Indirect identification of isoprenoid quinones in Escherichia coli by LC-MS with atmospheric pressure chemical ionization in negative mode

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1 J. Basic Microbiol. 44 (24) 6, DOI: 1.2/jobm ( 1 College of Environmental Science and Engineering, Ocean University of China, Qingdao, 2663, China; 2 State Key Laboratory of Environmental Aquatic Chemistry, Research Center for ECO- Environmental Sciences, Chinese Academy of Sciences, 85, Beijing, China; 3 Department of Urban and Environmental Science, Peking University, Beijing 871, China) Indirect identification of isoprenoid quinones in Escherichia coli by LC-MS with atmospheric pressure chemical ionization in negative mode MENGCHUN GAO 1,2, *, HONGJUN LIU 1, MIN YANG 2, JIANYING HU 3 and BING SHAO 2 (Received 2 May 24/Accepted 2 July 24) A novel analytical method was applied for identification of isoprenoid quinones in Escherichia coli by liquid chromatography atmospheric press chemical ionization mass spectrometry in negative mode (LC-NI-APCI-MS). Extraction and clean-up of sample were carried out on Sep-Pak Plus Silica solidphase extraction cartridges. Ubiquinone-7 (UQ-7), Ubiquinone-8 (UQ-8) and Mequinone-8 (MK-8) were determined directly using combined information on retention time, molecular ion mass, fragment ion masses and UV characteristic spectrometry without any standard reagent. It was found that UQ-8 was the major component of isoprenoid quinones in Escherichia coli under aerobic condition. Compared with UQ-8, the relative abundance of UQ-7 and MK-8 is only 15 and 14, respectively. The average recoveries of UQ-6, UQ-1 and vitamin K 1 in Escherichia coli were investigated by standard spiking experiment. The recoveries were achieved in the range from 94 to 16, and the relative standard deviations (RSD) of the triplicate analysis of the spiked samples (UQ-6, UQ-1 and vitamin K 1 ) ranged from 3 to 8. The detection limits of LC-NI-APCI-MS were estimated to be 5, 4 and.8 µg/g dry cell for UQ-6, UQ-1 and vitamin K 1, respectively. Isoprenoid quinones are essential for the respiratory electron transport system in bacteria and have attracted attention not only from physiological but also from chemotaxonomic viewpoints (JEFFRIES et al. 1969, YAMADA et al. 1976, COLLINS et al. 1981). Microbial isoprenoid quinones have usually been analyzed by high-performance liquid chromatography (HPLC) followed by preparative thin-layer chromatography or solid-phase extraction (SPE) (HU et al. 1999). HPLC peaks are assigned by comparison with authentic standards of isoprenoid quinones, which are hard to obtain. The LC-UV method is useful for determining isoprenoid quinones, however, the selectivity and sensitivity of the method is insufficient for minor quinones due to complex matrices. In recent years, LC-MS with various kinds of interface has made rapid advances and has become an important tool to analyze isoprenoid quinone mixture (MIYUKI et al. 1997). Up to now, only the frit-fab interface has been explored for the analysis of ubiquinone and menaquinone by LC-MS, however, the detection sensitivity is very low. At present, we have developed a simple and sensitive quinone analysis method using liquid chromatography atmospheric press chemical ionization mass spectrometry in negative mode (LC-NI-APCI- MS) without any authentic standards of isoprenoid quinone. Escherichia coli is a representative of facultative anaerobic bacteria, and composition of its quinone pool is also known to dramatically vary during aerobiosis-to-anaerobiosis transition in correlation with coherent changes in content of respiratory chain enzymes (SOBALLE * Corresponding author: Dr. M. GAO; mengchungao@hotmail.com 24 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim X/

2 Indirect identification of isoprenoid quinones in E. coli 425 et al. 1999). In this present study, we applied LC-NI-APCI-MS to identify isoprenoid quinones of Escherichia coli cultured under aerobic condition. Materials and methods Standard solutions and reagents: Ubiquinone standard (UQ-6 and UQ-1) and menaquinone standard (vitamin K 1 ) were obtained from SIGMA (St Louis, MO, USA). Standard stock solutions of mg/l were prepared in acetone (LC grade, FISHER, China) and stored at 4 C. Working standard solutions were prepared by diluting the stock solutions in acetone and stored in the same way. Methanol, isopropyl ether and hexane were LC grade (FISHER). Ultra pure water was prepared using Easypure UV compact Ultrapure system under a resistivity of 18.3 Ω/cm. The Sep-Pak Plus Silica cartridges (2 1 cm I.D.; polypropylene tubes; silica gel amount: 6 mg; particle diameter: µm) were purchased from Waters (MILFORD, MA, USA). Growth media and conditions: Escherichia coli was cultured in culture medium supplemented with.5 yeast extract,.1 tryptone, and.4 maltose at ph 7.4 and 37 C. Batch cultures were incubated up to cells/ml in 1 l glass flasks with 5 ml of growth medium vigorously shaken at 2 r.p.m. Extraction of quinones: Isoprenoid quinones in Escherichia coli were extracted using the previously described method (HU et al. 1999, KURISU et al. 22). The sample was centrifuged after 1 ml ultrapure water and 2 ml chloroform-methanol (2:1, v/v) were added to the centrifuge tube. The top layer (water layer) was discarded, and the middle and bottom layers were filtered. The filtrate was collected, and the filter cake together with the filter paper was then extracted three times with 3 ml of a chloroform-methanol mixture. The filtrate was concentrated almost to dryness on a rotary evaporator, and the residues were further extracted twice with 2 ml hexane. The hexane extract, containing the ubiquinones and menaquinones, was purified using two Sep-Pak plus silica cartridges joined in series. Typically, 5 ml hexane was passed through two Sep-Pak plus silica cartridges and the hexane extract was then loaded on Sep-Pak plus silica cartridges at a flow of 24 ml/min. Menaquinones and ubiquinones were eluted with 2 ml of 2 and 1 diethyl ether in hexane solution from the above Sep-Pak Plus Silica cartridges, respectively. The eluate was dried under a gentle stream of nitrogen, and re-dissolved in 2 ml of acetone for future analysis. Liquid chromatography and mass spectrometry: Liquid chromatography was carried out on an Alliance 269 HPLC (Waters, USA) equipped with a quaternary gradient pump, an auto-sampler with 2. µl injection loop, a photodiode array detector and a reversed-phase column (ZORBAX-ODS, 4.6 mm I.D. 25 mm, Alliance and Agilent, USA). A mixture of methanol and isopropyl ether (83:17, v/v) was used as the mobile phase at a flow rate of.8 ml/min. A platform ZMD single-quadruple mass spectrometer (MICROMASS, Manchester, UK) was used with a Z-Spray ion-source fitted with a pneumatically assisted atmospheric pressure chemical ionization probe. The conditions of mass spectrometry in negative ion mode were shown as follows: APCI capillary voltage at 3.5 kv; cone voltage at 45 V; source block temperature at 15 C; APCI mass spectrometry temperature 35 C; cone gas flow 3 l/h; desolvation gas flow 4 l/h. Results and discussion Analysis of a standard sample mixture Fig. 1 exhibits the UV spectrum of UQ-6 and vitamin K 1. UQ-6 shows a characteristic UV spectrum with λ max at 25 nm and 275 nm, and vitamin K 1 shows absorptions at λ max 24, 248 and 27 nm. Fig. 2 shows the APCI-MS mass spectrum of UQ-6 in the negative mode at a cone voltage of 45 V and 8 V, respectively. At a cone voltage of 45 V, the molecular ion [M] at 59.4 was found as the most abundant ion, which was different from the conventional 24 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3 426 M. GAO et al. Vitami n K nm 25 UQ nm 29 Fig. 1 UV spectrum of vitamin K 1 and UQ-6 quasi-molecular ion [M-H]. When the cone voltage was gradually increased from 45 to 8 V, fragment ions due to an in-source collision induced dissociation (CID) were found. In addition to the molecular ion M at 59.4 with 23 relative abundance, we found the base peak ion at ([M-CH 3 ] ) together with another fragment ion at 56.4 with a relative abundance of 2. The mass spectra of vitamin K 1 were also investigated in NI-APCI mode (Fig. 3). The most abundant ion at a cone voltage of 45 V was the molecular ion M at With 45V 59.4 [M] - 8V [M-CH 3 ] - UQ [M-CH [M] - 3 O] Fig. 2 Mass spectrum of UQ-6 at cone voltages of 45 V and 8 V. UQ-6 concentration: 1 mg/l; injection volume: 2 µl 45V 45.7 [M] V [M] Vitamin K 1 V [M] Fig. 3 Mass spectrum of vitamin K 1 at cone voltages of 45, 8, and V. Vitamin K 1 concentration: 1 mg/l; injection volume: 2 µl 24 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

4 Indirect identification of isoprenoid quinones in E. coli 427 the gradual increase of cone voltage from 45 to V, two fragment ions were found. One was the base peak ion at 21.2, other was at Thus, NI-APCI-MS provides most useful information for the analysis of UQ-6 and vitamin K 1. Analysis of Escherichia coli Fig. 4 shows the total ion current chromatogram of the sample extracted from Escherichia coli with an injection of 2 µl. As shown in the chromatogram, three peaks of isoprenoid quinones were completely separated. Compared with the major peak (peak 2), the relative amount of peak 1 and peak 3 was 15 and 14, respectively. The UV spectrum of peak 1 and peak 2 showed characteristic absorption maximum at λ max 25 and 275 nm, which were similar to those of UQ-6. The UV spectrum of peak 3 with maximum at λ max 24, 248 and 27 nm was similar to that of vitamin K 1. Peak2 Peak3 Peak1 Time (min) Fig. 4 LC-NI-APCI-MS chromatogram of the sample extracted from Escherichia coli. APCI capillary voltage at 3.5 kv; cone voltage at 45 V; source block temperature at 15 C; APCI mass spectrometry temperature 35 C; cone gas flow 3 l/h; desolvation gas flow 4 l/h; injection volume 2 µl Fig. 5 exhibits the APCI-MS mass spectrum of peak 1 in the negative ion mode, the molecular ion [M] at was found as the most abundant ion at a cone voltage of 45 V, which had the same value to the molecular mass of UQ-7. Fig. 5(b) showed the mass spectrum of peak 1 at a cone voltage of 8 V. In addition to the molecular ion, a fragment ion (5 relative abundance) at ([M-CH 3 ] ) and another fragment ion (32 relative abundance) at ([M-CH 2 O] ) were found which proved that peak 1 was due to UQ-7. Similarly, peak 2 was identified as UQ-8 by molecular ion mass, fragment ion masses and the characteristic UV spectrum. The mass spectrum of peak 3 was also investigated in the negative ion mode. At a cone voltage of 45 V, the most abundant ion was found at which is the same value as the molecular mass of MK-8. When the cone voltage was increased to 8 V (Fig. 6), peak 3 delivered two fragment ions at 21.5 and with a relative abundance of 29 and 58, respectively, and was identified as MK WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

5 428 M. GAO et al. a [M] - 45V b 8V [M] - [M-CH 3 ] [M-CH 2 O] Fig. 5 Mass spectrum of peak 1 in negative mode (a) cone voltage of 45 V; (b) cone voltage of 8 V; other analytical conditions, see Fig [M ]- 8V Fig. 6 Mass spectrum of peak 3 in negative mode at the cone voltage of 8 V; other analytical conditions, see Fig. 4 Method validation The average recoveries of UQ-6, UQ-1 and vitamin K 1 in Escherichia coli were investigated by standard spiking experiments (see Table 1). The recovery rates for UQ-6, UQ-1 and vitamin K 1 were achieved in the range from 94 to 16, and the relative standard deviations (RSD) of the triplicate analysis of the spiked samples (UQ-6, UQ-1 and vitamin K 1 ) ranged from 3 to 8. Considering the concentration times and recoveries of UQ-6, UQ-1 and vitamin K 1, the detection limits of LC-NI-APCI-MS were estimated to be 5, 4 and.8 µg/g for UQ-6, UQ-1 and vitamin K 1, respectively. 24 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

6 Indirect identification of isoprenoid quinones in E. coli 429 Table 1 Mean recoveries, relative standard deviations (RSD) and detection limits of the method obtained in selected ion monitoring mode Mean recoveries ± RSD UQ-6 94 ± 3.4 UQ-1 95 ± 5 5. Vitamin K 1 98 ± 8.12 Detection limits of the method (µg/g dry cell) LC-NI-APCI-MS method was used to analyze isoprenoid quinones with satisfactory sensitivity, selectivity, and reproducibility. The isoprenoid quinones in Escherichia coli could be detected simultaneously by a single analysis with a small amount containing the quinones from 2 3 mg of dry cells. The method surmounts the shortcomings of the conventional methods such as LC-UV and Frit-FAB-LC-MS, and can give the accurate evaluation of isoprenoid quinones in Escherichia coli without any standard reagent. Acknowledgements Funding for this research was provided by the National Natural Science Foundation of China (Grants 52385), and State Hi-Tech Research and Development Project of the Ministry of Science and Technology, China (Grants 22AA6131). References COLLINS, M. D. and JONES D., Distribution of isoprenoid quinone structural types in bacteria and their taxonomic implications. Microbiol. Rev., 45, HU, H.-Y., FUJIE, K. and URANO, K., Development of a novel solid phase extraction method for the analysis of bacterial quinones in activated sludge with a higher reliability. J. Biosci. Bioeng., 87, JEFFRIES, L., CAWTHORNE, M. A., HARRIS, M., COOK, B. and DIPLOCK, A. T., Menaquinone determination in the taxonomy of Micrococcaceae. J. Gen. Microbiol., 54, KURISU, F., SATOH, H., MINO, T. and MATSUO, T., 22. Microbial community analysis of thermophilic contact oxidation process by using ribosomal RNA approaches and the quinone profile method. Wat. Res., 36, MIYUKI, NISHIJIMA, MIHO, ARAKI-SAKAI and HIROSHI, SANO, Identification of isoprenoid quinones by frit-fab liquid chromatography-mass spectrometry for the chemotaxonomy of microorganisms. J. Microbiol. Method, 28, SOBALLE, B. and POOLE, R. K., Microbial ubiquinone: Multiple roles in respiration, gene regulation and oxidative stress management. Microbiol., 145, YAMADA, Y., NOJIRI, M., MASTUYAMA, M. and KONDO, K., Coenzyme Q system in the classification of the ascoporogenous yeast genera Debaryomyces, Saccharomyces, Kluyveromyces and Endomycopsis. J. Gen. Appl. Microbiol., 22, Mailing address: Dr. MENGCHUN GAO, College of Environmental Science and Engineering, Ocean University of China, No. 5 Yushan Road, Qingdao, 2663, China Tel.: ; Fax: ; mengchungao@hotmail.com 24 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim