Isolation and chemical synthesis of a major, novel biliary bile acid in the common wombat (Vombatus ursinus): 15α-hydroxylithocholic acid

Abstract

The major bile acids present in the gallbladder bile of the common Australian wombat (Vombatus ursinus) were isolated by preparative HPLC and identified by NMR as the taurine N-acylamidates of chenodeoxycholic acid (CDCA) and 15α-hydroxylithocholic acid (3α,15α-dihydroxy-5β-cholan-24-oic acid). Taurine-conjugated CDCA constituted 78% of biliary bile acids, and (taurine-conjugated) 15α-hydroxylithocholic acid constituted 11%. Proof of structure of the latter compound was obtained by its synthesis from CDCA via a Δ14 intermediate. The synthesis of its C-15 epimer, 15β-hydroxylithocholic acid (3α,15β-dihydroxy-5β-cholan-24-oic acid), is also reported. The taurine conjugate of 15α-hydroxylithocholic acid was synthesized and shown to have chromatographic and spectroscopic properties identical to those of the compound isolated from bile. It is likely that 15α-hydroxylithocholic acid is synthesized in the wombat hepatocyte by 15α-hydroxylation of lithocholic acid that was formed by bacterial 7α-dehydroxylation of CDCA in the distal intestine. Thus, the wombat appears to use 15α-hydroxylation as a novel detoxification mechanism for lithocholic acid.

Keywords

• bile acid synthesis
• detoxification of bile acids
• bile acid metabolism in marsupials
• 15-hydroxylation
• lithocholic acid