Changed ratios of glucocorticoids/mineralocorticoids caused by point mutations in the putative I-helix regions of CYP11B1 and CYP11B2.

Computer modelling and site-directed mutagenesis were employed to investigate the structural basis for the regioselectivity of steroid hydroxylation and to determine whether point mutations of CYP11B1 and CYP11B2 can result in changes in the ratio of glucocorticoids/mineralocorticoids. Single replacement of CYP11B2 residues for CYP11B1-specific residues at positions 296, 301, 302, 320, and 335, belonging to the putative I-helix, gave rise to slightly increased 11 beta-hydroxylase activities. Replacement of 3 amino acids of CYP11B2 by the corresponding residues in CYP11B1 was sufficient to increase cortisol formation from about 5% to 85% of the CYP11B1 wild type level. The aldosterone synthase activities of the mutants were decreased to varying degrees indicating that point mutations at positions 296, 301, 302, and 335 could potentially cause hypoaldosteronism. Replacement of Val-320 of CYP11B1 by alanine, the corresponding residue found in CYP11B2, led to production of aldosterone by this mutant enzyme. This observation suggests that glucocorticoid-remediable hyperaldosteronism could also be due to point mutations located in the CYP11B1 sequence.