Dionne F T, Dube J Y, Tremblay R R
Can J Biochem. 1977 Sep;55(9):995-1000. doi: 10.1139/o77-148.
In our studies of the binding of steroids in rat skeletal muscles, we have shown the existence of a saturable '4S' dihydrotestosterone-binding protein of low affinity (Kd = 1.16 X 10(-6) M). Competition studies led us to believe that the binding of dihydrotesterone (DHT) was due to 3 alpha-hydroxysteroid dehydrogenase (3 alpha-OHSD) enzyme responsible for the conversion of DHT to androstanediol (Adiol). Indeed, both the binding and the enzymatic activity are inhibited by various 3-keto steroids. In addition, the dissociation constant (Kd) and the Michaelis constant (Km) of the enzyme are similar. However, experiments with ammonium sulfate fractions of the cytosol have shown a partial separation of the binding and of the enzymatic activity. On the other hand, we have established that DHT (3 micrometer) is almost completely metabolized to Adiol during a 2-h incubation of 0 degrees C even in the absence of added coenzymes. Furthermore the '4S' protein binds Adiol more strongly than DHT and this binding is not saturable. Finally the binding behaviour of both DHT and Adiol with either muscle cytosol or rat albumin is similar when subjected to ammonium sulfate fractionation and sucrose density gradient centrifugation. In conclusion, the skeletal muscle 3 alpha-OHSD rapidly metabolizes DHT into Adiol which then binds strongly to a nonspecific binding protein, presumably rat serum albumin. Thus it can be said that the observed saturability of DHT binding is only apparent.
在我们对大鼠骨骼肌中类固醇结合的研究中，我们已经证明存在一种低亲和力（Kd = 1.16×10⁻⁶ M）的可饱和“4S”双氢睾酮结合蛋白。竞争研究使我们相信双氢睾酮（DHT）的结合是由于负责将DHT转化为雄烷二醇（Adiol）的3α-羟基类固醇脱氢酶（3α-OHSD）。事实上，结合和酶活性都受到各种3-酮类固醇的抑制。此外，该酶的解离常数（Kd）和米氏常数（Km）相似。然而，用胞质溶胶的硫酸铵分级分离实验表明结合和酶活性有部分分离。另一方面，我们已经确定，即使在没有添加辅酶的情况下，在0℃孵育2小时期间，DHT（3微摩尔）几乎完全代谢为Adiol。此外，“4S”蛋白与Adiol的结合比与DHT的结合更强，并且这种结合是不饱和的。最后，当进行硫酸铵分级分离和蔗糖密度梯度离心时，DHT和Adiol与肌肉胞质溶胶或大鼠白蛋白的结合行为相似。总之，骨骼肌3α-OHSD迅速将DHT代谢为Adiol，然后Adiol与一种非特异性结合蛋白（可能是大鼠血清白蛋白）强烈结合。因此可以说，观察到的DHT结合的可饱和性只是表面现象。
