Plemenitas A, Lenasi H, Hudnik-Plevnik T
Institute of Biochemistry, Medical Faculty, University of Ljubljana, Slovenia.
J Steroid Biochem Mol Biol. 1993 Apr;45(4):281-5. doi: 10.1016/0960-0760(93)90343-u.
Plasma membrane associated binding sites for progesterone have been identified in the filamentous fungus Cochliobolus lunatus (C. lunatus). The Kd for progesterone determined by Scatchard analysis was 13.9 +/- 5.7 nM and the Bmax was 250-360 fmol/mg protein. A broad ligand specificity of these binding sites is suggested by the observation that all tested steroids, regardless of their capability to act as inducers of the 11 beta-steroid hydroxylase, competed at 250-fold excess with [3H]progesterone binding. A biological role of these plasma membrane associated steroid binding sites is nevertheless suggested since in protoplasts which were devoid of them, 11 beta-steroid hydroxylase could not be induced. Progesterone binding sites were present in the plasma membrane as well as in the cytosol and were detected in this fraction, in contrast to the plasma membrane fraction, only under special experimental conditions in respect to redox state. Kd and Bmax of cytosol binding sites were of the same order of magnitude compared to the plasma membrane progesterone binding sites. Ethisterone and 4-cholesten-3-one which cannot induce 11 beta-hydroxylase competed efficiently for plasma membrane binding sites; ethisterone, however also competed for cytosol binding sites and acted, in contrast with 4-cholesten-3-one, as antagonist in the induction of 11 beta-steroid hydroxylase in C. lunatus. On the basis of presented evidence we concluded that C. lunatus contains binding sites for steroids in the plasma membrane and in the cytosol and that both types of binding site are involved in the process of induction of enzymes which transform steroids in this fungus.
在丝状真菌新月弯孢霉（C. lunatus）中已鉴定出与质膜相关的孕酮结合位点。通过Scatchard分析确定的孕酮解离常数（Kd）为13.9±5.7 nM，最大结合量（Bmax）为250 - 360 fmol/mg蛋白质。所有测试的类固醇，无论其是否具有作为11β - 类固醇羟化酶诱导剂的能力，在250倍过量时都能与[3H]孕酮结合竞争，这表明这些结合位点具有广泛的配体特异性。然而，这些与质膜相关的类固醇结合位点的生物学作用仍被认为是存在的，因为在没有这些位点的原生质体中，11β - 类固醇羟化酶无法被诱导。孕酮结合位点存在于质膜以及胞质溶胶中，与质膜部分不同，仅在关于氧化还原状态的特殊实验条件下才能在该部分检测到。与质膜孕酮结合位点相比，胞质溶胶结合位点的Kd和Bmax处于相同的数量级。不能诱导11β - 羟化酶的炔雌醇和4 - 胆甾烯 - 3 - 酮有效地竞争质膜结合位点；然而，炔诺酮也竞争胞质溶胶结合位点，并且与4 - 胆甾烯 - 3 - 酮相反，在新月弯孢霉中作为11β - 类固醇羟化酶诱导的拮抗剂起作用。根据所提供的证据，我们得出结论，新月弯孢霉在质膜和胞质溶胶中含有类固醇结合位点，并且这两种类型的结合位点都参与了该真菌中转化类固醇的酶的诱导过程。
