Hydroxylation steps in the biosynthesis of adrenal steroids (Fig. 7.7) require NADPH, molecular oxygen, and an enzyme complex. The complex consists of a flavoprotein dehydrogenase, a nonhaem iron-containing protein (adrenodoxin), and cytochrome P450 (so called since a characteristic high absorption is observed when it complexes with carbon monoxide). This oxidase system is essentially similar for 11a-, 20,22-, 17a- and 18-hydroxylation, and it appears that specificity is achieved through differences in the respective P450 cytochromes. Hydroxylation is mediated by the introduction of one electron into the electron transport chain, with the introduction of one atom of oxygen into the steroid. The remaining oxygen atom combines with hydrogen to form water. This system is known as a ΓÇÿmixed-function oxidaseΓÇÖ.
