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Bile acids (BAs) are important end products of cholesterol metabolism. The main function of bile acids is the absorption of fats and fat-soluble vitamins. In addition, they are important for the maintenance of cholesterol homeostasis, excretion of toxic substances, induction of bile flow, acting as signalling molecules and intermediates between host and gut microbiome, ultimately influencing glucose homeostasis, lipid metabolism and energy expenditure. BAs are primarily synthesized from cholesterol in the liver and play house keeping roles in many physiological processes as described above. BAs are modulated by gut microbiome, and dysregulation of the homeostasis of circulating BAs pool is associated with liver and intestinal diseases, as well as neuropsychiatric symptoms.
In humans BAs comprise primary BAs synthesized in the liver (including cholic acid (CA) and chenodeoxycholic acid (CDCA)), secondary BAs produced in the gut (deoxycholic acid (DCA), lithocholic acid (LCA), and ursodeoxycholic acid (UDCA)) generated by deconjugation and dehydroxylation by intestinal bacteria, and tertiary BAs which are synthesized in both the liver and gut via modification of secondary BAs, such as sulfation, glucuronidation, and glucosidation. BAs are reabsorbed and transported back to the liver in a process known as enterohepatic circulation. BAs biotransformation is a complex process resulting in structural diversity of molecular species of BAs. Also, BAs present an extended concentration range in human biofluids and between physiological and disease phenotype (0.1-2000 nM). Thus, considering the biological significance of Bas and their complex profile, there is an apparent need for high quality quantitative results in various biospecimens and clinical applications.
Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is considered as the method of choice for the quantification of BAs and their conjugates.