Biosynthesis of the adrenocortical steroids is thus effected by the interaction of the various steroid hydroxylases: P450 scc, P450 C21, P450 C17a, and P450 C11b. In addition, 3b-hydroxysteroid dehydrogenase isomerase is required to change the structure in ring A to the important 4-ene-3-one configuration. P450 C21 appears to be capable only of effecting hydroxylation at C21 (for example, of 17- hydroxyprogesterone to 11-deoxycortisol), whereas other hydroxylases are multifunctional. Thus, P450 scc effects hydroxylation at C20 and C22, and also acts as a 20,22 lyase. P450 C17a mediates hydroxylation at C17a and also acts as a 17,20 lyase. This enzyme is absent from the zona glomerulosa cells, which limits the steroidogenic capacity of the cells to mineralocorticoid production. P450 C11 causes hydroxylation at C11, C18, and C19, and can also act in some species as an aromatase.
