Quantification of the 15 major human bile acids and their precursor 7α-hydroxy-4-cholesten-3-one in serum by liquid chromatography–tandem mass spectrometry

Bile acids are increasingly gaining attention since they were discovered to be activators of the transcription factor farnesoid X receptor (FXR) in addition to their well-established role in dietary lipid emulsification. Moreover, the differential activation potency of bile acids on FXR, which is due to structural variation of the ligands, generates the need for new analytical tools that are sensitive and specific enough to quantify the individual species of this complex class of compounds. Because bile acids undergo enterohepatic circulation, the additional assessment of a bile acid precursor as a marker for bile acid biosynthesis is used to differentiate between newly synthesised bile acids and bile acids reabsorbed from the intestine. This paper describes two new methods using liquid chromatography–tandem mass spectrometry (LC–MS/MS) for the quantification of the major unconjugated bile acids in human serum (cholic acid, chenodeoxycholic acid, deoxycholic acid, lithocholic acid and ursodeoxycholic acid) with their glycine- and taurine-conjugates as well as their precursor 7α-hydroxy-4-cholesten-3-one (C4). Intra- and inter-day variation was less than 12% and accuracy was between 84% and 102% for all analytes. Extraction recovery was between 78% and 100% for the bile acids whereas it was 62% for C4 and limit of quantification values ranged from 2 nmol/l to 50 nmol/l for all compounds. These two methods have the practical advantage of requiring low sample volume (100 μl serum for each method) and identical eluents, stationary phase as well as ionisation technique, so that they can be used in a combined way. Moreover, they provide information on the composition of the bile acid pool on one hand and on the relative amount of newly synthesised bile acids on the other, which taken together, gives new insights in the investigation of bile acid metabolism.