Stimulation of Luteal Mitochondrial Cholesterol Side-Chain Cleavage by Cardiolipin*

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Abstract
The effects of exogenous phospholipids upon steroid hormone synthesis by rat luteal mitochondria and mitochondrial acetone powders were studied. Cardiolipin, added as a dispersion, to intact mitochondria prepared from ovaries of PMS gonadotropin-hCG-primed immature rats produced a significant (up to 4-fold) stimulation of net steroid synthesis. A number of other phospholipids, including phosphatidylinositol- 4′-phosphate, either had no striking effect upon steroid synthesis or inhibited it. The stimulatory effects of cardiolipin were dose dependent, with maximal stimulation observed at 100–200 μM. Whether mitochondria were prepared from the ovaries of rats acutely treated with cycloheximide, hCG, or vehicle, cardiolipin stimulated steroid production to the same absolute value. Exogenous cardiolipin caused a pronounced increase (6-fold) in steroid synthesis by mitochondrial acetone powders, while phosphatidylcholine and phosphatidylinositol-4′-phosphate produced a much smaller increase. Although exogenous cardiolipin was clearly shown to stimulate mitochondrial steroidogenesis, treatments that acutely altered the capacity of rat luteal mitochondria to produce steroids in vitro, such as hCG (stimulation) or cycloheximide (inhibition) administration, did not affect the phospholipid composition of the mitochondrial fractions or the in vivo incorporation of 32PO4 into mitochondrial cardiolipin. We conclude that exogenous cardiolipin can be a potent stimulator of luteal mitochondrial steroidogenesis. While this polar lipid may be an endogenous activator of cholesterol side-chain cleavage, the acute effects of tropic hormones and protein synthesis inhibitors on steroidogenesis do not appear to be mediated by major changes in the mitochondrial content of cardiolipin.