Origin of circulating 18-oxocortisol in the normal human adrenal.

18-Oxocortisol is the product of the metabolism of 11-deoxycortisol by the mitochondrial enzyme aldosterone synthase (CYP11B2). The traditional concept is that the CYP11B2 is exclusively expressed in zona glomerulosa cells and the 17α-hydroxylase (CYP17A1) enzyme, required to synthesize 11-deoxycortisol, is in the zona fasciculata of the human adrenal. It has been postulated that the substrate for 18-oxocortisol is either cortisol from the circulation or from zona fasciculata cells adjacent to the zona glomerulosa.