title: A tandem mass spectrometric study of bile acids: interpretation of fragmentation pathways and differentiation of steroid isomers.
abstract: Bile acids are steroids with a pentanoic acid substituent at C-17. They are the terminal products of cholesterol excretion, and play critical physiological roles in human and animals. Bile acids are easy to detect but difficult to identify by using mass spectrometry due to their poly-ring structure and various hydroxylation patterns. In this study, fragmentation pathways of 18 free and conjugated bile acids were interpreted by using tandem mass spectrometry. The analyses were conducted on ion trap and triple quadrupole mass spectrometers. Upon collision-induced dissociation, the conjugated bile acids could cleave into glycine or taurine related fragments, together with the steroid skeleton. Fragmentations of free bile acids were further elucidated, especially by atmospheric pressure chemical ionization mass spectrometry in positive ion mode. Aside from universally observed neutral losses, eliminations occurred on bile acid carbon rings were proposed for the first time. Moreover, four isomeric 5β-cholanic acid hydroxyl derivatives (3α,6α-, 3α,7β-, 3α,7α-, and 3α,12α-) were differentiated using electrospray ionization in negative ion mode: 3α,7β-OH substituent inclined to eliminate H(2)O and CH(2)O(2) groups; 3α,6α-OH substituent preferred neutral loss of two H(2)O molecules; 3α,12α-OH substituent apt to lose the carboxyl in the form of CO(2) molecule; and 3α,7α-OH substituent exhibited no further fragmentation after dehydration. This study provided specific interpretation for mass spectra of bile acids. The results could contribute to bile acid analyses, especially in clinical assays and metabonomic studies.
authors: State Key Laboratory of Natural and Biomimetic Drugs, School of Pharmaceutical Sciences, Peking University, 38 Xueyuan Road, Beijing 100191, China.
authors: Xue
authors: X
authors: Qiao
authors: Min
authors: M
authors: Ye
authors: Chun-fang
authors: CF
authors: Liu
authors: Wen-zhi
authors: WZ
authors: Yang
authors: Wen-juan
authors: WJ
authors: Miao
authors: Jing
authors: J
authors: Dong
authors: De-an
authors: DA
authors: Guo
mesh_terms: D000818:Animals; D001647:Bile Acids and Salts / Q000032:analysis / Q000737:chemistry*; D002245:Carbon Dioxide / Q000737:chemistry; D003840:Deoxycholic Acid / Q000737:chemistry*; D005998:Glycine / Q000737:chemistry; D006801:Humans; D006900:Hydroxylation; D007536:Isomerism; D015394:Molecular Structure; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods; D053719:Tandem Mass Spectrometry; D013654:Taurine / Q000737:chemistry; D014867:Water / Q000737:chemistry
chemical_list: D001647:Bile Acids and Salts; D003840:Deoxycholic Acid; D014867:Water; D002245:Carbon Dioxide; C041069:carboxyl radical; D013654:Taurine; C010471:hyodeoxycholic acid; D005998:Glycine