Formation of novel C21-bile acids from cholesterol in the rat. Structure identification of the major Di- and trihydroxylated species.

We recently showed that previously unknown di- and trihydroxylated C21-bile acids are major degradation products of sitosterol and campesterol in bile-fistulated female Wistar rats. Using a mixture of 4-14C- and 22-3H-labeled cholesterol it was shown that such C21-bile acids are formed also from cholesterol in amounts up to about 25% of the total formation of bile acids. The C21-bile acids were formed from labeled cholesterol also in perfused rat liver, demonstrating that the liver is the site of synthesis. The major trihydroxylated C21-bile acids in bile were identified, by means of mass spectrometry, NMR, stereospecific dehydrogenases, and reagents, as 5 beta-pregnan-3 alpha, 11 beta, 15 beta-triol-21-oic acid and 5 beta-pregnan-3 alpha, 11 beta, 15 alpha-triol-21-oic acid. The corresponding 11-oxo-isomers were also present. A minor trihydroxylated C21-bile acid was identified as 5 beta-pregnan-3 alpha, 11 beta, 16-triol-21-oic acid. The major dihydroxylated C21-bile acid was identified by the same means as 5 alpha-pregnan-3 alpha, 12 alpha-diol-21-oic acid. Male rats converted 4-14C-cholesterol into C21-bile acids less efficiently than did female rats. None of the C21-bile acids from male rats contained a 15-hydroxyl group. It is speculated that the novel C21-bile acids are formed both from cholesterol and from plant sterols by an initial hydroxylation at C21 followed by peroxisomal or mitochondrial beta-oxidation. The presence of a hydroxyl group at C15 may facilitate this reaction. The above formation of C21-bile acids shows that mammalian liver is able to degrade the side chain of cholesterol beyond the C24 stage, even in the absence of a blocking group at C24. C21-bile acids, or one of their precursors, are hydroxylated in the liver by a hitherto unknown 11 beta-hydroxylase. The possible physiological importance of the C21-bile acids is discussed.