. 1978 Nov 15;90(1):45-51. doi: 10.1016/0009-8981(78)90082-7.
In the extrathyroidal deiodination of T4 the importance of T3 and rT3 for the peripheral action of thyroid hormones is well documented. With the development of a specific radioimmunoassay for 3,3'-T2, a deiodination product of both T3 and rT3, we were able to characterize these subsequent enzymatic reactions as well as the degradation of 3,3'-T2 in rat liver homogenate. It was found that the reaction T3 leads to 3,3'-T2 is slow compared to the conversion of T4 to T3. The pH activity profile shows a peak at 8.4. The reaction rT3 leads to 3,3'-T2 is very fast, with an apparent KM of 4 x 10(-7) M. The reaction velocity is significantly higher in acid than in alkaline pH. The deiodination of 3,3'-T2 is also faster in the acid than in the alkaline range. It is concluded that the outer ring of T4 is more readily deiodinated in acid and the inner ring in alkaline media, and that 3,3'-T2 is mainly degraded to 3-T1. In the acid pH range T3 may accumulate and in the alkaline range rT3 by the pH characteristics of these ractions. Therefore small shifts in pH can enhance the potent inhibitory action of rT3 on the 5'-deiodination of T4 adding another mechanism to the peripheral regulation of thyroid hormone activity.
在甲状腺外T4的脱碘过程中，T3和反T3（rT3）对甲状腺激素外周作用的重要性已有充分文献记载。随着针对T3和rT3的脱碘产物3,3'-二碘甲腺原氨酸（3,3'-T2）的特异性放射免疫测定法的发展，我们得以对大鼠肝匀浆中这些后续的酶促反应以及3,3'-T2的降解进行表征。结果发现，与T4转化为T3相比，T3转化为3,3'-T2的反应较慢。pH活性曲线在8.4处出现峰值。rT3转化为3,3'-T2的反应非常快，表观米氏常数（KM）为4×10⁻⁷ M。该反应在酸性条件下的速度明显高于碱性pH条件。3,3'-T2的脱碘在酸性条件下也比碱性范围内更快。结论是T4的外环在酸性条件下更容易脱碘，内环在碱性介质中更容易脱碘，并且3,3'-T2主要降解为3-碘甲腺原氨酸（3-T1）。在酸性pH范围内，T3可能会积累，而在碱性范围内，rT3会积累，这是由这些反应的pH特性决定的。因此，pH的微小变化可以增强rT3对T4 5'-脱碘的强效抑制作用，这为甲状腺激素活性的外周调节增加了另一种机制。
