A new, major C27 biliary bile acid in the Red-winged tinamou (Rhynchotus rufescens):(25R)-1β,3α,7α-trihydroxy-5β-cholestan-27-oic acid

Bile salts are amphipathic end products of cholesterol metabolism with multiple physiological functions. Most bile salts belong to one of three large classes: C27 bile alcohols, C27 bile acids, and C24 bile acids. For C24 bile acids, which are present in mammals, chenodeoxycholic acid, possessing an α-hydroxyl group at C-3 and at C-7, may be considered the root bile acid (1); in most species, a third hydroxyl group is added to the hydrophilic face of the planar bile acid molecule during bile acid biosynthesis. The most common sites of additional nuclear hydroxylation are at C-12 (cholic acid) or C-16 (2) (for which the name avicholic acid has been proposed) (3). Less-common sites of hydroxylation in major primary bile acids are at C-1 (4, 5), C-6 (6, 7), and C-15 (8). Hydroxylation at C-5 has also been reported to occur in pheasant biliary bile acids (9), and C-19 hydroxy bile acids have been identified in human urinary bile acids (10). Such additional nuclear hydroxylation of chenodeoxycholic acid may have biological utility in that it precludes the formation of lithocholic acid (11). Lithocholic acid is formed when chenodeoxycholic acid undergoes bacterial 7-dehydroxylation in the distal intestine (12). Lithocholic acid is a highly toxic bile acid in many mammalian species (13–16). This reasoning should be applicable to C27 bile acids, which are the major biliary bile acids in amphibians, some reptiles, and ancient birds (2). The root C27 bile acid would possess an α-hydroxyl group at C-3 and at C-7 and is 3α,7α-dihydroxy-5β-cholestan-27-oic acid (no trivial name has been proposed). The most common site of additional nuclear hydroxylation for C27 bile acids is at C-12 (2). To date, the only additional sites of nuclear hydroxylation that have been identified in C27 bile acids are at C-1 in alligators (17), C-15 in turtles (18), and C-16 in early evolving birds (19). The C-15 hydroxy bile acid is a 7-deoxy bile acid, suggesting that it is a secondary bile acid formed by bacterial 7-dehydroxylation of an unidentified precursor. Hydroxylation at C-15 could occur during primary bile acid biosynthesis or result from C-15 hydroxlation of the 7-deoxy bile acid, as occurs in the wombat (20). Whether such third site nuclear hydroxylation in primary C27 bile acid biosynthesis has biological utility is not known, as the toxicity of the C27 bile acid possessing only a 3α-hydroxyl group has not been examined. We report here the presence of a new C27 bile acid that is a major biliary bile acid in the Red-winged tinamou, (Rhynchotus rufescens), an ancient bird that is considered to be related to the ratites (rhea, ostrich, emu, cassowary, and kiwi) (21). The new bile acid was found to be (25R)-1β,3α,7α-trihydroxy-5β-cholestan-27-oic acid. This trihydroxy C27 bile acid, occurring in bile as its taurine N-acyl amidate, has not been previously described.