Determination of fetal bile acids in biological fluids from neonates by gas chromatography-negative ion chemical ionization mass spectrometry

Key discoveries in bile acid chemistry and biology and their clinical applications: history of the last eight decades

During the last 80 years there have been extraordinary advances in our knowledge of the chemistry and biology of bile acids. We present here a brief history of the major achievements as we perceive them. Bernal, a physicist, determined the X-ray structure of cholesterol crystals, and his data together with the vast chemical studies of Wieland and Windaus enabled the correct structure of the steroid nucleus to be deduced. Today, C24 and C27 bile acids together with C27 bile alcohols constitute most of the bile acid "family". Patterns of bile acid hydroxylation and conjugation are summarized. Bile acid measurement encompasses the techniques of GC, HPLC, and MS, as well as enzymatic, bioluminescent, and competitive binding methods. The enterohepatic circulation of bile acids results from vectorial transport of bile acids by the ileal enterocyte and hepatocyte; the key transporters have been cloned. Bile acids are amphipathic, self-associate in solution, and form mixed micelles with polar lipids, phosphatidylcholine in bile, and fatty acids in intestinal content during triglyceride digestion. The rise and decline of dissolution of cholesterol gallstones by the ingestion of 3,7-dihydroxy bile acids is chronicled. Scientists from throughout the world have contributed to these achievements.

Measurement of the transport activities of bile salt export pump using LC-MS

The high performance liquid chromatography-electrospray ionization-mass spectrometry method has been applied to the measurement of bile acid transport activities in membrane vesicles obtained from a human bile salt export pump expressing Sf9 cells. The amounts of bile acids transported using the human bile salt export pump expressing Sf9 cells were determined using liquid chromatography-electrospray ionization-mass spectrometry method and the values of the kinetic parameters were determined to be comparable with those obtained using radioisotope-labeled substrates. The developed method was highly useful for the measurements of bile acid transport activities.

Metabolism of 4 beta -hydroxycholesterol in humans

One of the major oxysterols in the human circulation is 4 beta-hydroxycholesterol formed from cholesterol by the drug-metabolizing enzyme cytochrome P450 3A4. Deuterium-labeled 4 beta-hydroxycholesterol was injected into two healthy volunteers, and the apparent half-life was found to be 64 and 60 h, respectively. We have determined earlier the half-lives for 7 alpha-, 27-, and 24-hydroxycholesterol to be approximately 0.5, 0.75, and 14 h, respectively. Patients treated with certain antiepileptic drugs have up to 20-fold increased plasma concentrations of 4 beta-hydroxycholesterol. The apparent half-life of deuterium-labeled 4 beta-hydroxycholesterol in such a patient was found to be 52 h, suggesting that the high plasma concentration was because of increased synthesis rather than impaired clearance. 4 beta-Hydroxycholesterol was converted into acidic products at a much slower rate than 7 alpha-hydroxycholesterol in primary human hepatocytes, and 4 beta-hydroxycholesterol was 7 alpha-hydroxylated at a slower rate than cholesterol by recombinant human CYP7A1. CYP7B1 and CYP39A1 had no activity toward 4 beta-hydroxycholesterol. These results suggest that the high plasma concentration of 4 beta-hydroxycholesterol is because of its exceptionally slow elimination, probably in part because of the low rate of 7 alpha-hydroxylation of the steroid. The findings are discussed in relation to a potential role of 4 beta-hydroxycholesterol as a ligand for the nuclear receptor LXR.

Electron-capture mass spectrometry: recent advances

Electron-capture (EC) is a sensitive and selective ionization technique for mass spectrometry (MS). In the most familiar form of EC, a susceptible analyte (electrophore) is detected after eluting from a gas chromatography (GC) column, where a low attomole detection limit for standards is routine. High-performance liquid chromatography can facilitate sample cleanup prior to detection by GC-EC-MS, but carryover and shifts in retention time for the "invisible" analyte can be difficulties. Solid-phase extraction avoids these difficulties, but the degree of cleanup and recovery can be problems. Alternative electrophoric derivatizing reagents are available to help deal with interferences, and new reagents such as "AMACE1" are emerging. Releasable forms of electrophores can be used as tags for labeling macromolecules, motivated by the desire to multiplex ligand-type assays. The conventional, gas-phase ion source for EC is not well-understood, especially the role of wall reactions. Using an electron monochromator to tune the electron energy adds to the selectivity and information provided by EC-MS. High-resolution and tandem EC-MS measurements are emerging. Electron-capture dissociation is a new technique to sequence small- to medium-sized peptides, having the advantage of providing more extensive sequence information relative to other MS techniques. Particle-beam EC-MS tends to be less sensitive than GC-EC-MS, but not always. Recently it was demonstrated that EC-MS can be accomplished on an ordinary laser desorption time-of-flight mass spectrometer, and also by using atmospheric pressure chemical ionization. Two applications are discussed here in detail: bile acids and oxidized phenylalanine. EC-MS is well-established as a useful technique for trace analysis in special cases, and the scope of its usefulness is broadening (qualitative analysis and detection of more polar and larger molecules), based on advances in both the chemical and instrumental aspects of this technique.

Profile of urinary bile acids in infants and children: developmental pattern of excretion of unsaturated ketonic bile acids and 7beta-hydroxylated bile acids

Unusual bile acids, such as unsaturated ketonic and 7beta-hydroxylated bile acids, have been detected in urine early in life. To elucidate the normal profiles of usual and unusual urinary bile acids in the neonatal and pediatric periods, we measured the concentrations of 28 kinds in urine from normal newborns, infants, and children by gas chromatography-mass spectrometry. The mean total bile acid/Cr ratio in 7-d-old infants was significantly higher than in subjects of other age groups (birth, 2-4 mo, 5-7 mo, 11-12 mo, 2-3 y, 9-14 y, and adult) (p < 0.05). Relatively large amounts of unusual bile acids were detected during infancy, especially during the period up to 1 mo of age. At that time, 1beta,3alpha,7alpha,12alpha-tetrahydroxy-5bet a-cholan-24-oic, 7alpha, 12alpha-dihydroxy-3-oxo-5beta-chol-1-en-24-oic, and 7alpha,12alpha-dihydroxy-3-oxo-4-cholen-24-oic acids were predominant among the unusual urinary bile acids present. Moreover, the levels of 3alpha,7beta,12alpha-trihydroxy-5beta-cholan+ ++-24-oic acid increased significantly after 2-4 mo of age. These results indicate that bile acid synthesis and metabolism in the liver of developing infants are significantly different from that occurring in the liver of adults. Significant amounts of urinary isomerized 7beta-hydroxylated bile acids were detected after late infancy, probably because of changes in the intestinal bacterial flora response to a change in nutrition. We describe, for the first time, evidence of the epimerization of the 7alpha-hydroxyl group of cholic acid, which may be unique to human development.

Determination of 3-oxo-delta4- and 3-oxo-delta4,6-bile acids and related compounds in biological fluids of infants with cholestasis by gas chromatography-mass spectrometry

A method has been developed for the determination of 3-oxo-delta4- and 3-oxo-delta4,6-bile acids and related bile acids in biological fluids of infants by gas chromatography-mass spectrometry (GC-MS) of the methyl ester-dimethylethylsilyl ether-methoxime derivatives. The 7alpha-hydroxylated 3-oxo-delta4-bile acids were partially dehydrated to give the 3-oxo-delta4,6-bile acids by trimethylsilyl or dimethylethylsilyl derivatization and other pretreatments under acidic or alkaline conditions for GC-MS analysis. To prevent dehydration, the 3-oxo-delta4-bile acids were derivatized to the oximes by treatment with O-methylhydroxylamine prior to pretreatments such as solid-phase extraction, enzymatic solvolysis and hydrolysis of the conjugates, and silylation with dimethylethylsilylimidazole. Calibration curves for the bile acids were linear over a range of 5-250 ng and the detection limit was 100 pg for each 3-oxo-delta4-bile acid. Recoveries of the bile acids and their glycine and taurine conjugates from bile acid-free urine and serum ranged from 94.2 to 105.9% of their added amounts. The bile acids in urine and serum of four patients with severe cholestatic liver disease were measured by the analytical method, and the 3-oxo-delta4-bile acids were determined to be the major bile acids (59-68%) in the urines associated with 3-oxo-delta4-steroid 5beta-reductase deficiency or dysfunction.