Mass spectrometry analysis of in vitro nitration of a recombinant human IgG1 monoclonal antibody

Identification of nitrated immunoglobulin variable regions in the HIV-infected human brain: implications in HIV infection and immune response

HIV can infiltrate the brain and lead to HIV-associated neurocognitive disorders (HAND). The pathophysiology of HAND is poorly understood, and there are no diagnostic biomarkers for it. Previously, an increase in inducible nitric oxide synthase levels and protein tyrosine nitration in the brain were found to correlate with the severity of HAND.1,2 In this study, we analyzed human brains from individuals who had HIV infection without encephalitis and with encephalitis/HAND and compared them to the brains of healthy individuals. We identified the nitrated proteins and determined the sites of modification using affinity enrichment followed by high-resolution and high-mass-accuracy nanoLC–MS/MS. We found that nitrated proteins were predominantly present in the HIV-infected individuals with encephalitis, and, interestingly, the modifications were predominantly located on immunoglobulin variable regions. Our molecular model indicated potential interactions with HIV envelope proteins and changes on the heavy and light chain interface upon the nitration and nitrohydroxylation of these residues. Therefore, our findings suggest a role for these modifications in the immune response, which may have implications in disease pathogenesis.

Selective chemoprecipitation and subsequent release of tagged species for the analysis of nitropeptides by liquid chromatography-tandem mass spectrometry

Tyrosine nitration is a low-abundance post-translational protein modification that requires appropriate enrichment techniques to enable proteomic analyses. We report a simple yet highly specific method to enrich nitropeptides by chemoprecipitation involving only two straightforward chemical modifications of the nitropeptides before capturing the obtained derivatives with a strategically designed solid-phase active ester reagent. Specifically, capping of the aliphatic amines in the peptides is done first by reductive methylation to preserve the charge state of peptides for electrospray ionization mass spectrometric analysis, followed by reduction of nitrotyrosines to the corresponding aminotyrosines. These peptides are then immobilized on the solid-phase active ester reagent, whereas other peptides carrying no free amino groups are separated from the immobilized species by thoroughly washing the beads from which the tagged peptide derivatives can easily be released by acid-catalyzed hydrolysis at room temperature. The benefits of selective enrichment from a matrix of unmodified peptides for liquid chromatography-tandem mass spectrometry are demonstrated on three synthetic nitropeptides that are nitrated fragments of biologically relevant proteins. Identification of several in vitro nitrated human plasma proteins, also implicated under various pathological processes, by database searches from the enriched and tagged tryptic nitropeptides is presented as a practical application. We also show that converting the nitro-group to the small 4-formylbenzoylamido tag does not significantly alter fragmentation properties upon collision-induced dissociation compared with those of the native nitropeptides, and at the same time this derivatization actually improves electron capture dissociation due to conversion of the electron-predator nitro-group to this novel tag.

Stability of disubstituted copper complexes in the gas phase analyzed by electrospray ionization mass spectrometry

A series of nitrogen ligand (L)/copper complexes of the type [Cu(I)L](+), [Cu(II)L(X)](+) and [Cu(I)L(2)](+) (X = Cl(-), BF(4) (-), acac(-), CH(3)COO(-) and SO(3)CF(3) (-)) was studied in the gas phase by electrospray ionization mass spectrometry. The following ligands (L) were employed: 1,12-diazaperylene (dap), 1,1'-bisisoquinoline (bis), 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), 2,11-disubstituted 1,12-diazaperylenes (dap), 3,3'-disubstituted 1,1'- bisisoquinoline (bis), 5,8-dimethoxy-substituted diazaperylene (meodap), 6,6'- dimethoxy-substituted bisisoquinoline (meobis) and 2,9-dimethyl-1,10-phenanthroline (dmphen). Collision-induced decomposition measurements were applied to evaluate the relative stabilities of the different copper complexes. The influence of the spatial arrangement of the ligands, of the type of substituents and of the counter ion of the copper salts employed for the complexation was examined. Correlations were found between the binding constants of the [ML(2)](+) complexes in solution and the relative stabilities of the analogous complexes in the gas phase. Furthermore, complexation with the ligands 2,11-dialkylated 1,12-diazaperylenes [alkyl = ethyl (dedap) and isopropyl (dipdap)] was studied in the solvents CH(3)OH and CH(3)CN.

Application of FAIMS to anabolic androgenic steroids in sport drug testing

Mass spectrometric identification of anabolic androgenic steroids challenges standard doping-control methods. To reveal a doping offence the presence of prohibited anabolic androgenic steroids at trace levels in the picogram-per-millilitre range must be confirmed as reliable. Human urine samples containing epitrenbolone, metandienone metabolite (17beta -hydroxymethyl-17alpha-methyl-18-norandrost-1,4,13-trien-3-one), stanozolol, 16beta-hydroxystanozolol and 4beta-hydroxystanozolol were analysed using LC-FAIMS-MS/MS. These substances are prohibited in sport according to World Anti-Doping Agency (WADA) regulations. Glucuronides were hydrolysed and prepared by liquid-liquid extraction. Excellent recovery and precision were obtained for all compounds. Linear calibration results for epitrenbolone and metandienone metabolite were obtained and concentration information could be determined in the ranges of reliable response between 750-1200 and 100-600 pg/mL, respectively. Limits of detection were estimated at 25 pg/mL (stanozolol), 50 pg/mL (metandienone metabolite, 16beta-hydroxystanozolol), 100 pg/mL (4beta-hydroxystanozolol) and 500 pg/mL (epitrenbolone). The assay was applied to doping-control samples. For all analytes, LC-FAIMS-MS/MS resulted in excellent interference removal, which effectively extends the post-dose detection time.

Analysis of reduced monoclonal antibodies using size exclusion chromatography coupled with mass spectrometry

Size-exclusion chromatography (SEC) has been widely used to detect antibody aggregates, monomer, and fragments. SEC coupled to mass spectrometry has been reported to measure the molecular weights of antibody; antibody conjugates, and antibody light chain and heavy chain. In this study, separation of antibody light chain and heavy chain by SEC and direct coupling to a mass spectrometer was further studied. It was determined that employing mobile phases containing acetonitrile, trifluoroacetic acid, and formic acid allowed the separation of antibody light chain and heavy chain after reduction by SEC. In addition, this mobile phase allowed the coupling of SEC to a mass spectrometer to obtain a direct molecular weight measurement. The application of the SEC-MS method was demonstrated by the separation of the light chain and the heavy chain of multiple recombinant monoclonal antibodies. In addition, separation of a thioether linked light chain and heavy chain from the free light chain and the free heavy chain of a recombinant monoclonal antibody after reduction was also achieved. This optimized method provided a separation of antibody light chain and heavy chain based on size and allowed a direct measurement of molecular weights by mass spectrometry. In addition, this method may help to identify peaks eluting from SEC column directly.

A novel Fourier transform ion cyclotron resonance mass spectrometer with improved ion trapping and detection capabilities

A novel Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer has been developed for improved biomolecule analysis. A flared metal capillary and an electrodynamic ion funnel were installed in the source region of the instrument for improved ion transmission. The transfer quadrupole is divided into 19 segments, with the capacity for independent control of DC voltage biases for each segment. Restrained ion population transfer (RIPT) is used to transfer ions from the ion accumulation region to the ICR cell. The RIPT ion guide reduces mass discrimination that occurs as a result of time-of-flight effects associated with gated trapping. Increasing the number of applied DC bias voltages from 8 to 18 increases the number of ions that are effectively trapped in the ICR cell. The RIPT ion guide with a novel voltage profile applied during ion transfer provides a 3- to 4-fold increase in the number of ions that are trapped in the ICR cell compared with gated trapping for the same ion accumulation time period. A novel ICR cell was incorporated in the instrument to reduce radial electric field variation for ions with different z-axis oscillation amplitudes. With the ICR cell, called trapping ring electrode cell (TREC), we can tailor the shape of the trapping electric fields to reduce dephasing of coherent cyclotron motion of an excited ion packet. With TREC, nearly an order of magnitude increase in sensitivity is observed. The performance of the instrument with the combination of RIPT, TREC, flared inlet, and ion funnel is presented.

Separation and characterization of clindamycin phosphate and related impurities in injection by liquid chromatography/electrospray ionization mass spectrometry

A simple high-performance liquid chromatography/electrospray ionization tandem mass spectrometric (HPLC/ESI-MS/MS) method has been developed for the rapid identification of clindamycin phosphate and its degradation products or related impurities in clindamycin phosphate injection. Detection was performed by quadrupole time-of-flight mass spectrometry (Q-TOFMS) via an ESI source in positive mode. Clindamycin phosphate and its related substances lincomycin, 7-epilincomycin-2-phosphate, lincomycin-2-phosphate, clindamycin B, clindamycin B-2-phosphate, and clindamycin were identified simultaneously by HPLC/ESI-MS/MS results. Based on the MS/MS spectra of their quasi-molecular ions, the fragmentation pathways of clindamycin phosphate and its related substances were compared and proposed, which are specific and useful for the identification of the lincosamide antibiotics and related impurities. The method was rapid, sensitive and specific and can be used to identify clindamycin phosphate and its related impurities in clindamycin phosphate injection without control compounds.

Rapid-resolution liquid chromatography/mass spectrometry for determination and quantitation of polyphenols in grape berries

A rapid-resolution liquid chromatography/mass spectrometric (RRLC/MS) method for detection and quantitation of polyphenols in grape berry skins and seeds has been developed. Pulp-free berry skins were treated with liquid nitrogen and ground; seeds were also ground. Then, 3 g of samples were extracted with 30 mL of a mixture of methanol/water/formic acid 70:30:1 (v/v/v) under sonication and 1 microL of the final extract was injected into two 100 x 2.1 mm i.d., 1.8 microm Zorbax Eclipse plus C18 columns connected in series. Compounds were fractionated using a gradient elution of acidified acetonitrile/methanol 50:50 (v/v)/water. Columns were thermostatted at 70 degrees C. MS was carried out on an Agilent 6410 QqQ instrument equipped with an electrospray ionization source. Positive and negative MS/MS product ion scans were used for compound identification, whereas positive full scan MS in the m/z range 200-1400 was used for quantitation. By means of mass spectra comparison, various flavonols, flavan-3-ols, anthocyanins and stilbenes were identified. Quantitation was performed by external calibration, and concentration values were corrected for matrix effect that was evaluated in separate experiments. Semi-quantitative estimation was performed for compounds for which standards were not commercially available. Recoveries ranged from 90-102% with relative standard deviation (RSD) <5%, whereas the between samples RSD was in the range 4-12%. Two surrogate standards were used for quality control. The developed method was applied to analyze the polyphenol content of three Vitis vinifera table cultivars at physiological maturity and after proper preservation for 6 weeks. Results demonstrated that during preservation about half of the polyphenol content was lost.