ELECTRON-TRANSFER TO CYTOCHROME P-450SCC LIMITS CHOLESTEROL-SIDE-CHAIN-CLEAVAGE ACTIVITY IN THE HUMAN PLACENTA

The aim of this study was to determine whether electron transfer from adrenodoxin reductase and adrenodoxin limits the activity of cytochrom e P-450scc in mitochondria from the human placenta. Mitochondria were disrupted by sonication to enable exogenous adrenodoxin and adrenodoxi n reductase to deliver electrons to cytochrome P-450scc. After sonicat ion, the rate of pregnenolone synthesis was greatly decreased relative to that by intact mitochondria, due to dilution of endogenous adrenod oxin and adrenodoxin reductase into the incubation medium. The additio n of saturating concentrations of bovine or human adrenodoxin and bovi ne adrenodoxin reductase to the disrupted mitochondria gave an initial rate of pregnenolone synthesis that was 6.3-fold higher than that for intact mitochondria. Similar results were observed when 20 alpha-hydr oxycholesterol was used as substrate rather than endogenous cholestero l. The turnover number of cytochrome P-450scc in sonicated placental m itochondria supplemented with adrenodoxin and adrenodoxin reductase wa s comparable to that for the purified enzyme assayed under conditions where electron transfer was not limiting. Addition of exogenous adreno doxin and adrenodoxin reductase to sonicated mitochondria from the pig corpus luteum and rat adrenal had a much smaller effect on pregnenolo ne synthesis compared with intact mitochondria, than observed for the placenta. We conclude that in the human placenta, electron transfer to cytochrome P-450scc is limiting, permitting pregnenolone synthesis to proceed at only 16% maximum velocity.