Action of different nucleotides on the last step of aldosterone synthesis.

The effect of various nucleotides on the last step of aldosterone biosynthesis, the so-called "18 oxidation" (transformation of 18-hydroxycorticosterone to aldosterone), was studied by incubation of tritiated 18-hydroxycorticosterone with untreated duck adrenal mitochondria in vitro. The study was carried out in the absence or in the presence of antimycin A which blocks the respiratory chain. Results show that, when oxidative phosphorylation chain functions normally, GTP and CTP had no effect, UTP stimulated this reaction but ADP and ATP inhibited the transformation of 18-hydroxycorticosterone into aldosterone to the same extent. For this reason ATP is included in all controls for experiments studying the effect of ATP when "18 oxidation" is inhibited by antimycin A. When oxidative phosphorylation chain is inhibited by antimycin A, ATP is able to reverse the inhibition of "18 oxidation" induced by antimycin A, in the presence of succinate. Under these conditions UTP is not able to reverse the inhibition induced by antimycin A; GTP and CTP had no effect. Effects of ATP and UTP on the last step of aldosterone biosynthesis are related to different mechanisms. ATP clearly acts as an energy source for "18 oxidation" in the presence of succinate. The role of UTP must still be determined.