Kobayashi A, Shimazaki M, Kuwahara H, Ohashi H, Hashimoto R, Inoue A, Hamada N, Morii H, Shimazu A
Osaka City Med J. 1989 Nov;35(2):137-44.
It has been generally believed that reverse triiodothyronine (reverse T3, rT3) is biologically inactive. Nevertheless the serum rT3 level is very high in fetal life, when the serum calcium level is higher than in the maternal. For this hypercalcemia in fetal life, there is no convincing evidence that any major calcium regulating hormone is responsible. And the assumption that rT3 may be concerned in active transport of calcium at placenta could be suggested. In this study nuclear protein was isolated from human placenta by extraction with 0.4M KCl buffer and binding studies utilizing radioactive rT3 were carried out. Scatchard analysis presented a curvilinear pattern, suggesting two classes of receptors: One with a association constant (Ka) of 1.14 X 10(8) M-1 and a limited capacity (Bmax) of 32.0 X 10(-15) mol/100 micrograms DNA and the other with Ka = 4.34 X 10(6) and Bmax = 454 X 10(-15). Its relative affinities for several thyroid hormone analogues were calculated. If the affinity for rT3 was assigned l, that for triiodothyronine would be 1/40, thyroxine: 1/63, Triiodothyroacetic acid: 1/18. Sulfhydryl agents affecting these binding characteristics were also studied. The Ka was not substantially changed but the Bmax was notably decreased by dithiothreitol (43%) and 2-mercaptoethanol (26%). In liver nuclei, the binding characteristics of rT3 receptor were analyzed for comparison. In this case only a single class of low affinity-high capacity rT3 binding site with a Ka of 6.69 X 10(6) was detected, a finding which was apparently different from those in placenta.(ABSTRACT TRUNCATED AT 250 WORDS)
人们普遍认为，反式三碘甲状腺原氨酸（反T3，rT3）没有生物活性。然而，在胎儿期，血清rT3水平非常高，此时血清钙水平高于母体。对于胎儿期的这种高钙血症，没有令人信服的证据表明任何主要的钙调节激素与之有关。由此可以推测，rT3可能参与了胎盘处钙的主动转运。在本研究中，用0.4M KCl缓冲液从人胎盘中提取核蛋白，并利用放射性rT3进行结合研究。Scatchard分析呈现出曲线模式，表明存在两类受体：一类的缔合常数（Ka）为1.14×10⁸ M⁻¹，结合容量有限（Bmax）为32.0×10⁻¹⁵ mol/100微克DNA；另一类的Ka = 4.34×10⁶，Bmax = 454×10⁻¹⁵。计算了其对几种甲状腺激素类似物的相对亲和力。如果对rT3的亲和力设定为1，则对三碘甲状腺原氨酸的亲和力为1/40，对甲状腺素为1/63，对三碘甲状腺乙酸为1/18。还研究了影响这些结合特性的巯基试剂。二硫苏糖醇（43%）和2-巯基乙醇（26%）使Ka没有显著变化，但使Bmax明显降低。为作比较，分析了肝细胞核中rT3受体的结合特性。在这种情况下，仅检测到一类低亲和力-高容量的rT3结合位点，其Ka为6.69×10⁶，这一结果显然与胎盘中的不同。（摘要截短于250字）
