Yamanouchi H, Ishii S
Department of Biology, School of Education, Waseda University, Tokyo, Japan.
Gen Comp Endocrinol. 1990 May;78(2):231-41. doi: 10.1016/0016-6480(90)90010-j.
We found that the bullfrog testis has specific gonadotropin-binding sites of only a single type which have the same affinity for both follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the bullfrog. The affinity (the equilibrium constant of dissociation or Kd) and capacity (the number of binding sites) of the binding sites for FSH were 0.49 nM and 0.27 fmol/mg fresh tissue, respectively, and those for LH were 0.53 nM and 0.23 fmol/mg fresh tissue, respectively, when these parameters were determined independently. Specific binding of labeled FSH was completely displaced by both FSH and LH, and their competition curves were identical. However, when labeled LH was used, the slope of the curve for competition by FSH was much gentler than that of the curve for competition by LH. A large amount of FSH (about 5 micrograms/ml) was required to replace labeled LH and to reduce the binding of LH to the nonspecific binding level. To explain this result, we assumed the model of nonlinear positive cooperativity by heterologous hormones, in which the affinity for LH of the binding sites is increased in proportion to the number of sites occupied by FSH, raised to the cth power, where c is a constant. When the observed equilibrium parameters were employed and appropriate values for c and for Kf (the Kd for completely occupied binding sites) were chosen as 1.45 and 0.00045 nM, respectively, the theoretical curve for competition by FSH against labeled LH coincided almost exactly with the observed curve. The positive cooperative action of FSH on the binding of LH was also shown by association experiments, but not by dissociation experiments. The positive cooperative action of a heterologous hormone may be a device developed in anurans to overcome the problem that one of the two gonadotropins competitively inhibits the action of the other when both gonadotropins are secreted simultaneously.
我们发现牛蛙睾丸具有仅一种类型的特异性促性腺激素结合位点，该位点对牛蛙的促卵泡激素（FSH）和促黄体生成素（LH）具有相同的亲和力。当分别测定这些参数时，FSH结合位点的亲和力（解离平衡常数或Kd）和容量（结合位点数）分别为0.49 nM和0.27 fmol/mg新鲜组织，LH结合位点的亲和力和容量分别为0.53 nM和0.23 fmol/mg新鲜组织。标记的FSH的特异性结合被FSH和LH完全取代，并且它们的竞争曲线相同。然而，当使用标记的LH时，FSH竞争曲线的斜率比LH竞争曲线的斜率平缓得多。需要大量的FSH（约5微克/毫升）来取代标记的LH并将LH与非特异性结合水平的结合降低。为了解释这一结果，我们假设了异源激素的非线性正协同作用模型，其中结合位点对LH的亲和力与被FSH占据的位点数成正比增加，增加到c次方，其中c是一个常数。当采用观察到的平衡参数并分别选择c和Kf（完全占据结合位点的Kd）的合适值为1.45和0.00045 nM时，FSH与标记的LH竞争的理论曲线几乎与观察到的曲线完全一致。FSH对LH结合的正协同作用在结合实验中也得到了证实，但在解离实验中未得到证实。异源激素的正协同作用可能是无尾目动物发展出的一种机制，以克服当两种促性腺激素同时分泌时，其中一种促性腺激素会竞争性抑制另一种促性腺激素作用的问题。
