Quantitation of N6-Formyl-lysine Adduct Following Aristolochic Acid Exposure in Cells and Rat Tissues by Liquid Chromatography-Tandem Mass Spectrometry Coupled with Stable Isotope-Dilution Method

LC-MS/MS Quantitation of Formaldehyde-Glutathione Conjugates as Biomarkers of Formaldehyde Exposure and Exposure-Induced Antioxidants: A New Look on an Old Topic

Humans are continuously exposed to formaldehyde via both endogenous and exogenous sources. Prolonged exposure to formaldehyde is associated with many human diseases, such as lung cancer and leukemia. The goal of this study is to develop biomarkers to measure formaldehyde exposure, which could be used to predict the risk of associated diseases. As glutathione (GSH) is well-known for its crucial role in the detoxification of a wide variety of xenobiotics, including formaldehyde, we rigorously quantitated in this study the conjugates formed when formaldehyde reacted with GSH using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) coupled with an isotope dilution method. The results showed for the first time that (S)-1-(((R)-2-amino-3-(carboxymethylamino)-3-oxopropylthio)methyl)-5-oxopyrrolidine-2-carboxylic acid (PGF) and thioproline-glycine (SPro-Gly) are major metabolites in both nonenzymatic reactions and formaldehyde-exposed human cells. In particular, over 35% of the formaldehyde from external sources was found to convert to SPro-Gly in the exposed cells. Interestingly, data showed that these exposure-induced adducts exhibited good antioxidative properties, which can protect cells from hydrogen peroxide mediated oxidative insult. It is anticipated that the findings of this study could shed light on developing PGF and SPro-Gly as dietary supplements and on the development of noninvasive methods to assess health risks associated with formaldehyde exposure.

Polyurethane-Based Face Mask as a Sampling Device for Environmental Tobacco Smoke

Environmental tobacco smoke (ETS), also known as secondhand smoking, contains human carcinogens associated with the development of many human diseases, including stroke, heart disease, leukemia, and lung cancer. Due to these adverse health effects, a sensitive and selective method is crucial for assessing the health impacts of ETS. While current methods to evaluate ETS exposure are either invasive or nonspecific and insensitive, in this study, we assessed the use of polyurethane foam face masks as a sampling medium to collect tobacco smoke-specific nicotine and nitrosamines for estimating personal exposure to ETS. This method was used in conjunction with tandem mass spectrometry coupled with isotope-dilution detection. After validation by comparison with the National Institute for Occupational Safety and Health standard method (NIOSH 2551) for nicotine, we quantitated ETS exposure in indoor and outdoor environments. The analysis shows the applicability of the method for monitoring nicotine down to ∼0.20 mg/m³ near an outdoor smoking hotspot and up to ∼5.2 mg/m³ in a room with burning cigarettes, all with a time resolution as short as 5 min. In comparison with the NIOSH method, the newly developed method is convenient, inexpensive, and does not require a personal sampling pump, thus can facilitate large-scale ETS exposure monitoring.

[Preparation of molecularly imprinted polymers-functionalized silica nanoparticles for the separation and recognition of aristolochic acids]

Aristolochic acids (AAs), which is commonly found in Aristolochia and Asarum plants, has been widely used in several traditional medicine practices due to their anti-inflammatory, anti-malarial, and anti-hyperglycemic activities. Recently, researchers have found a “decisive link” between liver cancer and aristolochic acid after analyzing a large number of liver cancer samples around the world. Therefore, a highly sensitive and selective method is required for the analysis of AAs in traditional Chinese medicines (TCM). For the determination of AAs in TCM, pretreatment is indispensable because in actual TCM samples, AAs is present in trace amounts and the complex matrix exerts interference. In the past decades, molecularly imprinted polymers (MIPs) have attracted considerable attention as an alternative for the trace analysis in complicated matrices. In this study, MIP-coated SiO2 nanoparticles (SiO2@MIP NPs) was prepared for the determination of aristolochic acid by surface molecular imprinting using aristolochic acid Ⅰ (AAI ) as the template molecule, 2-vinylpyridine (VPY) as the functional monomer, and ethyleneglycol dimethacrylate (EGDMA) as the cross-linking agent. Core-shell-structure SiO2@MIP NPs were obtained by modifying vinyl groups on the surface of SiO2 NPs, coating MIPs films onto the silica surface via selective polymerization, and final extraction of template AAI and generation of the recognition site. To find a suitable functional monomer for the best imprinting effect, the interaction between the template and the functional monomers, including acrylic acid (AA), methyl acrylic acid (MAA), 2-vinyl pyridine (VPY), acrylamide (AM), and methylacrylamide (MAM) was investigated. Electrostatic interaction between AAI and VPY resulted in the maximum decrease in absorbance of AAI at 250 nm. Therefore, VPY was chosen for the preparation of MIP. The morphological and physical properties of the MIPs were characterized by transmission electron microscopy (TEM), Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetric analysis, and N2 adsorption and desorption surface analysis. TEM images showed that SiO2 NPs were monodispersed with diameter of about 200 nm. The clear core-shell structure of SiO2@MIP NPs was observed, and the thickness of MIPs coating was about 35 nm. The FT-IR spectra of SiO2 NPs, vinyl group modified SiO2 and SiO2@MIP NPs revealed that the vinyl group and organic MIP layer were successfully modified at SiO2 sequentially. The results of thermogravimetric analysis were consistent with the FT-IR data for different SiO2 NPs. The nitrogen gas adsorption-desorption experiments showed that SiO2@MIP NPs and non-imprinted polymer (SiO2@NIP NPs) have the same pore volumes, while the surface area and pore size of MIPs were slightly larger than those of NIPs. Therefore, the difference in adsorption between SiO2@MIP NPs and SiO2@NIP NPs resulted from the imprinted sites on the MIP surface, rather than the difference in their surface areas. The adsorption properties of SiO2@MIP NPs were demonstrated by kinetic, isothermal, and selective adsorption experiments. The results of these experiments displayed that SiO2@MIP NPs reached adsorption equilibrium within a short period (120 s) and possessed a much higher rebinding ability than SiO2@NIP NPs. To verify the selectivity of SiO2@MIP NPs for AAI, three structural analogues (viz. tanshinone ⅡA, 2-methoxy-5-nitrophenol, and benzoic acid) were selected. The results showed that the binding capacity of SiO2@MIP NPs was much higher than those of these analogues. SiO2@MIP NPs have high adsorption capacity (5.74 mg/g), high imprinting factor (4.9), good selectivity coefficient (2.3-6.6) towards the structural analogues. SiO2@MIP NPs was used as an adsorbent and combined with HPLC for the selective separation of AAI in TCM. The recoveries of Kebia trifoliate samples spiked with three levels of AAI (0.3, 0.5, and 1.0 μg/mL) ranged from 73% to 83%. The results suggested that the proposed SiO2@MIP NPs could be used for selective enrichment of AAI from real complex TCM samples.

Probing the Hidden Role of Mitochondrial DNA Damage and Dysfunction in the Etiology of Aristolochic Acid Nephropathy

Aristolochic acid nephropathy (AAN) is a unique type of progressive renal interstitial fibrotic disease caused by prolonged exposure to aristolochic acids (AAs) through AA-containing herbal medicines or AA-tainted food. Despite decades of research and affecting millions of people around the world, the pathophysiology of AAN remains incompletely understood. In this study, we tested the potential causative role of mitochondrial dysfunction in AAN development. Our findings revealed AA exposure induces an exposure concentration and duration dependent lowering of adenosine triphosphate in both cultured human kidney and liver cells, highlighting an AA exposure effect on mitochondrial energy production in the kidney and liver, which both are highly metabolically active and energy-demanding organs. Analysis with liquid chromatography-tandem mass spectrometry coupled with stable isotope dilution method detected high levels of mutagenic 8-oxo-2'-deoxyguanosine and 7-(deoxyadenosine-N⁶-yl)-aristolactam adduct on mitochondrial DNA isolated from AA-treated cells, unmasking a potentially important causative, but previously unknown role of mitochondrial DNA mutation in the pathophysiology of AAN development.

Quantification of N6-formylated lysine in bacterial protein digests using liquid chromatography/tandem mass spectrometry despite spontaneous formation and matrix effects

RATIONALE

N⁶-Formyl lysine is a well-known modification of histones and other proteins. It can also be formed as a damaged product from direct formylation of free lysine and accompanied by other lysine derivatives such as acetylated or methylated forms. In relation to the activity of cellular repair enzymes in protein turnover and to lysine metabolism, it is important to accurately quantify the overall ratio of modified lysine to free lysine.

METHODS

N⁶-Formyl lysine was quantified using liquid chromatography/tandem mass spectrometry (LC/MS/MS) with data collected in a non-targeted manner using positive mode electrospray ionization on a Q-Exactive HF⁺ Orbitrap mass spectrometer. Studies were performed with lysine and deuterated lysine spiked into protein digests and solvents to investigate the extent of spontaneous formation and matrix effects of formation of N⁶-formyl lysine.

RESULTS

We show that N⁶-formyl lysine, N²-formyl lysine, N⁶-acetyl lysine, and N²-acetyl lysine are all formed spontaneously during sample preparation and LC/MS/MS analysis, which complicates quantification of these metabolites in biological samples. N⁶-Formyl lysine was spontaneously formed and correlated to the concentration of lysine. In the sample matrix of protein digests, 0.03% of lysine was spontaneously converted into N⁶-formyl lysine, and 0.005% of lysine was converted into N⁶-formyl lysine in pure run solvent.

CONCLUSIONS

Spontaneous formation of N⁶-formyl lysine, N⁶-acetyl lysine, N²-formyl lysine, and N²-acetyl lysine needs to be subtracted from biologically formed lysine modifications when quantifying these epimetabolites in biological samples.

Quantitation of Protein Adducts of Aristolochic Acid I by Liquid Chromatography-Tandem Mass Spectrometry: A Novel Method for Biomonitoring Aristolochic Acid Exposure

Emerging evidence suggests that chronic exposure to aristolochic acids (AAs) is one of the etiological pathways leading to chronic kidney disease (CKD). Due to the traditional practice of herbal medicine and AA-containing plants being used extensively as medicinal herbs, over 100 million East Asians are estimated to be at risk of AA poisoning. Given that the chronic nephrotoxicity of AAs only manifests itself after decades of exposure, early diagnosis of AA exposure could allow for timely intervention and disease risk reduction. However, an early detection method is not yet available, and diagnosis can only be established at the end stage of CKD. The goal of this study was to develop a highly sensitive and selective method to quantitate protein adducts of aristolochic acid I (AAI) as a biomarker of AA exposure. The method entails the release of protein-bound aristolactam I (ALI) by heat-assisted alkaline hydrolysis, extraction of ALI, addition of internal standard, and quantitation by liquid chromatography-tandem mass spectrometric analysis. Accuracy and precision of the method were critically evaluated using a synthetic ALI-containing glutathione adduct. The validated method was subsequently used to detect dose-dependent formation of ALI-protein adducts in human serum albumin exposed to AAI and in proteins isolated from the tissues and sera of AAI-exposed rats. Our time-dependent study showed that ALI-protein adducts remained detectable in rats even at 28 days postdosing. It is anticipated that the developed method will fill the technical gap in diagnosing AA intoxication and facilitate the biomonitoring of human exposures to AAs.

Molecularly imprinted ratiometric fluorescent probe for visual and fluorescent determination of aristolochic acid I based on a Schiff-base fluorescent compound

A molecularly imprinted ratiometric fluorescent probe (MIRF probe) was synthesized for the determination of aristolochic acid I (AAI) based on the Schiff-base fluorescent compound N,N'-bis(o-carboxybenzylidene)-p-4,4'-diaminobiphenyl (BDDB). The BDDB was immobilized in the silica nanoparticle (BDDB@SiO₂) as an internal standard material. The blue-emitting BDDB@SiO₂ and the yellow-emitting carbon quantum dots (y-CDs) were wrapped in the molecularly imprinted polymer (MIP) to provide a reliable reference signal at 440 nm and a fluorescent response signal at 530 nm at the excitation wavelength of 365 nm, respectively. In the preparation of the MIP of the MIRF probe, 4-vinylbenzoic acid as the functional monomer and AAI as the template molecule were used. An imprinting factor of 2.25 was obtained. Under the optimum conditions, the fluorescent response signal at 530 nm was quenched gradually by AAI in the range 1.0 to 120.0 μmol/L, while the reference signal at 440 nm remained unchanged. The limit of detection was 0.45 μmol/L, and the fluorescent color of the MIRF probe changed gradually from yellow to green to blue, which illustrated that the developed probe had a specific AAI recognition ability, a good anti-interference ability, and a sensitively visual determination ability. The probe was successfully applied to the AAI determination in traditional Chinese medicine (TCM) Asarum. The results showed that it had satisfactory recoveries (95.5-107.3%) and low relative standard deviations (2.0%). Furthermore, this method has a potential for the onsite naked eye determination of AAI in TCM samples.Graphical abstract.