Purification and analysis of phospholipids in the inner mitochondrial membrane fraction of bovine corpus luteum, and properties of cytochrome P‐450scc incorporated into vesicles prepared from these phospholipids

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Abstract
Cytochrome P-450scc, which catalyzes the conversion of cholesterol to pregnenolone in steriodogenic tissues, can be incorporated into artificial phospholipid vesicles and cholesterol binding to the cytochrome is affected by the composition of the vesicles. The phospholipids from the inner mitochondrial membrane fraction of the bovine corpus luteum where the cytochrome is located were purified. The composition in mol% was 49% phosphatidylcholine, 34% phosphatidylethanolamine, 8.7% cardiolipin, 6.4% lysophosphatidylethanolamine and 1.5% phosphatidylinositol. The ratio of cholesterol to phospholipid (mol/mol) in the inner membrane fraction was 0.14-1. The Km for cholesterol of purified luteal cytochrome P-450scc incorporation into vesicles prepared from the total inner mitochondrial membrane phospholipids was 0.063 mol of cholesterol/mol of phospholipid. Removal of the cardiolipin component of the inner mitochondrial membrane phospholipids prior to preparation of vesicles caused a 4-fold increase in the Kd of cytochrome P-450 for cholesterol and a 2-fold increase in Km. In the inner mitochondrial membrane of the bovine corpus luteum the cholesterol concentration apparently is less than saturating for cytochrome P-450scc.