Human cecal bile acids: concentration and spectrum

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Abstract
To obtain information on the concentration and spectrum of bile acids in human cecal content, samples were obtained from 19 persons who had died an unnatural death from causes such as trauma, homicide, suicide, or drug overdose. Bile acid concentration was measured via an enzymatic assay for 3α-hydroxy bile acids; bile acid classes were determined by electrospray ionization mass spectrometry and individual bile acids by gas chromatography mass spectrometry and liquid chromatography mass spectrometry. The 3α-hydroxy bile acid concentration (μmol bile acid/ml cecal content) was 0.4 ± 0.2 mM (mean ± SD); the total 3-hydroxy bile acid concentration was 0.6 ± 0.3 mM. The aqueous concentration of bile acids (supernatant after centrifugation) was identical, indicating that most bile acids were in solution. By liquid chromatography mass spectrometry, bile acids were mostly in unconjugated form (90 ± 9%, mean ± SD); sulfated, nonamidated bile acids were 7 ± 5%, and nonsulfated amidated bile acids (glycine or taurine conjugates) were 3 ± 7%. By gas chromatography mass spectrometry, 10 bile acids were identified: deoxycholic (34 ± 16%), lithocholic (26 ± 10%), and ursodeoxycholic (6 ± 9), as well as their primary bile acid precursors cholic (6 ± 9%) and chenodeoxycholic acid (7 ± 8%). In addition, 3β-hydroxy derivatives of some or all of these bile acids were present and averaged 27 ± 18% of total bile acids, indicating that 3β-hydroxy bile acids are normal constituents of cecal content. In the human cecum, deconjugation and dehydroxylation of bile acids are nearly complete, resulting in most bile acids being in unconjugated form at submicellar and subsecretory concentrations.