Nguyen T T, Chapa F, DiStefano J J
Department of Computer Science, University of California, Los Angeles 90095-1596, USA.
Endocrinology. 1998 Nov;139(11):4626-33. doi: 10.1210/endo.139.11.6323.
Production of T3 from T4 in tissues is catalyzed by two 5'-deiodinases, type I (D1) and type II (D2), but the quantitative contribution of each pathway to whole body T3 production is not well established. In the presence of propylthiouracil (PTU), D1, but not D2, can be effectively blocked, providing an experimental probe for addressing this problem. Decades ago, this approach provided indirect estimates ranging from 23-44% contribution by D2, based on plasma T3 appearance rate comparisons (PAR3 = PCR3 [T3]p) in periodically T4-injected athyreotic rats vs. controls. Two, more recent studies, using constant infusions of T4 for replacement, achieved 22% and 65% estimates, respectively, from PAR3 comparisons. We have revisited this problem more directly and precisely, with two major differences in experiment design. We used direct whole body steady state measurements of T3 production, instead of indirect plasma-only data (PAR3). We also used (euthyroid) physiological doses of both T4 (0.9 microg/day x 100 g BW) and T3 (0.15 microg/day x 100 g BW) for replacement in two thyroidectomized rat groups, instead of T4 only, in a 7-day constant steady state, dual tracer infusion protocol. The first group also had chronically implanted 150-mg PTU pellets (TXR-PTU); the other had implanted 0.1 N NaOH placebo pellets (TXR-EU); each delivered their product at constant rates. A third euthyroid intact group was used as the controls. The completeness of D1 inhibition was ascertained in a fourth group, identically treated with 150-mg PTU pellets, in which negligible D1 activity was found in liver and kidney using labeled rT3 as substrate for the 5'-D assays and minimal (1 mM) dithiothreitol as cofactor. In the TXR-PTU group, the percentage of T4 converted to T3 was 11.8%, compared with 23.4% (P < 0.0005) in the TXR-EU group, and 22.7% (P = NS) in controls. Thus, in euthyroid steady state, D2 contributes about half of the T3 produced from T4.
组织中由T4生成T3的过程由两种5'-脱碘酶催化，即I型（D1）和II型（D2），但每条途径对全身T3生成的定量贡献尚未明确。在丙硫氧嘧啶（PTU）存在的情况下，D1可被有效阻断，而D2不受影响，这为解决该问题提供了一种实验手段。几十年前，基于周期性注射T4的甲状腺切除大鼠与对照组血浆T3出现率比较（PAR3 = PCR3 [T3]p），该方法间接估计D2的贡献在23%-44%之间。最近的两项研究，采用持续输注T4进行替代治疗，通过PAR3比较分别得出22%和65%的估计值。我们采用了两种主要的实验设计差异，更直接、精确地重新审视了这个问题。我们使用T3生成的直接全身稳态测量方法，而非仅间接的血浆数据（PAR3）。我们还在两个甲状腺切除大鼠组中，采用（甲状腺功能正常的）生理剂量的T4（0.9微克/天×100克体重）和T3（0.15微克/天×100克体重）进行替代治疗，而非仅使用T4，采用7天恒定稳态双示踪剂输注方案。第一组还长期植入150毫克PTU药丸（TXR-PTU）；另一组植入0.1N NaOH安慰剂药丸（TXR-EU）；每组均以恒定速率释放其产物。第三组甲状腺功能正常的完整大鼠用作对照。在第四组中，采用相同的150毫克PTU药丸处理，以标记的反式T3作为5'-D测定的底物，最小浓度（1毫摩尔）的二硫苏糖醇作为辅助因子，确定D1抑制的完整性，结果发现肝脏和肾脏中的D1活性可忽略不计。在TXR-PTU组中，T4转化为T3的百分比为11.8%，而TXR-EU组为23.4%（P < 0.0005），对照组为22.7%（P = 无显著差异）。因此，在甲状腺功能正常的稳态下，D2对由T4生成的T3贡献约一半。
