Leaver H A, Boyd G S
J Endocrinol. 1981 Oct;91(1):123-33. doi: 10.1677/joe.0.0910123.
The pool of cholesterol at the active site of the cholesterol side-chain cleavage enzyme was increased by preincubation with substrate. This process occurred at a linear time-dependent rate over 20 min, whereas uptake of cholesterol into mitochondria was complete by 1 min. Both endogenous mitochondrial cholesterol and added extramitochondrial cholesterol showed similar time-dependent substrate preincubation kinetics. The rate of metabolism of exogenous cholesterol was increased by injection of LH and decreased by treatment with cycloheximide. The rate of entry of cholesterol into the active site in both preparations, however, retained linear time-dependent kinetics. Calcium ions (2 mmol/L) increased the rate of steroidogenesis most in mitochondria preincubated for short periods and least in mitochondria preincubated for longer periods. These studies indicated that the pool of cholesterol at the active site of the enzyme can determine the rate of pregnenolone biosynthesis in the mitochondria of rat corpus luteum. Calcium ions, but not trophic hormones or cycloheximide, appeared to alter the access of cholesterol to the active site of the cholesterol side-chain cleavage enzyme during substrate preincubation.
通过与底物预孵育，胆固醇侧链裂解酶活性位点处的胆固醇池增加。该过程在20分钟内以线性时间依赖性速率发生，而胆固醇在线粒体内的摄取在1分钟内完成。内源性线粒体胆固醇和添加的线粒体外胆固醇均表现出相似的时间依赖性底物预孵育动力学。注射促黄体生成素可增加外源性胆固醇的代谢速率，而用环己酰亚胺处理则降低该速率。然而，在两种制剂中，胆固醇进入活性位点的速率均保持线性时间依赖性动力学。钙离子（2 mmol/L）对短期预孵育的线粒体中类固醇生成速率的增加作用最大，而对长期预孵育的线粒体中类固醇生成速率的增加作用最小。这些研究表明，酶活性位点处的胆固醇池可决定大鼠黄体线粒体中孕烯醇酮的生物合成速率。在底物预孵育期间，钙离子似乎改变了胆固醇进入胆固醇侧链裂解酶活性位点的途径，而营养激素或环己酰亚胺则不然。
