Oxygen availability as a regulatory factor of androgen synthesis by adrenocortical cells.

Molecular oxygen is an obligatory substrate of all cytochrome P-450 (cyt P-450) hydroxylases involved in the steroid biosynthetic pathways. However, oxygen-derived free radicals are highly destructive species resulting from cyt P-450-catalyzed steroid hydroxylation reactions. Cells in culture are usually exposed to an atmospheric pO2 that is well above the estimated in situ pO2 in vivo. It has been suggested that lowering pO2 might prevent the loss of biosynthetic enzymatic activities in bovine adrenocortical or Leydig cells in culture. The present study was performed to examine the effect of low pO2 pressure (1% oxygen in the gas phase) compared to routinely employed conditions (19% oxygen) on bovine adrenocortical cell steroidogenic activities under both basal and stimulated conditions. Lowering the pO2 showed no significant effect on the cultured adrenocortical cell proliferation rate or their ability to produce cortisol. By contrast, it resulted in a dramatic drop in androgen secretion and a slight increase in corticosterone synthesis. The mechanism involved in the qualitative modulation of steroid biosynthesis by oxygen availability was examined in some detail using purified steroid hydroxylase components. We found that cyt P-450(17 alpha, which can catalyze both the steroid 17 alpha-hydroxylation and the 17-20-lyase reaction is probably the major target explaining the oxygen effect. Indeed, cyt P-450(17 alpha) hydroxylase activity exhibits a clearly higher affinity for oxygen (Km, 22 microM) than its lyase activity (Km, 66 microM). These observations suggest that 1) oxygen availability is able to modulate the balance between androgen and corticosteroid pathways in bovine adrenocortical cell; and 2) adrenocortical cell functions studied in vitro under relatively high pO2 do not exactly reflect the in vivo situation.