The concentration of adrenodoxin reductase limits cytochrome P450scc activity in the human placenta

We have previously reported that cytochrome P450scc activity in the human p lacenta is limited by the supply of electrons to the P450scc [Tuckey, R. C. , Woods, S. T. & Tajbakhsh, M. (1997) Eur. J. Biochem. 244, 835-839]. The a im of the present study was to determine whether it is adrenodoxin reductas e, adrenodoxin or both which limits cytochrome P450scc activity and hence p rogesterone synthesis in the placenta. We found that the concentrations of adrenodoxin reductase and adrenodoxin in placental mitochondria were both c onsiderably lower than the concentrations of these proteins in the bovine a drenal cortex. When P450scc activity assays were carried out at high mitoch ondrial protein concentrations, we found that the addition of exogenous adr enodoxin reductase to sonicated mitochondria rescued pregnenolone synthesis to a level above that for intact mitochondria, showing that adrenodoxin is near-saturating in vivo. in contrast, pregnenolone synthesis by sonicated mitochondria was almost zero even after the addition of human adrenodoxin. This shows that the concentration of endogenous adrenodoxin reductase was i nsufficient to support appreciable rates of pregnenolone synthesis, even wh en concentrated mitochondrial samples were used. Comparative studies with h uman and bovine adrenodoxin reductase have revealed that a twofold higher c oncentration of human adrenodoxin reductase is required for maximal P450scc activity in the presence of saturating human adrenodoxin. Thus, not only i s the adrenodoxin concentration low in placental mitochondria, but the amou nt required for maximal P450scc activity is higher than that for the bovine reductase. Overall, the data indicate that the adrenodoxin reductase conce ntration limits the activity of P450scc in placental mitochondria and hence determines the rate of progesterone synthesis.