Liquid Ionization Mass Spectrometry Research Articles

Xanthophylls in Commercial Egg Yolks: Quantification and Identification by HPLC and LC-(APCI)MS Using a C30 Phase

The xanthophylls lutein and zeaxanthin have attracted a lot of interest since it was presumed that an increased nutritional uptake may prevent adult macula degeneration (AMD). Although egg yolks serve as an important dietary source of lutein and zeaxanthin, data on xanthophyll concentrations in commercial egg yolks are not available. Thus, an high-performance liquid chromatography-diode array detector (HPLC-DAD) method was developed allowing for simultaneous separation of eight xanthophylls used to fortify poultry feed. Peak identification was carried out by liquid chromatography-atmospheric pressure chemical ionization mass spectrometry [LC-(APCI)MS]. Egg yolks of four types of husbandry (seven batches each) were examined. Lutein and zeaxanthin were the predominant xanthophylls in egg yolks produced in accordance with ecological husbandry (class 0) because the concentrations of these xanthophylls ranged from 1274 to 2478 microg/100 g and from 775 to 1288 microg/100 g, respectively. Analysis of variance (ANOVA) proved that both mean lutein and mean zeaxanthin concentrations of eggs from class 0 were statistically discriminable from mean lutein and zeaxanthin concentrations from eggs of all other classes (P < 0.01). Mean concentrations of synthetic xanthophylls in eggs of classes 1 (free range), 2 (barn), and 3 (cage) were as follows: canthaxanthin, 707 +/- 284 microg/100 g; beta-apo-8'-carotenoic acid ethyl ester, 639 +/- 391 microg/100 g; and citranaxanthin, 560 +/- 231 microg/100 g. Experiments with boiled eggs proved that beta-apo-8'-carotenoic acid ethyl ester was the xanthophyll with the highest stability, whereas lutein was degraded to the largest extent (loss of 19%). Detailed knowledge about the xanthophyll amounts in eggs is indispensable to calculate the human uptake.

Differential Dimethyl Labeling of N-Termini of Peptides after Guanidination for Proteome Analysis

We describe an enabling technique for proteome analysis based on isotope-differential dimethyl labeling of N-termini of tryptic peptides followed by microbore liquid chromatography (LC) matrix-assisted laser desorption and ionization (MALDI) mass spectrometry (MS). In this method, lysine side chains are blocked by guanidination to prevent the incorporation of multiple labels, followed by N-terminal labeling via reductive amination using d(0),(12)C-formaldehyde or d(2),(13)C-formaldehyde. Relative quantification of peptide mixtures is achieved by examining the MALDI mass spectra of the peptide pairs labeled with different isotope tags. A nominal mass difference of 6 Da between the peptide pair allows negligible interference between the two isotopic clusters for quantification of peptides of up to 3000 Da. Since only the N-termini of tryptic peptides are differentially labeled and the a(1) ions are also enhanced in the MALDI MS/MS spectra, interpretation of the fragment ion spectra to obtain sequence information is greatly simplified. It is demonstrated that this technique of N-terminal dimethylation (2ME) after lysine guanidination (GA) or 2MEGA offers several desirable features, including simple experimental procedure, stable products, using inexpensive and commercially available reagents, and negligible isotope effect on reversed-phase separation. LC-MALDI MS combined with this 2MEGA labeling technique was successfully used to identify proteins that included polymorphic variants and low abundance proteins in bovine milk. In addition, by analyzing a mixture of two equal amounts of milk whey fraction as a control, it is shown that the measured average ratio for 56 peptide pairs from 14 different proteins is 1.02, which is very close to the theoretical ratio of 1.00. The calculated percentage error is 2.0% and relative standard deviation is 4.6%.

Clavulanic Acid Inactivation of SHV-1 and the Inhibitor-resistant S130G SHV-1 β-Lactamase: INSIGHTS INTO THE MECHANISM OF INHIBITION

Clavulanic acid is a potent mechanism-based inhibitor of TEM-1 and SHV-1beta-lactamases, enzymes that confer resistance to beta-lactams in many gram-negative pathogens. This compound has enjoyed widespread clinical use as part of beta-lactam beta-lactamase inhibitor therapy directed against penicillin-resistant pathogens. Unfortunately, the emergence of clavulanic acid-resistant variants of TEM-1 and SHV-1 beta-lactamase significantly compromise the efficacy of this combination. A single amino acid change at Ambler position Ser130 (Ser --> Gly) results in resistance to inactivation by clavulanate in the SHV-1 and TEM-1beta-lactamases. Herein, we investigated the inactivation of SHV-1 and the inhibitor-resistant S130G variant beta-lactamases by clavulanate. Using liquid chromatography electrospray ionization mass spectrometry, we detected multiple modified proteins when SHV-1 beta-lactamase is inactivated by clavulanate. Matrix-assisted laser desorption ionization-time of flight mass spectrometry was used to study tryptic digests of SHV-1 and S130Gbeta-lactamases (+/- inactivation with clavulanate) and identified peptides modified at the active site Ser70. Ultraviolet (UV) difference spectral studies comparing SHV-1 and S130Gbeta-lactamases inactivated by clavulanate showed that the formation of reaction intermediates with absorption maxima at 227 and 280 nm are diminished and delayed when S130Gbeta-lactamase is inactivated. We conclude that the clavulanic acid inhibition of the S130G beta-lactamase must follow a branch of the normal inactivation pathway. These findings highlight the importance of understanding the intermediates formed in the inactivation process of inhibitor-resistant beta-lactamases and suggest how strategic chemical design can lead to novel ways to inhibit beta-lactamases.

LC-DAD-APCI-MS-based screening and analysis of the absorption and metabolite components in plasma from a rabbit administered an oral solution of danggui

A valid chromatographic fingerprint method using liquid chromatography-diode array detection-atmospheric pressure chemical ionization mass spectrometry in negative mode (LC-DAD-APCI-MS) is proposed for studying the absorption and metabolites of a traditional Chinese medicine (TCM) Angelica sinensis (danggui) in rabbit plasma, after the rabbit is administered with danggui oral solution (DOS). More than thirty-two common components were detected in both DOS and rabbit plasma, which shows that the components in the DOS were absorbed into the body of the rabbit. Of these, senkyunolide I, senkyunolide H, Z-6,7-epoxyligustilide, 3-butylidene-7-hydroxyphthalide, Z-ligustilide, Z-butylidenephthalide, Diels-Alder dimers of ligustilide, linolenic acid, linoleic acid and falcarindiol were tentatively identified from their MS, UV spectra and retention behavior by comparing the results with the published literature. At least ten components were found in rabbit plasma but not in DOS, indicating that these components must be metabolites of some of the components in the original extract. The results prove that the proposed method can be used to rapidly analyze multiple constituents in TCMs, and to screen for bioactive compounds by comparing and contrasting the chromatographic fingerprints of DOS and plasma samples.

Identification and Quantification of Differentially Expressed Proteins in E-Cadherin Deficient SCC9 Cells and SCC9 Transfectants Expressing E-Cadherin by Dimethyl Isotope Labeling, LC−MALDI MS and MS/MS

A strategy based on isotope labeling of peptides and liquid chromatography matrix-assisted laser desorption ionization mass spectrometry (LC-MALDI MS) has been employed to accurately quantify and confidently identify differentially expressed proteins between an E-cadherin-deficient human carcinoma cell line (SCC9) and its transfectants expressing E-cadherin (SCC9-E). Proteins extracted from each cell line were tryptically digested and the resultant peptides were labeled individually with either d(0)- or d(2)-formaldehyde. The labeled peptides were combined and the peptide mixture was separated and fractionated by a strong cation exchange (SCX) column. Peptides from each SCX fraction were further separated by a microbore reversed-phase (RP) LC column. The effluents were then directly spotted onto a MALDI target using a heated droplet LC-MALDI interface. After mixing with a MALDI matrix, individual sample spots were analyzed by MALDI quadrupole time-of-flight MS, using an initial MS scan to quantify the dimethyl labeled peptide pairs. MS/MS analysis was then carried out on the peptide pairs having relative peak intensity changes of greater than 2-fold. The MS/MS spectra were subjected to database searching for protein identification. The search results were further confirmed by comparing the MS/MS spectra of the peptide pairs. Using this strategy, we detected and compared relative peak intensity changes of 5480 peptide pairs. Among them, 320 peptide pairs showed changes of greater than 2-fold. MS/MS analysis of these changing pairs led to the identification of 49 differentially expressed proteins between the parental SCC9 cells and SCC9-E transfectants. These proteins were determined to be involved in different pathways regulating cytoskeletal organization, cell adhesion, epithelial polarity, and cell proliferation. The changes in protein expression were consistent with increased cell-cell and cell-matrix adhesion and decreased proliferation in SCC9-E cells, in line with E-cadherin tumor suppressor activity. Finally, the accuracy of the MS quantification and subcellular localization for 6 differentially expressed proteins were validated by immunoblotting and immunofluorescence assays.

Quantitative Analysis of Protein Expression Changes Induced by Geldanamycin in NPM-ALK Positive Lymphoma Cells by Functional Proteomics.

A subset of anaplastic large cell lymphomas are characterized by the t(2;5)(p23;q35) chromosomal translocation. Fusion of the nucleophosmin (NPM) gene on chromosome 5q35 with the anaplastic lymphoma kinase (ALK) gene on chromosome 2p23 results in the constitutive expression of the NPM-ALK oncoprotein. Heat shock protein 90 (HSP90) is a molecular chaperone involved in the folding of a number of substrate proteins implicated in cancer. The list of known HSP90 client proteins is expanding and includes oncogenes, cell cycle-associated proteins, and kinases including NPM-ALK. The ansamycin-based anti-cancer drug, geldanamycin (GA) has been shown to inhibit HSP90 by interfering with its function as a molecular chaperone, resulting in ubiquitin-mediated proteasomal degradation of its client proteins. To date, the cellular effects of inhibition of HSP90 by GA remain largely uncharacterized. Here, we describe a functional proteomics approach to identify the differentially expressed proteins in NPM-ALK positive lymphoma cells exposed to GA. GA induced a dose- and time-dependent decrease in cell viability with an IC50 of 20μM, which was associated with a G2/M cell cycle arrest followed by caspase-3 mediated apoptosis. Equal amounts of total cell lysates from DMSO-control and GA-treated SUDHL-1 cells were labeled with either the light or heavy cleavable ICAT™ reagent. Samples were combined, digested with trypsin, and separated by strong cation exchange chromatography and avidin affinity chromatography. Peptides were subjected to liquid chromatography (LC), electrospray ionization (ESI) and tandem mass spectrometry (MS/MS). The resulting peptides were searched against the NCBI and Swiss-Prot databases using SEQUEST™ for protein identification and relative quantitation determined by XPRESS™. A total of 117 proteins were differentially expressed by greater than 1.5-fold of which 48 were overexpressed and 69 were underexpressed in GA-treated cells relative to control. The 48 overexpressed proteins included those that are involved in apoptosis, protein degradation, cell communication, and cell cycle regulation. Of the 69 underexpressed proteins, many included proteins involved in ubiquitin-proteasome function (proteasome beta 2 subunit, HSP90-organizing protein, CBL-C), cytokines (IL-14 precursor), and DNA repair. Notably, a number of previously reported targets of HSP90/70 (male germ cell-associated kinase and IL-26 precursor) were identified. Our results provide insights into the role of HSP90 in the pathogenesis of NPM-ALK positive lymphomas and demonstrate the diverse cellular functions that are deregulated as a consequence of HSP90 inhibition.

An optimized protocol for nano-LC-MALDI-TOF-MS coupling for the analysis of proteolytic digests of glycoproteins.

Matrix-assissted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analyses of complete proteolytic digests are often hampered by contaminations and the complexity of the sample. This results in suppression effects and the formation of adducts which are difficult to assign, thus leading to low scores in database searches. In particular, signals of post-translationally modified peptides such as glycopeptides are often of low intensity or completely suppressed. Online liquid chromatography electrospray ionization mass spectrometry (ESI-MS) can, in part, overcome this problem, but the analytes are completely consumed during the run. Coupling of nano-flow HPLC (nano-LC), microfractionation and MALDI-TOF-MS combines separation and high-sensitivity UV detection with the possibility of collecting fractionated peptides and preserving the sample for detailed mass spectrometric analyses. Here we report on an optimized protocol for nano-LC-MALDI-TOF-MS analyses of glycoproteins. This protocol improves spectral quality, resulting in better protein identification scores in database searches. Furthermore, post-translationally modified peptides could be detected with higher sensitivity by changing the experimental conditions, allowing assignment, localization and characterization of the respective carbohydrate substituents.

Cluster composition distribution at the liquid surface of alcohol–water mixtures and evaporation processes studied by liquid ionization mass spectrometry

Liquid ionization mass spectrometry (LPI-MS) gives information about hydrogen-bonded clusters at a liquid surface. By improving the way in which samples are introduced, mass spectra showing clusters at the liquid surface or in a gas phase were obtained for 20% (v/v) ethanol and methanol aqueous solutions. Observed cluster ions were expressed as (ROH) m (H 2O) n H +, R = CH 3 or C 2H 5, and mass spectra gave the molar ratios of methanol to water ( M/ W) or ethanol to water ( E/ W) close to the ratio corresponding to the concentration of the respective solution. Binary cluster ions containing two molecules of alcohol ( m = 2) were abundant for both solutions. The molar ratios calculated from the cluster compositions, m − n, of the most abundant cluster ions were also close to the ratio corresponding to the concentration of the solution. The results suggest that the composition distribution of cluster ions observed as LPI mass spectrum may be similar to the composition distribution of clusters existing at the liquid surface. The cluster compositions at the liquid surface vary very quickly due to evaporation of the liquid. The clusters in the vapor were also measured using another device for sample introduction. The evaporation processes occurring in the nano-space at and above the liquid surface were mainly the loss of water molecules from larger clusters. The following collisions between smaller clusters leaded to the association of alcohol molecules accompanied with further loss of water molecules, resulting in the increase of the number of alcohol molecules in each cluster. Even ethanol clusters, (C 2H 5OH) m H +, were formed from the aqueous ethanol solutions. Reasonable differences between ethanol–water and methanol–water were observed for the mass spectra measured in the gas phase. Liquid ionization mass spectrometry gives nano-scale information concerning the cluster compositions at the liquid surface and the evaporation processes.

Lanopylins A1, B1, A2 and B2, Novel Lanosterol Synthase Inhibitors from Streptomyces sp. K99‐5041.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Water Clusters in Gas Phases Studied by Liquid Ionization Mass Spectrometry

Studies of water clusters under atmospheric pressure are very important. In previous methods, however, clusters existing in vacuum at low temperature have been measured. Therefore, the size distributions of observed cluster ions are likely different from those existing at normal conditions. Liquid ionization (LPI) mass spectrometry we have developed provides information about hydrogen-bonded clusters in a gas phase and also at the liquid surface at ambient temperature under atmospheric pressure. Thus, LPI-MS was applied to study water clusters in gas phases. The results indicated that the sizes of water clusters in vapor increased with the temperature of liquid water and those in the air increased with the relative humidity, while they decreased with increasing the temperature of the gas phase. In general, the size distribution of water clusters shows one smooth envelope, except for a few magic numbered clusters. Adiabatic condensation is negligible to occur in this method and thus, LPI mass spectra give useful information about clusters in gas phases under the normal conditions.

Direct characterization of bitter acids in a crude hop extract by liquid chromatography-atmospheric pressurechemical ionization mass spectrometry

Direct characterization of bitter acids in a crude hop extract by liquid chromatography-atmospheric pressurechemical ionization mass spectrometry

Direct characterization of bitter acids in a crude hop extract by liquid chromatography-atmospheric pressure chemical ionization mass spectrometry

The applicability of on-line coupling of reversed-phase high-performance liquid chromatography to atmospheric pressure ionization tandem mass spectrometry for the separation and characterization of hop acids mixture from the crude extract of Humulus lupulus was investigated. The solvent system consisting of acetonitrile-aqueous formic acid was used to give proper separation of the six main hop bitter acids within 30 min. Further structural information about the components was acquired by collision-induced dissociation (CID). On the basis of analyses of the fragmentation patterns of the major α- and β-bitter acids respectively, identification of the minor ones was performed using selected reaction monitoring (SRM) with a group of qualitatively relevant selected precursor-product ion transitions for each bitter acid in a single high performance liquid chromatography (HPLC) run. Using this technique, six minor hop acids, including “adprelupulone” observed for the first time in natural resources, were detected along with the six major acids. This hyphenated techniques provides potency for rapid qualitative determination of analogs and homologs in mixtures.

Identification and quantification of zeaxanthin esters in plants using liquid chromatography-mass spectrometry.

It has been suggested that lutein and zeaxanthin may decrease the risk for age-related macular degeneration. Surprisingly, oleoresins rich in zeaxanthin are not yet available on the market. Several authors have reported enhanced stability of esterified xanthophylls, so plants containing zeaxanthin esters were investigated to establish valuable sources for the production of durable oleoresins. Liquid chromatography-atmospheric pressure chemical ionization mass spectrometry [LC-(APCI)MS] was used to unequivocally identify zeaxanthin esters of a standard mixture and in several plant extracts. Zeaxanthin esters were quantified on the basis of their respective molecular masses using zeaxanthin for calibration; total zeaxanthin was determined after saponification of aliquots of the extracts. Thus, dried wolfberries (Lycium barbarum), Chinese lanterns (Physalis alkekengi), orange pepper (Capsicum annuum), and sea buckthorn (Hippophae rhamnoides) proved to be valuable zeaxanthin ester sources. The present LC-MS method allows for an even more detailed analysis of zeaxanthin esters than reported previously.

Isotopic labeling and LC-APCI-MS quantification for investigating absorption of carotenoids and phylloquinone from kale (Brassica oleracea).

The ability to study bioavailability of nutrients from foods is an important step in determining the health impact of those nutrients. This work describes a method for studying the bioavailability of nutrients from kale (Brassica oleracea var. Acephala) by labeling the nutrients with carbon-13, feeding the kale to an adult volunteer, and analyzing plasma samples for labeled nutrients. Results showed that conditions for producing atmospheric intrinsically labeled kale had no detrimental effect on plant growth. Lutein, beta-carotene, retinol, and phylloquinone were analyzed using liquid chromatography-atmospheric pressure chemical ionization mass spectrometry. Analysis of plasma samples showed that labeled lutein peaked in plasma at 11 h (0.23 microM), beta-carotene peaked at 8 (0.058 microM) and 24 h (0.062 microM), retinol peaked at 24 h (0.10 microM), and phylloquinone peaked at 7 h (3.0 nM). This method of labeling kale with (13)C was successful for producing clearly defined kinetic curves for (13)C-lutein,(13)C-beta-carotene, (13)C-retinol, and (13)C-phylloquinone.

Lanopylins A1, B1, A2 and B2, novel lanosterol synthase inhibitors from Streptomyces sp. K99-5041.

From an actinomycete strain, Streptomyces sp. K99-5041, lanopylins A1, B1, A2 and B2 were isolated as new natural products that inhibited the reaction of recombinant human lanosterol synthase. The crude extract from the whole broth of this strain was fractionated by silica gel column chromatography to afford an active fraction that showed a single spot on TLC. Detailed analyses of this fraction with liquid chromatography-atmospheric pressure chemical ionization mass spectrometry revealed that it contained 20 homologous compounds with differing side chain lengths. The fraction was separated by preparative HPLC to afford four of these homologues, lanopylins A1, B1, A2 and B2. Detailed spectroscopic analyses of these isolated compounds led to the identification of their structures. Lanopylins A1 and B1 were (3E)-isohexadecylmethylidene-2-methyl-1-pyrroline and (3E)-hexadecylmethylidene-2-methyl-1-pyrroline, respectively, and lanopylins A2 and B2 were homologues with the insertion of one cis-ethylenylidene in the side chain of lanopylins A1 and B1, respectively. These compounds inhibited recombinant human lanosterol synthase with IC50 values of 15, 18, 33, and 41 microM, respectively.

Identification of unknown surfactants using electrospray mass spectrometry and NMR spectroscopy preceded by liquid ionization mass spectrometry

Commercially available but completely unknown surfactants used in the tin–lead plating industry were successfully identified by using electrospray mass spectrometry and NMR spectroscopy techniques, preceded by liquid ionization mass spectrometry used to obtain the preliminary information. The mass spectral data suggested that ethoxylated nonionic surfactants having a homologous distribution of molecular weights like 520, 564, 608, 652, 696, etc. were present. The NMR data suggested the presence of two aromatic rings and a quaternary carbon for the hydrophobe moiety instead of the well-known alkyl chains or alkylphenols. The unknown surfactants were finally concluded to be novel nonionic 4-(α,α-dimethylbenzyl)-phenol ethoxylates.

Clusters at the Liquid Surface of Ethanol Aqueous Solutions Studied by Liquid Ionization Mass Spectrometry

Clusters at the Liquid Surface of Ethanol Aqueous Solutions Studied by Liquid Ionization Mass Spectrometry

Rapid Screening for Inborn Errors of Metabolism Using Liquid Ionization Mass Spectrometry with Dried Blood Spots on Screening Cards

Rapid Screening for Inborn Errors of Metabolism Using Liquid Ionization Mass Spectrometry with Dried Blood Spots on Screening Cards

Detection of non-prescription heroin markers in urine with liquid chromatography-atmospheric pressure chemical ionization mass spectrometry.

The planned introduction of a prescription heroin program in Germany created a need for differentiation between non-prescription and prescribed diamorphine use. The following substances were chosen as markers of non-prescription heroin: acetylcodeine (AC); its metabolites codeine (C) and codeine 6-glucuronide (C6G); papaverine (P); and noscapine (N). Typical heroin markers diamorphine (DAM) and its metabolites monoacetylmorphine (MAM) and morphine (M) were also determined. The drugs were extracted from urine samples with solid-phase extraction (C18) using standard 200-mg columns and 96-well microplates (100 mg). The extracts were examined with liquid chromatography-atmospheric pressure chemical ionization mass spectrometry (positive ionization) in two isocratic systems. Selected ion monitoring procedures were applied for protonated molecular masses and characteristic fragments of drugs involved. The limits of detection were in the range of 0.5-1 ng/mL urine. The occurrence of selected heroin markers was investigated in 25 urine samples collected from heroin abusers (road traffic offenders and overdosed patients). C6G was found in all samples, C in 24 samples, N in 22 samples, MAM in 16 samples, P in 14 samples, DAM in 12 samples, and AC in 4 samples. The appearance of these compounds in urine reflects their pharmacokinetic properties and the composition of non-prescription heroin.

A liquid chromatography-mass spectrometry method for the quantification of bioavailability and bioconversion of beta-carotene to retinol in humans.

A method based on high-performance liquid chromatography-atmospheric pressure chemical ionization mass spectrometry (APCI LC-MS) was developed for the quantification of the bioavailability of retinyl palmitate and beta-carotene and the bioconversion of beta-carotene to retinol in humans. Following oral administration of [8,9,10,11,12,13,14,15,19,20-13C10]-retinyl palmitate and [12,13,14,15,20,12',13',14',15',20'-13C10]-beta-carotene at physiological doses to children between 8 and 11 years of age, blood samples were drawn and serum was prepared. Retinol and beta-carotene were extracted from 0.2- and 1.0-mL serum samples, respectively, and analyzed using reversed-phase HPLC with a C30 column interfaced to an APCI mass spectrometer. Unlike other LC-MS assays for carotenoids, no additional purification steps were necessary, nor was any derivatization of retinol or beta-carotene required. APCI LC-MS showed a linear detector response for beta-carotene over 4 orders of magnitude. Using selected ion monitoring to record the elution profile of protonated circulating beta-carotene at m/z 537 and [13C10]-beta-carotene at m/z 547, the limit of detection was determined to be 0.5 pmol injected on-column. To assess the ratio of labeled to unlabeled retinol, selected ion monitoring was carried out at m/z 269, 274, and 279. These abundant fragment ions corresponded to the loss of water from the protonated molecule of circulating retinol, [13C5]-retinol (metabolically formed from orally administered [13C10]-beta-carotene), and [13C10]-retinol (formed by hydrolysis of [13C10]-retinyl palmitate). The ratios of labeled to unlabeled retinol and the ratio of labeled to unlabeled beta-carotene were calculated. Combined with standard HPLC measurement of beta-carotene and retinol concentration and a mathematical model, these results showed that this simple LC-MS method can be used to quantify beta-carotene bioavailability and its bioconversion to retinol at physiologically relevant doses.