Identification of a novel bile acid in swans, tree ducks, and geese: 3a,7a,15a-trihydroxy-5b-cholan-24-oic acid

By HPLC, a taurine-conjugated bile acid with a retention time different from that of taurocholate was found to be present in the bile of the black-necked swan, Cygnus melanocoryphus. The bile acid was isolated and its structure, established by H and C NMR and mass spectrometry, was that of the taurine N-acyl amidate of 3a,7a,15a-trihydroxy5b-cholan-24-oic acid. The compound was shown to have chromatographic and spectroscopic properties that were identical to those of the taurine conjugate of authentic 3a,7a,15a-trihydroxy-5b-cholan-24-oic acid, previously synthesized by us from ursodeoxycholic acid. By HPLC, the taurine conjugate of 3a,7a,15a-trihydroxy-5b-cholan-24oic acid was found to be present in 6 of 6 species in the subfamily Dendrocygninae (tree ducks) and in 10 of 13 species in the subfamily Anserinae (swans and geese) but not in other subfamilies in the Anatidae family. It was also not present in species from the other two families of the order Anseriformes. 3a,7a,15a-Trihydroxy-5b-cholan-24-oic acid is a new primary bile acid that is present in the biliary bile acids of swans, tree ducks, and geese andmay be termed 15a-hydroxy-chenodeoxycholic acid. —Kakiyama, G. , T. Iida, T. Goto, N. Mano, J. Goto, T. Nambara, L. R. Hagey, C. D. Schteingart, and A. F. Hofmann. Identificationof anovel bile acid in swans, tree ducks, and geese: 3a,7a,15a-trihydroxy5b-cholan-24-oic acid. J. Lipid Res. 2006. 47: 1551–1558. Supplementary key words Anseriformes . bile acid evolution . bile acid conjugation Bile acids are the amphipathic end products of cholesterol metabolism. In the small intestine, luminal bile acids promote lipid absorption by forming mixed micelles with dietary lipids (1) and also have antimicrobial effects (2). In addition, plasma bile acids appear to promote thermogenesis by interacting with a G protein-coupled receptor present on the surface of adipocytes in brown adipose tissue (3). C24 bile acids are found in most mammals and are present in bile as N-acyl amidates (conjugates) of taurine or glycine (4). Individual C24 bile acids are distinguished by their pattern of hydroxylation on the C19 nucleus or the C5 side chain. The default hydroxylation pattern of bile acids is at carbon 3 (C-3) (because bile acids are formed from cholesterol) and at C-7 [because hydroxylation at C-7 (catalyzed by cholesterol 7a-hydroxylase or sterol 7-hydroxylase) is believed to be an essential step in the biosynthesis of all bile acids] (5). Thus, chenodeoxycholic acid (3a,7adihydroxy) may be considered the root C24 bile acid (6). In most mammals, the majority of biliary bile acids are trihydroxy bile acids (4, 7). Hydroxylation at a third nuclear site, presumably mediated by cytochrome P450 hydroxylases, varies considerably between species. Hydroxylation at C-12 (cholic acid) and at C-6 (hyocholic acid and the muricholic acid epimers) has long been known (7). The third most common site of hydroxylation is probably at C-16, a hydroxylation site discovered many years ago in snakes by Haslewood (7) but now known to occur frequently in avian species (8). Hydroxylation at C-1 also has been reported. In the Australian opossum, hydroxylation at C-1 is in the a-configuration (9), whereas in certain fruit doves and pigeons, C-1 hydroxylation is in the b-configuration (10). Identification of novel bile acids is based on nuclear magnetic resonance spectroscopy and mass spectrometry, but chemical synthesis is highly desirable for confirmation of the assigned structure. We recently reported the synthesis of 3a,7a,16a-trihydroxy-5b-cholan-24-oic acid (11). In the process of that work, we were able to synthesize 3a,7a,15a-trihydroxy-5b-cholan-24-oic acid and 3a,7a, 15b-trihydroxy-5b-cholan-24-oic acid. It seemed likely to us that bile acids hydroxylated at C-15 should occur in nature, because hydroxylation at C-15 of a bile acid analog sulfonate during enterohepatic cycling in the hamster had already been observed (12). We report here the isolation Manuscript received 30 March 2006 and in revised form 27 April 2006. Published, JLR Papers in Press, April 28, 2006. DOI 10.1194/jlr.M600149-JLR200 1 To whom correspondence should be addressed. e-mail: ahofmann@ucsd.edu Copyright D 2006 by the American Society for Biochemistry and Molecular Biology, Inc. This article is available online at http://www.jlr. org Journal of Lipid Research Volume 47, 2006 1551 of a new natural bile acid, 3a,7a,15a-trihydroxy-5b-cholan24-oic acid, from the bile of the black-necked swan, where it occurs as its taurine conjugate. We also show that this novel bile acid is present in the biliary bile acids of swans, tree ducks, and geese.