Cytochrome P450 and Steroidogenesis
Walter L. Miller
Department of Pediatrics, University of California, San Francisco
Many steroids were isolated and characterized chemically by 1950, but their biosynthetic pathways were unknown. Ryan & Engel reported steroid 21-hydroxylation by adrenal microsomes, inhibited by CO and reversed by light (1956, 57); Klingenberg reported P450 spectra in 1957; and Cooper, Estabrook et al., showed 21-hydroxylation was P450-mediated (1963-65). Harding et al., reported P450 spectra in adrenal mitochondria in 1964, and in 1966 Omura et al., reported mitochondrial steroid 11-hydroxylation required two proteins now known as ferredoxin (FDX) and ferredoxin reductase (FDXR). These specific steroidogenic steps in different organelles were the first reactions shown to be P450-catalyzed.
Steroidogenic pathways, enzymes, and genes vary among species. Most studies of steroidogenesis utilize the adrenal, as steroidogenic cells comprise >80% of its mass, facilitating preparation of specific factors. The adrenal has functional zones that express mitochondrial P450scc (CYP11A1 gene), catalyzing the 3 reactions that convert cholesterol to pregnenolone; a disease where this activity is diminished led to the discovery of the steroidogenic acute regulatory protein, StAR. The Zona Glomerulosa (ZG), converts pregnenolone to the potent mineralocorticoid, aldosterone, because the ZG expresses aldosterone synthase (P450c11AS, CYP11B2) but does not express 17-hydroxylase (P450c17, CYP17A1), thus excluding synthesis of glucocorticoids and sex steroids. The Zona Fasciculata (ZF) expresses P450c11b (CYP11B1) and P450c17, but not P450c11AS, permitting synthesis of cortisol in most vertebrates (the rodent ZF fails to express P450c17, resulting in corticosterone production). P450c17 may also catalyze 17,20-lyase activity, converting 21-carbon (C21) steroids to C19 precursors of sex steroids. Whether 17,20-lyase activity occurs depends on factors favoring electron transfer to P450c17: i) the abundance of P450 oxidoreductase (POR), the redox partner for all Type 2 P450s; ii) the presence of cytochrome b5 (b5); iii) Ser/Thr phosphorylation of P450c17 (probably by p38a). The mature primate adrenal has a Zona Reticularis (ZR) that produces abundant C19 steroids at the time of ‘adrenarche’, the onset of adrenal androgen synthesis beginning before puberty. However, the well-studied C19 steroids DHEA, DHEAS, and androstenedione do not activate the androgen receptor: they identify but do not mediate adrenarche.
Work from 2013-2018 identified the adrenal androgen as 11keto-testosterone (11KT), made in the ZR by a newly described pathway requiring P450c11b. Also recently described is the adrenal/testicular ‘backdoor pathway’ to dihydrotestosterone that bypasses DHEA/S and androstenedione; this pathway is important in fetal sexual development and in hyperandrogenic disorders. Genetic diseases have been central to identifying enzymes and pathways and continue to reveal new physiology.
