Printer Friendly

Gas chromatography--tandem mass spectrometry method for the simultaneous determination of oxysterols, plant sterols, and cholesterol precursors.

Cholesterol is an essential constituent of cell membranes and myelin sheaths. It is required for adrenal and gonadal steroidogenesis and hepatic bile acid formation. Cells synthesize cholesterol, commencing with acetyl coenzyme A (CoA). (3) Initially 3-hydroxy-3methylglutaryl (HMG)-CoA is formed, followed by mevalonic acid, which is then metabolized via a series of isoprenoid intermediates to squalene. From squalene the sterol skeleton is formed by ring closure, followed by the synthesis of cholesterol precursors and finally cholesterol.

The analysis of cholesterol precursors like lathosterol and desmosterol plays an important role in the diagnosis of inherited disorders of cholesterol biosynthesis or malformation syndromes such as desmosterolosis, Smith-Lemli-Opitz syndrome, and others (1-4). Additionally, malabsorption of cholesterol by the intestinal mucosa or defects in cholesterol metabolism can be responsible for severe health effects. Most aberrations in cholesterol metabolism can be identified by the analysis of the sterol profile in plasma/serum. This analysis should include not only cholesterol and its precursors, but also plant sterols such as sitosterol and campesterol, and cholesterol oxidation products. Cholesterol can be oxidized in vivo enzymatically and nonenzymatically. There is substantial published evidence about the pathologic effects of oxysterols in atherogenesis, neurodegeneration, and inflammation. Comprehensive reviews of this topic have been published (5-11).

Several methods for sterol analysis from various biological matrices have been proposed, mostly based on gas chromatography (GC) (12-16), or liquid chromatography (LC) (2, 17-21) coupled with mass spectrometry (MS).

LC-tandem MS (LC-MS/MS) methods based on atmospheric pressure, chemical ionization, or photoionization without laborious derivatization procedures have been described in the literature but exhibit insufficient separation and quite high limits of detection (18, 21, 22). Derivatization, for example as picolinyl esters (23) or Girard P hydrazones (19), in LC-based methods enhanced the performance and decreased the detection limit but appeared more laborious than GC-based methods. The main advantage of Girard P derivatization can be seen in more detailed structure information due to [MS.sub.3] (MS/MS/MS) applicability (24-26). Honda, et al. have reported on the simultaneous LC-MS-based determination of cholesterol precursors and plant sterols (2). A similar methodology has been applied for oxysterols (23).

The GC-based methods benefit from excellent peak resolution but suffer from long run times and the need for derivatization. The separation of 24-, 25-, and 27-hydroxycholesterol is important. 24-Hydroxycholesterol is enzymatically formed by cholesterol 24-hydroxylase. Almost all circulating 24-hydroxycholesterol originates from the brain by a flux across the blood-brain-barrier (27-29). In contrast, almost all cells in the body contain the enzyme sterol 27hydroxylase, especially hepatocytes and macrophages, encoded by CYP27A1 contributing via side-chain oxidation to cholesterol elimination as an alternative to the HDL-mediated reversed cholesterol transport. 25-Hydroxycholesterol regulates the sterol regulatory element binding protein pathway for cholesterol-dependent transcriptional regulation (30).

On the basis of GC coupled to triple-quadrupole MS (GC-MS/MS), we developed a rapid analytical method for the simultaneous determination of 11 sterol components in human plasma.

Materials and Methods

CHEMICALS AND SOLUTIONS

Hexane, methanol, sodium chloride, and potassium hydroxide were purchased from VWR. N-Methyl-N-trimethylsilyl-trifluoracetamide was obtained from Macherey-Nagel. 7[beta]-hydroxycholesterol, 24(S)-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol(25R); 7-ketocholesterol; lanosterol; campesterol-[D.sub.6]; and sitosterol-[D.sub.6] were purchased from Avanti Polar Lipids. Campesterol, sitosterol, stigmasterol, desmosterol, lathosterol, 7-dehydrocholesterol, and butylated hydroxytoluene were obtained from Sigma-Aldrich Chemie. 7-Ketocholesterol-[D.sub.7] was purchased from Toronto Research Chemicals and 24-hydroxycholesterol-[D.sub.10] and 27-hydroxycholesterol-[D.sub.6] from Sugaris.

SAMPLES AND SAMPLE PREPARATION

EDTA-plasma was obtained from blood collected from healthy volunteers by standard venipuncture techniques. Samples were immediately centrifuged at 2000g for 10 min. To prevent autoxidation, butylated hydroxytoluene was added to all plasma samples at a concentration of 50 [micro]g/mL. Plasma samples were stored in aliquots at -20[degrees]C.

Sample preparation was based on the method of Dzeletovic et al. (14) with some modifications. To 400 [micro]L of sample, calibrator, or control, 40 [micro]L of a mixture of deuterium-labeled internal standards (5 [micro]g/mL 24-hydroxycholesterol-[D.sub.10], 5 [micro]g/mL 27-hydroxycholesterol-[D.sub.6], 25 [micro]g/mL 7ketocholesterol-[D.sub.7], 10 [micro]g/mL campesterol-[D.sub.6], and 10 [micro]g/mL sitosterol-[D.sub.6] in methanol) was added. To cleave ester bonds, alkaline hydrolysis was performed with 2 mL freshly prepared 1 mol/L potassium hydroxide in ethanol (5.61 g in 100 mL) for 60 min at 25[degrees]C under continuous agitation. Afterward, the reaction solution was adjusted to pH 7 with phosphoric acid and 2 mL sodium chloride solution. Sterols were extracted with 2 X 3 mL and 1 X 1 mL n-hexane. The solvent was evaporated to dryness under reduced pressure. The residue was dissolved in 50 [micro]L of the reagent N-methyl-N-trimethylsilyl-trifluoracetamide for trimethylsilylation. The derivatization reaction was performed for 60 min at 60[degrees]C. The derivatized samples were transferred to GC vials for direct injection.

GC--TRIPLE-QUADRUPOLE MS

GC-MS/MS was performed on an Agilent 7890A gas chromatograph equipped with a multimode inlet, a HP-5ms column (30 m, 0.25 mm, 0,25 [micro]m df), and an Agilent 7000B GC-MS/MS. The oven temperature program was as follows: 200[degrees]C for 0.5 min, 50[degrees]C/min to 290[degrees]C, 5[degrees]C/min to 295[degrees]C, and 10[degrees]C/min to 320[degrees]C, where the temperature was maintained for 2.5 min. The carrier gas was helium with a constant flow of 1.2 mL/min. One microliter was injected in pulsed-splitless mode with an injector temperature of 275[degrees]C. The transfer line to the ion source was held at 280[degrees]C. Unless stated otherwise, chemical ionization was operated in positive mode with the following settings: 20% ammonia (quality 6.0) as reagent gas, source temperature 250[degrees]C, MS1, MS2 quadrupole temperature 150[degrees]C, and collision gas nitrogen with 1.5 mL/min flow. A solvent delay was set for the first 6 min.

All analytes were monitored in the multiple reaction monitoring (MRM) mode.

CALIBRATION AND QUANTIFICATION

Pooled plasma samples from healthy volunteers were diluted with 0.1 mol/L sodium chloride in water in a ratio of 1:10. Aliquots of this diluted plasma were supplemented with a combined standard solution to obtain 5 calibrators in appropriate concentration ranges. The standard substances were dissolved in chloroform/methanol (50/50) and stock solutions were prepared. Calibrators were prepared on the day of analysis by addition of different concentrations of standards to diluted EDTA-plasma. The concentrations of the calibrators are provided in Table 1 in the Data Supplement that accompanies the online version of this article at http://www.clinchem.org/content/vol58/issue11. The calibrators underwent the same sample preparation procedure as the samples.

Data analysis was performed with Mass Hunter software (Agilent Technologies).

Results

MODE OF IONIZATION

A second phase of mass fragmentation analysis in mass spectrometry, MRM, improves the analytical sensitivity of the method because it allows better discrimination from background. Additionally, coelution in the chromatographic separation necessitates such a step to gain analytical specificity. In most cases selective ion monitoring in GC-MS methods is used to achieve analytical specificity. However, sterol molecules consist of the same steroid backbone, resulting in very similar ionization patterns when electron impact ionization is used. Good chromatographic resolution is therefore needed to separate the components if the same selected ion monitoring must be used for more than 1 species. To benefit from a second fragmentation step, precursor ions with high abundance should be produced. For this reason the optimal ionization mode was first determined in our experiments. 24(S)-hydroxycholesterol and 27-hydroxycholesterol were chosen as examples to find the best ionization mode. Electron impact (EI) is the most common ionization mode in GC-MS. For the 27-hydroxycholesteroltrimethylsilyl (TMS) derivative the species-specific ions (m/z 416.2; 455.4) are less abundant in EI and in principle not suitable for further fragmentation. Positive chemical ionization (PCI) with ammonia as reagent gas resulted in 2 main signals corresponding to (M+TMS)+ and (M+TMS-17) +. Based on MS 1 scans the appropriate precursor ions were chosen, product ions generated, and the most intense MRM defined and optimized for both EI and PCI. A plasma sample from a healthy donor underwent sample preparation and the signal-to-noise (S/N) ratios of the MRM signals were compared. Higher S/N ratios were obtained by PCI [24(S)-hydroxycholesterol, MRM m/z 473.7/367.3, S/N = 610; 27-hydroxycholesterol, MRM m/z 474.1/457.1, S/N = 1380] in contrast to EI [24(S)hydroxycholesterol, MRM m/z 412.6/159.0, S/N = 161; 27-hydroxycholesterol, MRM 416.5/214.8, S/N = 99]. For that reason PCI was always applied except for 7[beta]-hydroxycholesterol. 7[beta]-Hydroxycholesterol exhibited far more abundant molecular ions in negative CI (NCI).

QUANTIFICATION

The MRMs including retention times of the analytes and internal standards are provided in Table 1. Quadrupoles Q1 and Q2 were set to wide resolution. The presented MRMs were used as quantifier transitions, meaning that the peak areas of these signals were used for quantification. Qualifier transitions were also defined for 24(S)-hydroxycholesterol, 27-hydroxycholesterol, 7-ketocholesterol, and the appropriate deuterated internal standards. Neither specific nor sensitive MRMs that could be used as qualifiers were found for the other compounds. We divided the MS program into 3 time periods, 0-7.4 min, 7.4-8.05 min, and 8.05-8.3 min to enhance the analytical sensitivity by setting dwell times up to 50 ms. A second run was necessary, in NCI mode, to detect 7[beta]-hydroxycholesterol.

All peaks exhibit good symmetry without tailing or fronting (see online Supplemental Fig. 1).

ASSAY CHARACTERISTICS

We generated calibration lines by the addition of different concentrations of the analytes to diluted EDTA-plasma. Stability of labeling of internal standard was verified in each batch analysis with a blank run of internal standard only. A 5-point calibration was achieved for all analytes. The concentration ranges were chosen to cover the complete endogenous concentrations in human plasma. The calibration curves were linear throughout the calibration range for all analytes, with [R.sup.2] higher than 0.990. The regression coefficients and the limit of detection are shown in online Supplemental Table 1.

The imprecision of the method was determined in nonsupplemented plasma and serum samples. Intraassay and interassay CVs are presented in Table 2. The CVs show good precision both in plasma and serum except for 7[beta]-hydroxycholesterol, which can be formed during the preparation procedure by autoxidation of cholesterol. To simplify the preparation step, we omitted a cholesterol removal by solid-phase extraction. Because of this sensitivity to autoxidation and the time-consuming second run for NCI, 7[beta]-hydroxycholesterol was excluded from the list of parameters. Furthermore, 7[alpha]-hydroxycholesterol could not be separated from the cholesterol excess by our GC program and was therefore not analyzed.

We calculated recovery using 2 supplemented plasma samples of different concentrations (added amount in the range of the second and third calibration concentration). Recoveries were between 88% and 117% without any trend. The good recoveries also indicate that matrix effects had no influence. Despite the fact that calibration lines were conducted with diluted plasma, the added amounts were found correctly in supplemented samples, which were crude plasma, not diluted.

[FIGURE 1 OMITTED]

Extraction efficiencies were evaluated by the addition of standards after liquid/liquid extraction. The same amounts as used for the recovery study were added in parallel to the hexane extracts. The mean (SD) recovery was 90% (6.7%) (data not shown).

The reproducibility of the GC-MS/MS measurement was investigated by multiple injections from the same vial. The variances attributable to the measurement were very low, with CVs between 0.8% and 6.8%. The TMS derivatives were stable up to 3 days in hermetically sealed vials.

Application in Human Studies

HYPERCHOLESTEROLEMIA

We acquired plasma samples from healthy volunteers and from patients with hypercholesterolemia and applied the developed method. In a first attempt the sterol profiles of patients (n = 17) with LDL cholesterol >160 mg/dL (>4.14 mmol/L) and triglycerides <300 mg/dL (<3.39 mmol/L) were compared to a normotriglyceridemic control group with LDL cholesterol <100 mg/dL (<2.59 mmol/L) (n = 20). Fig. 1 illustrates the differences in the cholesterol precursors lanosterol, desmosterol, lathosterol, and 7-dehydrocholesterol together with the plant sterols campesterol and sitosterol. The patients with high LDL cholesterol (>160 mg/dL; >4.14 mmol/L) had higher values of cholesterol precursors, indicating higher cholesterol biosynthesis, whereas the plant sterol concentrations reflecting cholesterol absorption were similar for these groups. Some patients had notably higher values (Fig. 1) of cholesterol precursors than the control group. In these patients cholesterol is synthesized in considerably higher quantities than in controls, and these individuals would be ideal candidates for statin (HMG-CoA reductase inhibition) therapy. In contrast, patients with high sitosterol and stigmasterol values can be considered as hyperabsorbers of cholesterol, and therefore treatment with absorption inhibitors or resins would be the appropriate therapy to lower the LDL cholesterol. However, a detailed analysis to stratify several forms of hypercholesterolemia for the type of drug treatment was not within the aim of this study. Nevertheless, our method is applicable to the quantification of plasma markers for differential diagnosis of abnormalities in cholesterol biosynthesis and/or cholesterol absorption. Additional information can be obtained by the plasma concentrations of oxysterols, e.g., 7-ketocholesterol. 7-ketocholesterol may indicate increased concentrations of ox-LDL as biomarkers of oxidative stress and the risk of coronary heart disease (31-33). Fig. 2 shows the correlation between glycohemoglobin concentrations and 7-ketocholesterol in type 2 diabetes patients, representing another patient group for cholesterol lowering therapy.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

THYROID DYSFUNCTION

A comparative analysis of patients with hypothyroidism (n = 11) and hyperthyroidism (n = 11) showed increased concentrations of the cholesterol precursors desmosterol, lathosterol, and 7-dehydrocholesterol in hyperthyroid patients (Fig. 3). Cholesterol synthesis and degradation are both increased by thyroid hormones, leading to an increase in hepatic LDL receptor number and accelerated LDL clearance. In contrast, phytosterols (i.e., sitosterol and stigmasterol) are reduced in hyperthyroidism owing to gastrointestinal effects of thyroid hormones, which promote gut motility resulting in hyperdefecation. Conversely, impaired bowel transit occurs in hypothyroidism (3).

RARE GENETIC DISEASES

A number of inherited disorders show abnormal sterol profiles. For example, in cerebrotendinous xanthomatosis no or almost no 27-hydroxycholesterol is formed owing to a defect of the sterol 27-hydroxylase gene (34).In an affected patient (35) we could prove the absence of 27-hydroxycholesterol, whereas the other sterols had concentrations within reference intervals. In fact, a sterol profile would rapidly identify this disorder.

Because of recent reports about the relevance of oxysterols in Niemann-Pick type C disease, 7-ketocholesterol can be of diagnostic value despite its possible autoxidative origin (36) (37). We measured a 7-ketocholesterol concentration of 420 [micro]g/L in a patient with Niemann-Pick type C disease (homozygous for Ile1061 Thr), which was increased compared to the 7-ketocholesterol reference range of 8.5-146.9 [micro]g/L (n = 62).

In a patient with Tangier disease (38) (hereby a TD3 II:3 [pedigree 2]) all sterol concentrations were within the reference range.

Discussion

We developed a reliable GC-MS/MS method for the simultaneous determination of cholesterol precursors, plant sterols, and oxysterols. The use of specific MRMs allowed us to analyze coeluting or nonresolved compounds. This was the case for desmosterol and 7-dehydrocholesterol, and sitosterol and lanosterol as well as the known interference from lathosterol and zymosterol. The MRM of sitosterol was completely absent for lanosterol and vice versa. In pure desmosterol derivatives falsely high concentrations of 7-dehydrocholesterol, corresponding to 0.03% of the measured concentration of desmosterol, were obtained. In contrast, in pure 7-dehydrocholesterol derivatives falsely high concentrations of desmosterol, corresponding to 1% of the measured concentration of 7-dehydrocholesterol, were obtained. Therefore, a qualifying transition of m/z 473.8/367.3 for desmosterol was included with a quantifier/qualifier ratio of 3.5% as indicator of signal purity. However, an interference of 7-dehydrocholesterol on desmosterol and vice versa is only possible in very high plasma concentrations of these compounds.

A third couple of coeluting compounds are lathosterol and zymosterol. Acimovic, et al. described elutions at similar retention times (15). In our experiments there was no interference between the zymosterol signal and the lathosterol MRMs, even for the pure compounds. Since zymosterol concentrations in human plasma are too low to be detected by our method, zymosterol was not implemented in the panel of analytes.

The main advantage of our method is the rapid and simultaneous determination of a set of cholesterol precursors, plant sterols and oxysterols The GC run time of 8.5 min is comparable to LC methods, but GC provides much better peak resolution. The separation of 24(S)-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol is critical. Concentrations of cholesterol precursors, oxysterols, and plant sterols determined in our work are in the same range as those published in the literature, except for 7-dehydrocholesterol. Honda, et al. (2) ascribe this discrepancy to thermal instability in GC separation.

The hyphenation of GC and tandem MS has successfully enhanced the performance of approved GC methods, both in analytical sensitivity and in time savings. Because the main applications of sterol profiling in human plasma can be seen in neurodegenerative diseases, atherosclerosis, and cholesterol metabolism disorders, we excluded the implementation of cholesterol epoxides, and other autoxidative species.

To discriminate hyperabsorber from hypersynthesizer patients with a single analysis, our method offers a valuable tool for direct therapeutic stratification. Furthermore, a sterol profile identifies metabolic overload--dependent uncoupling of cholesterol biosynthesis. The method contributes to the diagnosis of some rare genetic diseases like cerebrotendinous xanthomatosis, Niemann-Pick type C disease, and Smith-LemliOpitz-Syndrome in a single run.

In summary, profiling of sterols as biomarkers requires rapid and reliable methods. GC-MS/MS is a reliable tool that fulfills the demands of sterol profiling in the lower picogram per milliliter concentration range.

Author Contributions: All authors confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article.

Authors' Disclosures or Potential Conflicts of Interest: Upon manuscript submission, all authors completed the author disclosure form. Disclosures and/or potential conflicts of interest:

Employment or Leadership: None declared.

Consultant or Advisory Role: H.H. Kluenemann, Actelion.

Stock Ownership: None declared.

Honoraria: H.H. Kluenemann, honoraria for presentations about Niemann-Pick Type C.

Research Funding: Actelion; the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 202272, IP-Project LipidomicNet; and BMBF under grant agreement no. 0315494C SysMBo to the Institute for Clinical Chemistry and Laboratory Medicine University Hospital Regensburg. Expert Testimony: None declared.

Role of Sponsor: The funding organizations played no role in the design of study, choice of enrolled patients, review and interpretation of data, or preparation or approval of manuscript.

Acknowledgments: The authors thank the Niemann-Pick-Selbsthilfegruppe e.V. Deutschland.

References

(1.) Goedeke L, Fernandez-Hernando C. Regulation of cholesterol homeostasis. Cell Mol Life Sci 2011; 69:915-30.

(2.) Honda A, Yamashita K, Miyazaki H, Shirai M, Ikegami T, Xu GR, et al. Highly sensitive analysis of sterol profiles in human serum by LC-ESI-MS/MS. J Lipid Res 2008;49:2063-73.

(3.) Gardner DG, Shoback, D. Greenspan's basic and clinical endocrinology. 8th ed. New York: McGraw-Hill; 2007.

(4.) Porter FD, Herman GE. Malformation syndromes caused by disorders of cholesterol synthesis. J Lipid Res 2011;52:6-34.

(5.) Brown AJ, Jessup W. Oxysterols and atherosclerosis. Atherosclerosis 1999;142:1-28.

(6.) Brown AJ, Jessup W. Oxysterols: sources, cellular storage and metabolism, and new insights into their roles in cholesterol homeostasis. Mol Aspects Med 2009;30:111-22.

(7.) Schroepfer GJ Jr. Oxysterols: modulators of cholesterol metabolism and other processes. Physiol Rev 2000;80:361-554.

(8.) Vaya J, Schipper HM. Oxysterols, cholesterol homeostasis, and Alzheimer disease. J Neurochem 2007;102:1727-37.

(9.) Leoni V Oxysterols as markers of neurological disease--a review. Scand J Clin Lab Invest 2009; 69:22-5.

(10.) Vejux A, Lizard G. Cytotoxic effects of oxysterols associated with human diseases: induction of cell death (apoptosis and/or oncosis), oxidative and inflammatory activities, and phospholipidosis. Mol Aspects Med 2009;30:153-70.

(11.) Bjorkhem I, Heverin M, Leoni V, Meaney S, Diczfalusy U. Oxysterols and Alzheimer's disease. Acta Neurol Scand Suppl 2006;185:43-9.

(12.) Menendez-Carreno M, Garcia-Herreros C, Astiasaran I, Ansorena D. Validation of a gas chromatography-mass spectrometry method for the analysis of sterol oxidation products in serum. J Chromatogr B Analyt Technol Biomed Life Sci 2008;864:61-8.

(13.) Ahmida HS, Bertucci P, Franzo L, Massoud R, Cortese C, Lala A, Federici G. Simultaneous determination of plasmatic phytosterols and cholesterol precursors using gas chromatography-mass spectrometry (GC-MS) with selective ion monitoring (SIM). J Chromatogr B Analyt Technol Biomed Life Sci 2006;842:43-7.

(14.) Dzeletovic S, Breuer O, Lund E, Diczfalusy U. Determination of cholesterol oxidation products in human plasma by isotope dilution-mass spectrometry. Anal Biochem 1995;225:73-80.

(15.) Acimovic J, Lovgren-Sandblom A, Monostory K, Rozman D, Golicnik M, Lutjohann D, Bjorkhem I. Combined gas chromatographic/mass spectrometric analysis of cholesterol precursors and plant sterols in cultured cells. J Chromatogr B Analyt Technol Biomed Life Sci 2009;877: 2081-6.

(16.) Luzon-Toro B, Zafra-Gomez A, Ballesteros O. Gas chromatographic-mass spectrometric determination of brain levels of alpha-cholest-8-en-3beta-ol (lathosterol). J Chromatogr B Analyt Technol Biomed Life Sci 2007;850:177-82.

(17.) Lembcke J, Ceglarek U, Fiedler GM, Baumann S, Leichtle A, Thiery J. Rapid quantification of free and esterified phytosterols in human serum using APPI-LC-MS/MS. J Lipid Res 2005;46:21-6.

(18.) Karuna R, von Eckardstein A, Rentsch KM. Dopant assisted-atmospheric pressure photoionization (DA-APPI) liquid chromatography-mass spectrometry for the quantification of 27hydroxycholesterol in plasma. J Chromatogr B Analyt Technol Biomed Life Sci 2009;877:261-8.

(19.) Griffiths WJ, Wang Y, Alvelius G, Liu S, Bodin K, Sjovall J. Analysis of oxysterols by electrospray tandem mass spectrometry. J Am Soc Mass Spectrom 2006;17:341-62.

(20.) Griffiths WJ, Wang YQ. Analysis of neurosterols by GC-MS and LC-MS/MS. J Chromatogr B Analyt Technol Biomed Life Sci 2009;877:2778-805.

(21.) Burkard I, Rentsch KM, von Eckardstein A. Determination of 24S- and 27-hydroxycholesterol in plasma by high-performance liquid chromatography-mass spectrometry. J Lipid Res 2004;45:776-81.

(22.) DeBarber AE, Lutjohann D, Merkens L, Steiner RD. Liquid chromatography-tandem mass spectrometry determination of plasma 24Shydroxycholesterol with chromatographic separation of 25-hydroxycholesterol. Anal Biochem 2008;381:151-3.

(23.) Honda A, Yamashita K, Hara T, Ikegami T, Miyazaki T, Shirai M, et al. Highly sensitive quantification of key regulatory oxysterols in biological samples by LC-ESI-MS/MS. J Lipid Res 2009;50: 350-7.

(24.) Karu K, Hornshaw M, Woffendin G, Bodin K, Hamberg M, Alvelius G, et al. Liquid chromatography-mass spectrometry utilizing multi-stage fragmentation for the identification of oxysterols. J Lipid Res 2007;48:976-87.

(25.) Griffiths WJ, Hornshaw M, Woffendin G, Baker SF, Lockhart A, Heidelberger S, et al. Discovering oxysterols in plasma: a window on the metabolome. J Proteome Res 2008;7:3602-12.

(26.) Karu K, Turton J, Wang Y, Griffiths WJ. Nano liquid chromatography-tandem mass spectrometry analysis of oxysterols in brain: monitoring of cholesterol autoxidation. Chem Phys Lipids 2011; 164:411-24.

(27.) Bjorkhem I. Crossing the barrier: oxysterols as cholesterol transporters and metabolic modulators in the brain. J Intern Med 2006;260:493508.

(28.) Bjorkhem I, Meaney S. Brain cholesterol: long secret life behind a barrier. Arterioscler Thromb Vasc Biol 2004;24:806-15.

(29.) Leoni V, Caccia C. Oxysterols as biomarkers in neurodegenerative diseases. Chem Phys Lipids 2011;164:515-24.

(30.) Russell DW. Oxysterol biosynthetic enzymes. Biochim Biophys Acta 2000;1529:126-35.

(31.) Hitsumoto T, Takahashi M, Iizuka T, Shirai K. Clinical significance of serum 7-ketocholesterol concentrations in the progression of coronary atherosclerosis. J Atheroscler Thromb 2009;16: 363-70.

(32.) Endo K, Oyama T, Saiki A, Ban N, Ohira M, Koide N, et al. Determination of serum 7-ketocholesterol concentrations and their relationships with coronary multiple risks in diabetes mellitus. Diabetes Res Clin Pract 2008;80:63-8.

(33.) Ferderbar S, Pereira EC, Apolinario E, Bertolami MC, Faludi A, Monte O, et al. Cholesterol oxides as biomarkers of oxidative stress in type 1 and type 2 diabetes mellitus. Diabetes Metab Res Rev 2007;23:35-42.

(34.) Sundaram SS, Bove KE, Lovell MA, Sokol RJ. Mechanisms of disease: inborn errors of bile acid synthesis. Nat Clin Pract Gastroenterol Hepatol 2008;5:456-68.

(35.) Matysik S, Orso E, Black A, Ahrens N, Schmitz G. Monitoring of 7alpha-hydroxy-4-cholesten-3one during therapy of cerebrotendinous xanthomatosis: a case report. Chem Phys Lipids 2011;164:530-4.

(36.) Porter FD, Scherrer DE, Lanier MH, Langmade SJ, Molugu V, Gale SE, et al. Cholesterol oxidation products are sensitive and specific blood-based biomarkers for Niemann-Pick C1 disease. Sci Transl Med 2010;2:56ra81.

(37.) Jiang X, Sidhu R, Porter FD, Yanjanin NM, Speak AO, te Vruchte DT, et al. A sensitive and specific LC-MS/MS method for rapid diagnosis of Niemann-Pick C1 disease from human plasma. J Lipid Res 2011;52:1435-45.

(38.) Bodzioch M, Orso E, Klucken J, Langmann T, Bottcher A, Diederich W, et al. The gene encoding ATP-binding cassette transporter 1 is mutated in Tangier disease. Nat Genet 1999;22:347-51.

S. Matysik, [1] H.H. Kliinemann, [2] and G. Schmitz [1] *

* Address correspondence to this author at: Institute for Clinical Chemistry, University Hosptial, Regensburg Franz-Josef- Strass-Allee 11 D-93053, Regensburg, Germany. Fax +49-941-9446202; e-mail gerd.schmitz@klinik.uniregensburg.de.

[1] University Hospital Regensburg, Regensburg, Germany; [2] Department of Psychiatry and Psychotherapy, University of Regensburg, Regensburg, Germany.

[3] Nonstandard abbreviations: CoA, coenzyme A; HMG, 3-hydroxy-3-methylglutaryl; GC, gas chromatography; LC, liquid chromatography; MS, mass spectrometry; LC-MS/MS, LC-tandem MS; MS3, MS/MS/MS; GC-MS/MS, GC coupled to triple-quadrupole MS; MRM, multiple reaction monitoring mode; EI, electron impact; TMS, trimethylsilyl; PCI, positive chemical ionization; S/N, signal-to-noise; NCI, negative CI.

Received May 10, 2012; accepted August 23, 2012.

Previously published online at DOI: 10.1373/clinchem.2012.189605
Table 1. MS parameters of the compounds.

Compound                               MRM       Analyte/  CE, eV
                                                 IS

Time period 1
  7[beta]-Hydroxycholesterol (a)   545.9/456.9   Target     10
  Desmosterol                      385.4/368.3   Target     10
  7-Dehydrocholesterol             457.5/367.4   Target     10
  Lathosterol                      369.0/215.0   Target     10
  Campesterol D6                   406.1/389.3   IS          5
  Campesterol                      400.1/383.3   Target      5
  Stigmasterol                     411.7/395.0   Target      5
  Sitosterol D6                    420.0/403.3   IS          5
  Lanosterol                       409.0/205.0   Target     10
  Sitosterol                       414.0/397.4   Target      5
Time period 2
  24-Hydroxycholesterol D10        483.7/376.9   IS         10
  24-Hydroxycholesterol            473.7/366.9   Target     10
  25-Hydroxycholesterol            473.7/366.9   Target     10
  7-Ketocholesterol d7             480.4/390.5   IS         10
  7-Ketocholesterol                473.4/383.5   Target     10
Time period 3                                               10
  27-Hydroxycholesterol D6         480.1/463.1   IS         10
  27-Hydroxycholesterol            474.1/457.1   Target     10

Compound                           Dwell time,   RT, min
                                   ms

Time period 1
  7[beta]-Hydroxycholesterol (a)   30            6.720
  Desmosterol                      30            6.617
  7-Dehydrocholesterol             30            6.622
  Lathosterol                      30            6.682
  Campesterol D6                   10            6.863
  Campesterol                      10            6.888
  Stigmasterol                     30            7.031
  Sitosterol D6                    10            7.339
  Lanosterol                       30            7.356
  Sitosterol                       10            7.365
Time period 2
  24-Hydroxycholesterol D10        50            7.658
  24-Hydroxycholesterol            50            7.722
  25-Hydroxycholesterol            50            7.837
  7-Ketocholesterol d7             50            7.958
  7-Ketocholesterol                50            7.979
Time period 3
  27-Hydroxycholesterol D6         50            8.213
  27-Hydroxycholesterol            50            8.230

(a) Negative CI gave higher intensities, 563.9/401.4.

Table 2. Imprecision of the GC-MS/MS method.

                                              Plasma 1

                                              Intra-assay  Interassay
                                 Mean value,    CV, %        CV, %
Compound                            ng/mL       (n = 5)      (n = 6)

7/[beta]-Hydroxycholesterol (a)      54           6.8          2.8
Desmosterol                         1300          2.9         11.5
7-Dehydrocholesterol                 484          9.3         n.a.b
Lathosterol                         1195          2.0          6.7
Campesterol                         2179          4.5          9.9
Stigmasterol                         17           4.2          9.3
Lanosterol                           130          4.2          8.4
Sitosterol                          1311          4.7          6.8
24-Hydroxycholesterol                28           3.9         11.8
25-Hydroxycholesterol                            n.a.         n.a.
7-Ketocholesterol                    66           3.1         14.5
27-Hydroxycholesterol                76           4.4          9.2

                                              Serum 1

                                              Intra-assay  Interassay
Compound                         Mean value,    CV, %        CV, %
                                    ng/mL       (n = 5)      (n = 6)
7/[beta]-Hydroxycholesterol (a)
Desmosterol                          125          5.8         25.5
7-Dehydrocholesterol                1356          2.1         15.7
Lathosterol                          363         17.5         10.4
Campesterol                          811          3.8         12.5
Stigmasterol                        2676          1.6         14.9
Lanosterol                           168          2.9         11.2
Sitosterol                           94          11.2          8.1
24-Hydroxycholesterol               2273          0.6         13.9
25-Hydroxycholesterol                31           5.2          6.9
7-Ketocholesterol                     8           9.8         10.8
27-Hydroxycholesterol                163          3.0         12.3
                                     66           1.5         11.9

                                         Plasma 2

                                              Intra-assay
                                 Mean value,    CV, %
Compound                            ng/mL       (n = 5)

7/[beta]-Hydroxycholesterol (a)      18          10.8
Desmosterol                         1432          1.9
7-Dehydrocholesterol                 318         29.7
Lathosterol                          764         12.2
Campesterol                         3922          3.1
Stigmasterol                         56           4.5
Lanosterol                           42           7.5
Sitosterol                          2212          4.8
24-Hydroxycholesterol                50           1.8
25-Hydroxycholesterol               n.a.         n.a.
7-Ketocholesterol                    24           8.0
27-Hydroxycholesterol                130          2.9

(a) Negative CI mode.

(b) NA, not analyzed.
COPYRIGHT 2012 American Association for Clinical Chemistry, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2012 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:Lipids, Lipoproteins, and Cardiovascular Risk Factors
Author:Matysik, S.; Klunemann, H.H.; Schmitz, G.
Publication:Clinical Chemistry
Article Type:Report
Geographic Code:4EUGE
Date:Nov 1, 2012
Words:4840
Previous Article:Mediator probe PCR: a novel approach for detection of real-time PCR based on label-free primary probes and standardized secondary universal...
Next Article:Troponin I measured by a high-sensitivity assay in patients with suspected reversible myocardial ischemia: data from the Akershus Cardiac Examination...
Topics:

Terms of use | Copyright © 2017 Farlex, Inc. | Feedback | For webmasters