Analyses of the CYP11B gene family in the guinea pig suggest the existence of a primordial CYP11B gene with aldosterone synthase activity

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Abstract
In this study we describe the isolation of three genes of the CYP11B family of the guinea pig. CYP11B1 codes for the previously described 11β‐hydroxylase [Bülow, H.E.,Möbius, K., Bähr, V. & Bernhardt, R. (1996) Biochem. Biophys. Res. Commun. 221, 304–312] while CYP11B2 represents the aldosterone synthase gene. As no expression for CYP11B3 was detected this gene might represent a pseudogene. Transient transfection assays show higher substrate specificity for its proper substrate for CYP11B1 as compared to CYP11B2, which could account for the zone‐specific synthesis of mineralocorticoids and glucocorticoids, respectively. Thus, CYP11B2 displayed a fourfold higher ability to perform 11β‐hydroxylation of androstenedione than CYP11B1, while this difference is diminished with the size of the C17 substituent of the substrate. Furthermore, analyses with the electron transfer protein adrenodoxin indicate differential sensitivity of CYP11B1 and CYP11B2 as well as the three hydroxylation steps catalysed by CYP11B2 to the availability of reducing equivalents. Together, both mechanisms point to novel protein intrinsic modalities to achieve tissue‐specific production of mineralocorticoids and glucocorticoids in the guinea pig. In addition, we conducted phylogenetic analyses. These experiments suggest that a common CYP11B ancestor gene that possessed both 11β‐hydroxylase and aldosterone synthase activity underwent a gene duplication event before or shortly after the mammalian radiation with subsequent independent evolution of the system in different lines. Thus, a differential mineralocorticoid and glucocorticoid synthesis might be an exclusive achievement of mammals.