Structural Identification and Kinetic Analysis of the in Vitro Products Formed by Reaction of Bisphenol A-3,4-quinone with N-Acetylcysteine and Glutathione

Small molecule deoxynyboquinone triggers alkylation and ubiquitination of Keap1 at Cys489 on Kelch domain for Nrf2 activation and inflammatory therapy

Activation of nuclear factor erythroid 2-related factor 2 (Nrf2) by Kelch-like ECH-associated protein 1 (Keap1) alkylation plays a central role in anti-inflammatory therapy. However, activators of Nrf2 through alkylation of Keap1-Kelch domain have not been identified. Deoxynyboquinone (DNQ) is a natural small molecule discovered from marine actinomycetes. The current study was designed to investigate the anti-inflammatory effects and molecular mechanisms of DNQ via alkylation of Keap1. DNQ exhibited significant anti-inflammatory properties both in vitro and in vivo. The pharmacophore responsible for the anti-inflammatory properties of DNQ was determined to be the α, β-unsaturated amides moieties by a chemical reaction between DNQ and N-acetylcysteine. DNQ exerted anti-inflammatory effects through activation of Nrf2/ARE pathway. Keap1 was demonstrated to be the direct target of DNQ and bound with DNQ through conjugate addition reaction involving alkylation. The specific alkylation site of DNQ on Keap1 for Nrf2 activation was elucidated with a synthesized probe in conjunction with liquid chromatography-tandem mass spectrometry. DNQ triggered the ubiquitination and subsequent degradation of Keap1 by alkylation of the cysteine residue 489 (Cys489) on Keap1-Kelch domain, ultimately enabling the activation of Nrf2. Our findings revealed that DNQ exhibited potent anti-inflammatory capacity through α, β-unsaturated amides moieties active group which specifically activated Nrf2 signal pathway via alkylation/ubiquitination of Keap1-Kelch domain, suggesting the potential values of targeting Cys489 on Keap1-Kelch domain by DNQ-like small molecules in inflammatory therapies.

The Key Role of GSH in Keeping the Redox Balance in Mammalian Cells: Mechanisms and Significance of GSH in Detoxification via Formation of Conjugates

Glutathione (GSH) is a ubiquitous tripeptide that is biosynthesized in situ at high concentrations (1-5 mM) and involved in the regulation of cellular homeostasis via multiple mechanisms. The main known action of GSH is its antioxidant capacity, which aids in maintaining the redox cycle of cells. To this end, GSH peroxidases contribute to the scavenging of various forms of ROS and RNS. A generally underestimated mechanism of action of GSH is its direct nucleophilic interaction with electrophilic compounds yielding thioether GSH S-conjugates. Many compounds, including xenobiotics (such as NAPQI, simvastatin, cisplatin, and barbital) and intrinsic compounds (such as menadione, leukotrienes, prostaglandins, and dopamine), form covalent adducts with GSH leading mainly to their detoxification. In the present article, we wish to present the key role and significance of GSH in cellular redox biology. This includes an update on the formation of GSH-S conjugates or GSH adducts with emphasis given to the mechanism of reaction, the dependence on GST (GSH S-transferase), where this conjugation occurs in tissues, and its significance. The uncovering of the GSH adducts' formation enhances our knowledge of the human metabolome. GSH-hematin adducts were recently shown to have been formed spontaneously in multiples isomers at hemolysates, leading to structural destabilization of the endogenous toxin, hematin (free heme), which is derived from the released hemoglobin. Moreover, hemin (the form of oxidized heme) has been found to act through the Kelch-like ECH associated protein 1 (Keap1)-nuclear factor erythroid 2-related factor-2 (Nrf2) signaling pathway as an epigenetic modulator of GSH metabolism. Last but not least, the implications of the genetic defects in GSH metabolism, recorded in hemolytic syndromes, cancer and other pathologies, are presented and discussed under the framework of conceptualizing that GSH S-conjugates could be regarded as signatures of the cellular metabolism in the diseased state.

Toxicity and mutagenicity studies of 6PPD-quinone in a marine invertebrate species and bacteria

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-quinone), an oxidation product of the tire additive, 6PPD, has been associated with high mortality of salmonids (0.1 μg/L). The objective of this study was to determine the acute toxicity using neonates and mutagenicity (micronuclei in hemolymph of exposed adults) of 6PPD-quinone in the marine amphipod Parhyale hawaiensis. Also, we studied its mutagenicity in the Salmonella/microsome assay using five strains of Salmonella with and without metabolic system (rat liver S9, 5%). 6PPD-quinone did not present acute toxicity to P. hawaiensis from 31.25 to 500 μg/L. Micronuclei frequency increased after 96 h-exposure to 6PPD-quinone (250 and 500 μg/L) when compared to the negative control. 6PPD-quinone also showed a weak mutagenic effect for TA100 only in the presence of S9. We conclude that 6PPD-quinone is mutagenic to P. hawaiensis and weakly mutagenic to bacteria. Our work provides information for future risk assessment of the presence of 6PPD-quinone in the aquatic environment.

From polyphenol to o-quinone: Occurrence, significance, and intervention strategies in foods and health implications

Polyphenol oxidation is a chemical process impairing food freshness and other desirable qualities, which has become a serious problem in fruit and vegetable processing industry. It is crucial to understand the mechanisms involved in these detrimental alterations. o-Quinones are primarily generated by polyphenols with di/tri-phenolic groups through enzymatic oxidation and/or auto-oxidation. They are highly reactive species, which not only readily suffer the attack by nucleophiles but also powerfully oxidize other molecules presenting lower redox potentials via electron transfer reactions. These reactions and subsequent complicated reactions are capable of initiating quality losses in foods, such as browning, aroma loss, and nutritional decline. To attenuate these adverse influences, a variety of technologies have emerged to restrain polyphenol oxidation via governing different factors, especially polyphenol oxidases and oxygen. Despite tremendous efforts devoted, to date, the loss of food quality caused by quinones has remained a great challenge in the food processing industry. Furthermore, o-quinones are responsible for the chemopreventive effects and/or toxicity of the parent catechols on human health, the mechanisms by which are quite complex. Herein, this review focuses on the generation and reactivity of o-quinones, attempting to clarify mechanisms involved in the quality deterioration of foods and health implications for humans. Potential innovative inhibitors and technologies are also presented to intervene in o-quinone formation and subsequent reactions. In future, the feasibility of these inhibitory strategies should be evaluated, and further exploration on biological targets of o-quinones is of great necessity.

A new toxicity mechanism of N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone: Formation of DNA adducts in mammalian cells and aqueous organisms

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPDQ), one of the oxidation products of rubber antioxidant 6PPD, has been identified as a novel toxicant to many organisms. However, an understanding of its underlying toxicity mechanisms remained elusive. In this study, we reported that 6PPDQ could react with deoxyguanosine to form one isomer of 3-hydroxy-1, N²-6PPD-etheno-2'-deoxyguanosine (6PPDQ-dG). Next, by employing an ultra-performance liquid chromatography-electrospray ionization-tandem mass spectrometry (UPLC-ESI-MS/MS) method, we found that 6PPDQ-dG could be detected in genomic DNA from 6PPDQ-treated mammalian cells and Chlamydomonas reinhardtii. We observed positive correlations between concentrations of exogenous 6PPDQ and the amounts of 6PPDQ-dG, and a recovery period after removal of 6PPDQ also led to decreased levels of the adduct in both organisms, which suggested potential repair pathways for this adduct in mammalian cells and unicellular algae. Additionally, we extracted the genomic DNA from tissues of frozen capelin and observed substantial amounts of the adduct in roe and gills, as well as livers at a relatively lower level. These results provided insights into the target organs and tissues that 6PPDQ might accumulate or harm fish. Overall, our study provides a new understanding of the mechanisms of toxicity of 6PPDQ in mammalian cells and aqueous organisms.

Temperature affects the kinetics but not the products of the reaction between 4-methylbenzoquinone and lysine

Quinones are electrophilic compounds that can undergo Michael addition or Schiff base reaction with nucleophilic amines, but the effect of temperature has not been systematically studied. The aim of this study was to characterize how temperature affects the reaction mechanism and kinetics of 4-methylbenzoquinone (4MBQ) with lysine (Lys), N^α-acetyl Lys or N^ε-acetyl Lys. The products were identified and characterized by LC-MS/MS, which revealed formation of Michael addition products, Schiff base, and a di-adduct in Lys and N^α-acetyl Lys-containing reaction mixtures. The product profiles were not affected by temperature in the range of 15-100 °C. NMR analysis proved that Michael addition of N^α-acetyl Lys occurred on the C5 position of 4MBQ. Rate constants for the reactions studied by stopped-flow UV-vis spectrophotometry under pseudo-first-order conditions where the amines were present in excess in the range 15 °C to 45 °C showed the α-amino groups of Lys are more reactive than the ε-groups. The kinetics results revealed that the temperature dependence of reaction rates followed the Arrhenius law, with activation energies in the order: Lys < N^ε-acetyl Lys < N^α-acetyl Lys. Our results provide detailed knowledge about the temperature dependence of the reaction between Lys residues and quinones under conditions relevant for storage of foods.

Landomycins as glutathione-depleting agents and natural fluorescent probes for cellular Michael adduct-dependent quinone metabolism

Landomycins are angucyclines with promising antineoplastic activity produced by Streptomyces bacteria. The aglycone landomycinone is the distinctive core, while the oligosaccharide chain differs within derivatives. Herein, we report that landomycins spontaneously form Michael adducts with biothiols, including reduced cysteine and glutathione, both cell-free or intracellularly involving the benz[a]anthraquinone moiety of landomycinone. While landomycins generally do not display emissive properties, the respective Michael adducts exerted intense blue fluorescence in a glycosidic chain-dependent manner. This allowed label-free tracking of the short-lived nature of the mono-SH-adduct followed by oxygen-dependent evolution with addition of another SH-group. Accordingly, hypoxia distinctly stabilized the fluorescent mono-adduct. While extracellular adduct formation completely blocked the cytotoxic activity of landomycins, intracellularly it led to massively decreased reduced glutathione levels. Accordingly, landomycin E strongly synergized with glutathione-depleting agents like menadione but exerted reduced activity under hypoxia. Summarizing, landomycins represent natural glutathione-depleting agents and fluorescence probes for intracellular anthraquinone-based angucycline metabolism.

Antioxidants other than vitamin C may be detected by glucose meters: Immediate relevance for patients with disorders targeted by antioxidant therapies

Owing to their ease of use, glucose meters are frequently used in research and medicine. However, little is known of whether other non-glucose molecules, besides vitamin C, interfere with glucometry. Therefore, we sought to determine whether other antioxidants might behave like vitamin C in causing falsely elevated blood glucose levels, potentially exposing patients to glycemic mismanagement by being administered harmful doses of glucose-lowering drugs. To determine whether various antioxidants can be detected by seven commercial glucose meters, human blood samples were spiked with various antioxidants ex vivo and their effect on the glucose results were assessed by Parkes error grid analysis. Several of the glucose meters demonstrated a positive bias in the glucose measurement of blood samples spiked with vitamin C, N-acetylcysteine, and glutathione. With the most interference-sensitive glucose meter, non-blood solutions of 1 mmol/L N-acetylcysteine, glutathione, cysteine, vitamin C, dihydrolipoate, and dithiothreitol mimicked the results seen on that glucose meter for 0.7, 1.0, 1.2, 2.6, 3.7 and 5.5 mmol/L glucose solutions, respectively. Glucose meter users should be alerted that some of these devices might produce spurious glucose results not only in patients on vitamin C therapy but also in those being administered other antioxidants. As discussed herein, the clinical relevance of the data is immediate in view of the current use of antioxidant therapies for disorders such as the metabolic syndrome, diabetes, cardiovascular diseases, and coronavirus disease 2019.

In vitro metabolic activation of triphenyl phosphate leading to the formation of glutathione conjugates by rat liver microsomes

The present study investigated the metabolism of the flame retardant and plasticizer chemical, triphenyl phosphate (TPHP), in a rat liver microsome-based in vitro assay with glutathione (GSH) in order to elucidate metabolic pathways leading to formation of conjugates. A highly sensitive and efficient method was developed for the detection and characterization of GSH reactive metabolites using LC-Q-TOF-MS/MS both in the negative and positive electrospray ionization modes. Seven GSH conjugates formed as a result of microsomal incubation, which were identified as S-conjugates based on MS/MS spectra, and confirmed by subsequent time-dependent incubation assays. With the exception of hydrolysis reactions leading to formation of a diester metabolite, diphenyl phosphate (DPHP), the results demonstrated that Phase I epoxidation on phenyl ring of TPHP leading to mono- and di-hydroxylated TPHP metabolites, which can further conjugate with GSH. Depending on hydroxylated TPHP formation, an o-hydroquinone intermediate formed in vitro via Phase I metabolism, and the o-benzoquinone form reacted with GSH and also formed GSH conjugates. The present study showed that via hydroxylated TPHP Phase I formation that GSH conjugates are important Phase II metabolites for TPHP metabolism in vitro. Some GSH conjugates may be valuable candidate biomarkers for monitoring TPHP exposure in biota.