Abstract
We describe a highly sensitive and specific method for the quantification of serum 7alpha-hydroxy-4-cholesten-3-one (C4), which has been used as a biomarker for bile acid biosynthesis. This method is based upon a stable isotope dilution technique by liquid chromatography-tandem mass spectrometry (LC-MS/MS). C4 was extracted from human serum (2-50 mul) by a salting-out procedure, derivatized into the picolinoyl ester (C4-7alpha-picolinate), and then purified using a disposable C(18) cartridge. The resulting picolinoyl ester derivative of C4 was quantified by LC-MS/MS using the electrospray ionization mode. The detection limit of the C4 picolinoyl ester was found to be 100 fg (signal-to-noise ratio = 10), which was approximately 1,000 times more sensitive than the detection limit of C4 with a conventional HPLC-ultraviolet method. The relative standard deviations between sample preparations and between measurements by our method were calculated to be 5.7% and 3.9%, respectively, by one-way layout analysis. The recovery experiments were performed using serum spiked with 20.0-60.0 ng/ml C4 and were validated by a polynomial equation. The results showed that the estimated concentration with 95% confidence limit was 23.1 +/- 2.8 ng/ml, which coincided completely with the observed X(0) +/- SD = 23.3 +/- 1.0 ng/ml with a mean recovery of 93.4%. This method provides highly reliable and reproducible results for the quantification of C4, especially in small volumes of blood samples.
Highlights
We describe a highly sensitive and specific method for the quantification of serum 7a-hydroxy-4-cholesten-3one (C4), which has been used as a biomarker for bile acid biosynthesis
It was subsequently reported that serum concentrations of 7a-hydroxycholesterol [12] and C4 [13] reflected cholesterol 7a-hydroxylase (CYP7A1) activities and bile acid synthesis in humans
Picolinic acid and 2-methyl-6-nitrobenzoic anhydride were purchased from Tokyo Kasei Kogyo (Tokyo, Japan), and 4-dimethylaminopyridine and triethylamine were obtained from Wako Pure Chemical Industries (Osaka, Japan)
Summary
We describe a highly sensitive and specific method for the quantification of serum 7a-hydroxy-4-cholesten-3one (C4), which has been used as a biomarker for bile acid biosynthesis. This method is based upon a stable isotope dilution technique by liquid chromatography-tandem mass spectrometry (LC-MS/MS). A breakthrough of this problem was achieved in 1987 by Bjorkhem et al [9] They quantified the plasma concentrations of 7a-hydroxycholesterol, the immediate product of CYP7A1, by gas chromatographymass spectrometry with selected ion monitoring (GC-SIM) and reported that this measurement reflected hepatic CYP7A1 activities in humans. It was subsequently reported that serum concentrations of 7a-hydroxycholesterol [12] and C4 [13] reflected CYP7A1 activities and bile acid synthesis in humans
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