| Research Abstract |
Aldosterone is the most potent mineralocorticoid that regulates the sodium-potassium balance of the body fluid of animals. It is secreted from the adrenal cortex of mammals or from the interrenal tissue of amphibians, such as frogs. The final steps of biosynthesis of aldosterone are catalyzed by steroid 11beta/18- hydroxylase (P450(11beta)) and aldosterone synthase (P450(aldo)). In bovine adrenal cortex, the two enzyme activities were shown to reside in a single enzyme molecule. The circumstances are somewhat different in the cases of rat, mouse and human adrenal cortices, in which two similar, but definitely different, enzyme species, one catalyzing 11/18-hydroxylation of deoxycorticosterone and the other catalyzing aldosterone synthesis from corticosterone, have been identified. These findings raise an interesting question how the two enzymes were differentiated during animal evolution. To answer the question, it is important to gather more information on the molecular nature of these enzymes of other animal species. Here we isolated a cDNA encoding P450(11beta) from a library constructed from the interrenal tissues of bullfrog (Rana catesbeiana). The precursor protein of frog P450(11beta) is composed of 517 amino acids, the N-terminal 45 amino acidpeptide of which is presumably split when it is transported into mitochondria. Similarities of the frog enzyme to rat P450(11beta) and rat P450(aldo) are 47% and 51%, respectively. The enzyme, when expressed in COS-7 cells, has the biosynthesizing activity of aldosterone. Thus the molecular nature of aldosterone synthase of frog is similar to that of cattle in the sense that the single enzyme molecule catalyzes all the reactions from deoxycorticosterone through aldosterone.
|
