Schwartz H L, Surks M I, Oppenheimer J H
J Clin Invest. 1971 May;50(5):1124-30. doi: 10.1172/JCI106584.
Studies of the rate of extrathyroidal conversion of thyroxine (T4) to 3,5,3'-triiodo-L-thyronine (T3) were carried out in rats. Total body homogenates were prepared and extracted with ethanol 48, 72, and 96 hr after the intravenous injection of (125)I-T4. (131)I-T3 was added, and the paper chromatographic purification of T3 was effected by serial elution and rechromatography in three paper and one thin-layer cycles. The ratio of (131)I-T3 and (125)I-T3 counting rates in the final chromatograms, which was identical in three different paper chromatography systems, was used to calculate the proportion of (125)I-T3 to (125)I-T4 in the original homogenates. In order to discount the effects of in vitro monodeiodination of T4 during extraction and chromatography, we killed control animals immediately after injection of (125)I-T4 and processed them in a similar fashion to the experimental groups. The average ratio of (125)I-T3 to (125)I-T4 in carcass extracts of animals killed between 48 and 96 hr after isotopic injection was 0.08 whereas the average ratio of (125)I-T3 to (125)I-T4 in chromatograms of control animals was 0.01. On the basis of the proposed model, calculations indicated that about 17% of the secreted T4 was converted to T3. Assuming values cited in the literature for the concentration of nonradioactive T3 in rat plasma, these findings would suggest that about 20% of total body T3 is derived by conversion from T4. Moreover, since previous estimates have suggested that in the rat, T3 has about 3 to 5 times greater biologic activity than T4, these results also raise the possibility that the hormonal activity of T4 may be dependent in large part on its conversion to T3.A necessary assumption in calculating T4 to T3 conversion in this and other studies is that the 3' and 5' positions are randomly labeled with radioiodine in phenolic-ring iodine-labeled T4. Evidence supporting this assumption was obtained in the rat by comparing the amount of labeled T3 produced after injection of phenolic and nonphenolic-ring iodine-labeled T4.
在大鼠身上进行了甲状腺素（T4）向3,5,3'-三碘-L-甲状腺原氨酸（T3）的甲状腺外转化率的研究。静脉注射（125）I-T4后48、72和96小时制备大鼠全身匀浆并用乙醇提取。加入（131）I-T3，通过在三张纸和一个薄层色谱中进行连续洗脱和再色谱法对T3进行纸色谱纯化。最终色谱图中（131）I-T3和（125）I-T3计数率的比值（在三种不同的纸色谱系统中相同）用于计算原始匀浆中（125）I-T3与（125）I-T4的比例。为了消除提取和色谱过程中T4体外单碘化的影响，我们在注射（125）I-T4后立即处死对照动物，并以与实验组相似的方式处理它们。同位素注射后48至96小时处死的动物的尸体提取物中（125）I-T3与（125）I-T4的平均比值为0.08，而对照动物色谱图中（125）I-T3与（125）I-T4的平均比值为0.01。根据所提出的模型计算表明，约17%分泌的T4转化为T3。假设采用文献中大鼠血浆中非放射性T3浓度的引用值，这些发现表明全身T3的约20%来自T4的转化。此外，由于先前的估计表明在大鼠中，T3的生物活性比T4大约高3至5倍，这些结果也增加了T4的激素活性可能在很大程度上依赖于其向T3转化的可能性。在本研究及其他研究中计算T4向T3转化率的一个必要假设是，在酚环碘标记的T4中，3'和5'位被放射性碘随机标记。通过比较注射酚环和非酚环碘标记的T4后产生的标记T3的量，在大鼠身上获得了支持该假设的证据。
