Relationship of cholesterol supply to luteal mitochondrial steroid synthesis.

We examined the relationship of cholesterol availability to steroid synthesis by rat luteal mitochondria. Cholesterol supply to luteal mitochondria was manipulated in vitro by incubating organelles with cholesterol-phospholipid dispersions. The molar ratio of cholesterol to phospholipid in dispersions determined the cholesterol content of the mitochondria, and there was a direct relationship (r = 0.92) between mitochondrial cholesterol content and in vitro steroid biosynthesis. Mitochondria of rats treated with choriogonadotropin 1 h before being killed produced more steroid from endogenous substrate than mitochondria of salinetreated rats. Mitochondria of rats given cycloheximide 1 h before being killed produced one-half as much progestin from endogenous substrate as did mitochondria from saline-treated animals. In the presence of cholesterol-rich dispersions (cholesterol/phospholipid = 2:2, mol/mol), mitochondria from salineand choriogonadotropin-treated rats synthesized significantly more steroid than they did from endogenous substrate. However, mitochondria from cycloheximide-treated rats were unresponsive to exogenous substrate. Effects of in uiuo cycloheximide treatment on mitochondrial steroid synthesis were not observed if organelles were subjected to ultrasonic disruption prior to incubation, or intact organelles were incubated in the presence of oxygenated sterols. Our findings indicate that the cholesterol content of mitochondria per se is not the sole determinant of steroidogenic capacity. Movement of mitochondrial cholesterol to the side chain cleavage system appears to be the primary factor, and this process is inhibited by cycloheximide and probably stimulated by gonadotropin.