Aldosterone biosynthesis and cytochrome P-450 11β: evidence for two different forms of the enzyme in rats

Abstract

Whereas cytochrome P-450 11β has been recently shown to catalyze the two-step conversion of corticosterone to aldosterone in the bovine and porcine adrenal cortex, the identity of the enzyme involved in the two final steps of aldosterone biosynthesis in the rat adrenal cortex is as yet unknown. Mitochondria from capsular adrenals of sodium-deficient, potassium-replete rats converted corticosterone to 18-hydroxycorticosterone and aldosterone at markedly higher rates than mitochondria from capsular adrenals of sodium-replete, potassium-deficient rats. However, the same preparations exhibited no difference in the 11β-hydroxylase activity, i.e. the conversion of deoxycorticosterone to corticosterone. Only mitochondria of zona glomerulosa from rats with stimulated aldosterone biosynthesis contained a 49K. protein which showed a strong cross-reactivity with a monoclonal antibody raised against purified bovine cytochrome P-450 11β. By contrast, a crossreactive protein with a molecular weight of 51K was found in mitochondria of zona fasciculata and in mitochondria of zona glomerulosa from rats with a suppressed aldosterone biosynthesis. These findings indicate the existence of two different forms of cytochrome P-450 11β in the rat adrenal cortex, with only one of them, i.e. the 49K form, being capable of catalyzing the two final steps of aldosterone biosynthesis in situ.