Eckstein B, Greenbaum O, Cohen S
Endocrinology. 1985 Dec;117(6):2376-82. doi: 10.1210/endo-117-6-2376.
It has been shown that 3 h after the preovulatory gonadotropin surge, an abrupt decrease occurs in follicular C-17,20-lyase (lyase) activity which causes a decrease in C19-steroid production. To determine the reason for the reduced lyase activity, we used rats that were induced to ovulate by means of PMSG administration. In these rats, a 54% decrease in lyase activity occurred at the peak of the LH surge. When the gonadotropin surge and ovulation were blocked by pentobarbitone the decrease was prevented. Administration of LH to the pentobarbitone-blocked rats reduced lyase activity to well below the level reached after the endogenous gonadotropin surge. In cycling proestrous rats as well, human CG (hCG) decreased the lyase activity, as measured in isolated follicles 3 h after hCG administration. Out of three potential inhibitory steroids for lyase activity; progesterone, 3 alpha,17 alpha-dihydroxy-5 alpha-pregnen-20-one, and 17 alpha,20 alpha-dihydroxy-4-pregnen-3-one, only the last compound inhibited competitively ovarian lyase activity. The inhibition constant (Ki) value was 29 microM. In order to explore which of the two activities of the lyase complex is regulated by the gonadotropin, a double label double substrate experiment was conducted using [14C]progesterone with [3H]17 alpha-hydroxyprogesterone (17 alpha-OHP). With this assay procedure, we could determine in the same experiment the site of stimulation, the preferred substrate, and the amount of conversion. The conversion of progesterone to 17 alpha-OHP, as well as the conversion to androstenedione were significantly inhibited throughout the reaction by the gonadotropin. Thus, the changes in ovarian lyase after hCG mimic those of 17 alpha-hydroxylase. The labeling pattern of androstenedione showed that the ovarian lyase complex catalyzed the conversion preferentially from progesterone. Whereas the 3H/14C ratio in androstenedione varied between 0.29 to 0.76, the ratios in the 17 alpha-OHP were from 5 to 22. Thus, the exogenous 17 alpha-OHP did not equilibrate with the product formed from progesterone. The effect of the hCG was to decrease the preference of progesterone over 17 alpha-OHP as substrate. It is concluded that: the LH of the surge inhibits both the 17 alpha-hydroxylase and lyase activities. The ovarian lyase complex shows a preference for progesterone as a substrate rather than 17 alpha-OHP. 17 alpha-OHP is not an obligatory intermediate in androstenedione production in ovarian tissue. hCG affects the ovarian lyase complex by shifting the relative preference of substrates towards 17 alpha-OHP.
研究表明，在排卵前促性腺激素峰出现3小时后，卵泡C-17,20-裂解酶（裂解酶）活性急剧下降，导致C19-类固醇生成减少。为了确定裂解酶活性降低的原因，我们使用了通过注射孕马血清促性腺激素（PMSG）诱导排卵的大鼠。在这些大鼠中，促黄体生成素（LH）峰出现时，裂解酶活性下降了54%。当用戊巴比妥阻断促性腺激素峰和排卵时，这种下降被阻止。给戊巴比妥阻断的大鼠注射LH，可使裂解酶活性降至内源性促性腺激素峰后达到的水平以下。在处于发情前期的周期大鼠中，人绒毛膜促性腺激素（hCG）给药3小时后，在分离的卵泡中检测到，hCG也降低了裂解酶活性。在三种可能抑制裂解酶活性的类固醇中；孕酮、3α,17α-二羟基-5α-孕烯-20-酮和17α,20α-二羟基-4-孕烯-3-酮，只有最后一种化合物竞争性抑制卵巢裂解酶活性。抑制常数（Ki）值为29微摩尔。为了探究裂解酶复合物的两种活性中哪一种受促性腺激素调节，使用[14C]孕酮与[3H]17α-羟基孕酮（17α-OHP）进行了双标记双底物实验。通过这种测定方法，我们可以在同一实验中确定刺激位点、首选底物以及转化量。在整个反应过程中，促性腺激素显著抑制了孕酮向17α-OHP的转化以及向雄烯二酮的转化。因此，hCG后卵巢裂解酶的变化与17α-羟化酶的变化相似。雄烯二酮的标记模式表明，卵巢裂解酶复合物优先催化孕酮的转化。虽然雄烯二酮中的3H/14C比值在0.29至0.76之间变化，但17α-OHP中的比值为5至22。因此，外源性17α-OHP与由孕酮形成的产物未达到平衡。hCG的作用是降低孕酮相对于17α-OHP作为底物的偏好性。结论是：峰期的LH抑制17α-羟化酶和裂解酶活性。卵巢裂解酶复合物显示出对孕酮作为底物的偏好性而非17α-OHP。17α-OHP不是卵巢组织中雄烯二酮生成的必需中间体。hCG通过将底物的相对偏好性转向17α-OHP来影响卵巢裂解酶复合物。
