Karpyza E I, Kiklevich I E, Ermolenko M N, Sviridov O V
Biokhimiia. 1993 Feb;58(2):285-94.
Binding processes in in vitro systems modelling specific interactions between transport and receptor proteins, thyroid hormones of human placental tissue and the washing blood, have been studied. These systems included syncytiotrophoblast villous membranes, thyroxine (T4), triiodothyronine (T3) and iodothyronine-binding proteins: transthyretin, albumin, immunoglobulins (Ig) M and G, apolipoprotein A-I, and the T4-binding globulin purified from human retroplacental serum. All of the transport proteins at concentrations close to the Ka values of their complexes with thyroid hormones produced inhibitory effects on the binding of [125I]T3 or [125I]T4 to the thyroid hormone membrane receptor. The dependence of [125I]T4 membrane binding on IgM concentration in the system was characteristic of all proteins studied. In the case of T3 such dependence was unique for IgM, and included the phase of the IgM stimulatory effect (10(-11)-10(-9) M) and the phase of inhibition (10(-8)-10(-7) M). In the presence of 30 pM IgM, the concentration of the membrane T3-binding sites increased by 75% with a 2.2-fold decrease of the association constant (Ka). It was shown that IgM interacts specifically with two classes of binding sites on the plasma membranes with Ka(1) = 5.0 x 10(9) M-1, Bmax(1) = 34 fmol/mg of membrane protein and Ka(2) = 2.7 x 10(7) M-1, Bmax(2) = 2.0 pmol/mg of membrane protein. It is suggested that the stimulatory effect of IgM is caused by increases in the number of T3-binding sites on the placental villous membranes as a result of complex formation between IgM and its membrane receptor which displays an enhanced T3-binding activity.
对体外系统中模拟转运蛋白与受体蛋白、人胎盘组织甲状腺激素及洗涤血液之间特异性相互作用的结合过程进行了研究。这些系统包括合体滋养层绒毛膜、甲状腺素（T4）、三碘甲状腺原氨酸（T3）和碘甲状腺原氨酸结合蛋白：转甲状腺素蛋白、白蛋白、免疫球蛋白（Ig）M和G、载脂蛋白A-I，以及从人胎盘后血清中纯化的T4结合球蛋白。所有转运蛋白在其与甲状腺激素复合物的Ka值附近的浓度下，均对[125I]T3或[125I]T4与甲状腺激素膜受体的结合产生抑制作用。[125I]T4膜结合对系统中IgM浓度的依赖性是所有研究蛋白的特征。对于T3而言，这种依赖性是IgM所特有的，包括IgM刺激效应阶段（10^(-11)-10^(-9)M）和抑制阶段（10^(-8)-10^(-7)M）。在存在30pM IgM的情况下，膜T3结合位点的浓度增加了75%，而结合常数（Ka）降低了2.2倍。结果表明，IgM与质膜上的两类结合位点特异性相互作用，其Ka(1)=5.0×10^9M^(-1)，Bmax(1)=34fmol/mg膜蛋白，Ka(2)=2.7×10^7M^(-1)，Bmax(2)=2.0pmol/mg膜蛋白。有人认为，IgM的刺激效应是由于IgM与其膜受体之间形成复合物，导致胎盘绒毛膜上T3结合位点数量增加，而该膜受体表现出增强的T3结合活性。
