Aldosterone Biosynthesis by Cell-Free Systems

Abstract

Aldosterone is synthesized by homogenized adrenocortical tissue from endogenous and from exogenous precursors. According to Kahnt and Neher (1965), the endogenous precursor of aldosterone production by bovine adrenocortical tissue homogenate is most likely free cholesterol. After centrifugal fractionation of sheep (Raman et al. 1966), bullfrog (Psychoyos et al. 1966) and rat (Marusic and Mulrow 1967 b) adrenocortical homogenates, only the mitochondrial fractions were found to actively convert corticosterone to 18-hydroxycorticosterone and aldosterone. Aupetit et al. (1977 b) demonstrated that the conversion of added 1.8-hydroxycorticosterone to aldosterone was also restricted to the mitochondrial fraction of duck adrenocortical tissue rather than being localized in both the microsomal and the mitochondrial fraction as had been previously reported by Aupetit (1972). In sheep adrenal mitochondria, the site of both reactions was found to be the internal membrane (Antreassian et al. 1979). Attempts by Raman et al. (1966) and by Sandor et al. (1972) to bring the mitochondrial enzyme system involved in the two final steps of aldosterone biosynthesis into a soluble form were unsuccessful. But Rapp and Dahl (1976) succeeded in obtaining, by ultrasonification from rat capsular adrenal mitochondria, cytochrome P-450 particles capable of catalyzing the conversion of corticosterone to 18-hydroxycorticosterone and aldosterone. The conversion of tritiated corticosterone to aldosterone and 18-hydroxycorticosterone by a purified bovine adrenocortical cytochrome P-45011β preparation — reconstituted to an active enzyme system by the addition of adrenodoxin and adrenodoxin reductase — was reported by Wada et al. (1984, 1985).