Diacetyl and related flavorant α-Diketones: Biotransformation, cellular interactions, and respiratory-tract toxicity

Rational design of a zinc-dependence secondary alcohol dehydrogenase to boost its oxidation activity for α-hydroxyketones biosynthesis

α-Hydroxyketones, which have significant industrial applications, can be sustainably synthesized through the oxidation of secondary alcohols using secondary alcohol dehydrogenase (SADH). However, the activity of the SADH oxidation reaction is generally low, making it unsuitable for large-scale production. In this study, a rational design approach was employed to computationally engineer SADH derived from Ogataea parapolymorpha (OpSADH), significantly enhancing its oxidation activity towards (R)-1,2-propanediol (PDO). The mutant M2 (S222T/S316A) exhibited a 14.2-fold increase in specific enzyme activity compared to the wild type (WT) and was employed as a catalyst for high-concentration hydroxyacetone production, facilitated by an NAD+ regeneration system. By applying an appropriate Zn2+ ion force field in molecular dynamics (MD) simulations, it was found that two mutation sites could stabilize the conformations of NAD+ and PDO, thereby revealing the molecular mechanism behind the enhanced activity of this metalloenzyme mutant. Notably, we first uncovered the dynamic mechanism by which four key residues (Cys39, His75, Glu76, and Glu175) and PDO within the active pocket contribute to the formation of coordination bonds with Zn2+. The findings of this study provide robust support for researching the catalytic mechanisms and dynamics processes of metalloenzymes.

Synthesis of Indolyl Phenyl Diketones through Visible-Light-Promoted Ni-Catalyzed Intramolecular Cyclization/Oxidation Sequence of Ynones

The combination of visible light catalysis and Ni catalysis has enabled the synthesis of indolyl phenyl diketones through the cyclization/oxidation process of ynones. This reaction proceeded under mild and base-free conditions and showed a broad scope and feasibility for gram-scale synthesis. Several natural products and biologically interesting molecules could be readily postfunctionalized by this method.

Diacetyl inhalation impairs airway epithelial repair in mice infected with influenza A virus

Bronchiolitis obliterans (BO) is a debilitating disease of the small airways that can develop following exposure to toxic chemicals as well as respiratory tract infections. BO development is strongly associated with diacetyl (DA) inhalation exposures at occupationally relevant concentrations or severe influenza A viral (IAV) infections. However, it remains unclear whether lower dose exposures or more mild IAV infections can result in similar pathology. In the current work, we combined these two common environmental exposures, DA and IAV, to test whether shorter DA exposures followed by sublethal IAV infection would result in similar airways disease. Adult mice exposed to DA vapors 1 h/day for 5 consecutive days followed by infection with the airway-tropic IAV H3N2 (HKx31) resulted in increased mortality, increased bronchoalveolar lavage (BAL) neutrophil percentage, mixed obstruction and restriction by lung function, and subsequent airway remodeling. Exposure to DA or IAV alone failed to result in significant pathology, whereas mice exposed to DA + IAV showed increased α-smooth muscle actin (αSMA) and epithelial cells coexpressing the basal cell marker keratin 5 (KRT5) with the club cell marker SCGB1A1. To test whether DA exposure impairs epithelial repair after IAV infection, mice were infected first with IAV and then exposed to DA during airway epithelial repair. Mice exposed to IAV + DA developed similar airway remodeling with increased subepithelial αSMA and epithelial cells coexpressing KRT5 and SCGB1A1. Our findings reveal an underappreciated concept that common environmental insults while seemingly harmless by themselves can have catastrophic implications on lung function and long-term respiratory health when combined.

Review of evidence relating to occupational exposure limits for alpha-diketones and acetoin, and considerations for deriving an occupational exposure limit for 2,3-pentanedione

Alpha-diketones, notably diacetyl, have been used as flavoring agents. When airborne in occupational settings, exposures to diacetyl have been associated with serious respiratory disease. Other α-diketones, such as 2,3-pentanedione, and analogues such as acetoin (a reduced form of diacetyl), require evaluation, particularly, in light of recently available toxicological studies. The current work reviewed mechanistic, metabolic, and toxicology data available for α-diketones. Data were most available for diacetyl and 2,3-pentanedione, and a comparative assessment of their pulmonary effects was performed, and an occupational exposure limit (OEL) was proposed for 2,3-pentanedione. Previous OELs were reviewed and an updated literature search was performed. Respiratory system histopathology data from 3-month toxicology studies were evaluated with benchmark dose (BMD) modelling of sensitive endpoints. This demonstrated comparable responses at concentrations up to 100 ppm, with no consistent overall pattern of greater sensitivity to either diacetyl or 2,3-pentanedione. In contrast, based on draft raw data, no adverse respiratory effects were observed in comparable 3-month toxicology studies that evaluated exposure to acetoin at up to 800 ppm (highest tested concentration), indicating that acetoin does not present the same inhalation hazard as diacetyl or 2,3-pentanedione. To derive an OEL for 2,3-pentanedione, BMD modelling was conducted for the most sensitive endpoint from 90-day inhalation toxicity studies, namely, hyperplasia of nasal respiratory epithelium. On the basis of this modelling, an 8-hour time-weighted average OEL of 0.07 ppm is proposed to be protective against respiratory effects that may be associated with chronic workplace exposure to 2,3-pentanedione.

Xenobiotics Delivered by Electronic Nicotine Delivery Systems: Potential Cellular and Molecular Mechanisms on the Pathogenesis of Chronic Kidney Disease

Chronic kidney disease (CKD) is characterized as sustained damage to the renal parenchyma, leading to impaired renal functions and gradually progressing to end-stage renal disease (ESRD). Diabetes mellitus (DM) and arterial hypertension (AH) are underlying diseases of CKD. Genetic background, lifestyle, and xenobiotic exposures can favor CKD onset and trigger its underlying diseases. Cigarette smoking (CS) is a known modified risk factor for CKD. Compounds from tobacco combustion act through multi-mediated mechanisms that impair renal function. Electronic nicotine delivery systems (ENDS) consumption, such as e-cigarettes and heated tobacco devices, is growing worldwide. ENDS release mainly nicotine, humectants, and flavorings, which generate several byproducts when heated, including volatile organic compounds and ultrafine particles. The toxicity assessment of these products is emerging in human and experimental studies, but data are yet incipient to achieve truthful conclusions about their safety. To build up the knowledge about the effect of currently employed ENDS on the pathogenesis of CKD, cellular and molecular mechanisms of ENDS xenobiotic on DM, AH, and kidney functions were reviewed. Unraveling the toxic mechanisms of action and endpoints of ENDS exposures will contribute to the risk assessment and implementation of proper health and regulatory interventions.

Methylglyoxal and high glucose inhibit VEGFR2 phosphorylation at specific tyrosine residues

Diabetes is characterized by hyperglycemia and a significant risk of vascular complications. Vascular endothelial growth factor (VEGF) and its main receptor VEGFR2 (KDR), which is highly expressed in vascular endothelial cells, are essential mediators of vascular maintenance and angiogenesis. During glycolysis after high calorie food intake, methylglyoxal (MGO) is formed and MGO blood levels are elevated in diabetes. MGO reacts with arginine residues to generate MG-H1 or with lysine residues to carboxyethyl lysine which are common components of advanced glycation end-products. Therefore, the question arises whether hyperglycemic conditions affect VEGF signaling via a ligand-independent direct modification of signaling components. As a first step, the effect of MGO on VEGFR2 activation was investigated in cultured endothelial cells from human umbilical vein by determination of VEGFR2 phosphorylation at selected tyrosine residues by ELISA and immunoblotting using phospho-specific antibodies. Phosphorylation of VEGFR2-Y996, VEGFR2-Y1054, or VEGFR2-Y1175 reached a maximum 5 min after stimulation of endothelial cells with VEGF. Phosphorylation was significantly inhibited by 100 µM MGO and to a lesser extent by high glucose treatment. 2,3-Pentanedione and glyoxal were investigated for comparison. In summary, VEGFR2 phosphorylation is sensitive to MGO or high glucose concentrations which may be relevant in the pathophysiology of microvascular disease in diabetes.

Post-translational modifications to hemidesmosomes in human airway epithelial cells following diacetyl exposure

Diacetyl (DA; 2,3-butanedione) is a highly reactive alpha (α)-diketone. Inhalation exposure to DA can cause significant airway epithelial cell injury, however, the mechanisms of toxicity remain poorly understood. The purpose of these experiments was to assess for changes in abundance and distribution of hemidesmosome-associated proteins following DA exposure that contribute to DA-induced epithelial toxicity. Human bronchial epithelial cells were grown in submerged cultures and exposed to three occupationally-relevant concentrations of DA (5.7, 8.6, or 11.4 mM) for 1 h. Following DA exposure, epithelial cells were cultured for 4 days to monitor for cell viability by MTT and WST-1 assays as well as for changes in cellular distribution and relative abundance of multiple hemidesmosome-associated proteins, including keratin 5 (KRT5), plectin (PLEC), integrin alpha 6 (ITGα6) and integrin beta 4 (ITGβ4). Significant toxicity developed in airway epithelial cells exposed to DA at concentrations ≥ 8.6 mM. DA exposure resulted in post-translational modifications to hemidesmosome-associated proteins with KRT5 crosslinking and ITGβ4 cleavage. Following DA exposure at 5.7 mM, these post-translational modifications to KRT5 resolved with time. Conversely, at DA concentrations ≥ 8.6 mM, modifications to KRT5 persisted in culture with decreased total abundance and perinuclear aggregation of hemidesmosome-associated proteins. Significant post-translational modifications to hemidesmosome-associated proteins develop in airway epithelial cells exposed to DA. At DA concentrations ≥ 8.6 mM, these hemidesmosome modifications persist in culture. Future work targeting hemidesmosome-associated protein modifications may prevent the development of lung disease following DA exposure.

A Computational Approach for Respiratory Hazard Identification of Flavor Chemicals in Tobacco Products

Flavor chemicals contribute to the appeal and toxicity of tobacco products, including electronic nicotine delivery systems (ENDS). The assortment of flavor chemicals available for use in tobacco products is extensive. In this study, a chemistry-driven computational approach was used to evaluate flavor chemicals based on intrinsic hazardous structures and reactivity of chemicals. A large library of 3012 unique flavor chemicals was compiled from publicly available information. Next, information was computed and collated based on their (1) physicochemical properties, (2) global harmonization system (GHS) health hazard classification, (3) structural alerts linked to the chemical's reactivity, instability, or toxicity, and (4) common substructure shared with FDA's harmful and potentially harmful constituents (HPHCs) flavor chemicals that are respiratory toxicants. Computational analysis of the constructed flavor library flagged 638 chemicals with GHS classified respiratory health hazards, 1079 chemicals with at least one structural alert, and 2297 chemicals with substructural similarity to FDA's established and proposed list of HPHCs. A subsequent analysis was performed on a subset of 173 chemicals in the flavor library that are respiratory health hazards, contain structural alerts as well as flavor HPHC substructures. Four general toxicophore structures with an increased potential for respiratory toxicity were then identified. In summary, computational methods are efficient tools for hazard identification and understanding structure-toxicity relationship. With appropriate context of use and interpretation, in silico methods may provide scientific evidence to support toxicological evaluations of chemicals in or emitted from tobacco products.

Diacetyl exposure disrupts iron homeostasis in animals and cells

OBJECTIVE

Several mechanisms have been proposed for the biological effect of diacetyl. We tested the postulate that animal and cell exposures to diacetyl are associated with a disruption in iron homeostasis.

MATERIALS AND METHODS

Male, Sprague-Dawley rats were intratracheally-instilled with either distilled water or diacetyl. Seven days after treatment, animals were euthanized and the lungs removed, fixed, and embedded. Sections were cut and stained for iron, collagen, and ferritin. Human epithelial (BEAS-2B) and monocytic (THP-1) cells were exposed in vitro to ferric ammonium citrate (FAC), diacetyl, and both FAC and diacetyl. Cell non-heme iron concentrations and ferritin levels were quantified using inductively coupled plasma optical emission spectroscopy and an immunoassay respectively.

RESULTS

After exposure of animals to diacetyl, there were airway polypoid lesions which stained positively for both iron and the intracellular storage protein ferritin. Trichrome stain showed a deposition of collagen immediately adjacent to accumulated metal following diacetyl exposure. In in vitro cell exposures, FAC increased non-heme iron concentration but co-incubations of FAC and diacetyl elevated levels to significantly greater values. Levels of ferritin were increased with exposures of BEAS-2B and THP-1 cells to FAC but were similarly greater after co-exposure with FAC and diacetyl.

CONCLUSIONS

Results of animal and cell studies support a disruption of iron homeostasis by diacetyl. It is proposed that, following internalization, diacetyl complexes intracellular sources of iron. The cell recognizes a loss of its requisite iron to diacetyl and imports greater concentrations of the metal.

Moonlighting in drug metabolism

Drug metabolizing enzymes catalyze the biotransformation of many of drugs and chemicals. The drug metabolizing enzymes are distributed among several evolutionary families and catalyze a range of detoxication reactions, including oxidation/reduction, conjugative, and hydrolytic reactions that serve to detoxify potentially toxic compounds. This detoxication function requires that drug metabolizing enzymes exhibit substrate promiscuity. In addition to their catalytic functions, many drug metabolizing enzymes possess functions unrelated to or in addition to catalysis. Such proteins are termed 'moonlighting proteins' and are defined as proteins with multiple biochemical or biophysical functions that reside in a single protein. This review discusses the diverse moonlighting functions of drug metabolizing enzymes and the roles they play in physiological functions relating to reproduction, vision, cell signaling, cancer, and transport. Further research will likely reveal new examples of moonlighting functions of drug metabolizing enzymes.

Copper-Catalyzed Ring-Opening Defluorinative Alkylation of Siloxydifluorocyclopropanes: Synthesis of γ-Fluoro-δ-Ketoesters and γ,δ-Diketonitriles

In view of the importance of both fluorinated synthons and homoenolate equivalents, synthetic application of difluorocyclopropanols is desired but remains challenging due to their thermodynamic instability. Herein, we use siloxydifluorocyclopropanes as difluorocyclopropanol precursors to carry out new Cu-catalyzed ring-opening defluorinative alkylation. With α-bromo carboxylic esters as coupling partners, the reaction affords γ-fluoro-δ-ketoesters via a Cu^I/Cu^II catalytic cycle. Interestingly, by the use of α-bromoamides, the ring-opening defluorinative alkylation is followed by an additional intramolecular C-N oxidative coupling to deliver a lactam intermediate, which further undergoes defluorination, hydrolysis, ring opening, and dehydration cascade via a Cu^I/Cu^II/Cu^III catalytic pathway, leading to γ,δ-diketonitriles as the final products.

Functional analysis of BPSS2242 reveals its detoxification role in Burkholderia pseudomallei under salt stress

A bpss2242 gene, encoding a putative short-chain dehydrogenase/oxidoreductase (SDR) in Burkholderia pseudomallei, was identified and its expression was up-regulated by ten-fold when B. pseudomallei was cultured under high salt concentration. Previous study suggested that BPSS2242 plays important roles in adaptation to salt stress and pathogenesis; however, its biological functions are still unknown. Herein, we report the biochemical properties and functional characterization of BPSS2242 from B. pseudomallei. BPSS2242 exhibited NADPH-dependent reductase activity toward diacetyl and methylglyoxal, toxic electrophilic dicarbonyls. The conserved catalytic triad was identified and found to play critical roles in catalysis and cofactor binding. Tyr162 and Lys166 are involved in NADPH binding and mutation of Lys166 causes a conformational change, altering protein structure. Overexpression of BPSS2242 in Escherichia coli increased bacterial survival upon exposure to diacetyl and methylglyoxal. Importantly, the viability of B. pseudomallei encountered dicarbonyl toxicity was enhanced when cultured under high salt concentration as a result of BPSS2242 overexpression. This is the first study demonstrating that BPSS2242 is responsible for detoxification of toxic metabolites, constituting a protective system against reactive carbonyl compounds in B. pseudomallei..

Airway basal cell injury after acute diacetyl (2,3-butanedione) vapor exposure

RATIONALE

Diacetyl (DA; 2,3-butanedione) is a chemical found commonly in foods and e-cigarettes. When inhaled, DA causes epithelial injury, though the mechanism of repair remain poorly understood. The objective of this study was to evaluate airway basal cell repair after DA vapor exposure.

METHODS

Primary human bronchial epithelial cells were exposed to DA or PBS for 1 h. Lactate dehydrogenase, cleaved caspase 3/7 and trans-epithelial electrical resistance were measured prior to and following exposure. Exposed cultures were analyzed for the airway basal cell markers keratin 5 and p63 as well as ubiquitin and proteasome activity. Cultures were also treated with a proteasome inhibitor (MG132).

RESULTS

DA vapor exposure caused a transient decrease in trans-epithelial electrical resistance in all DA-exposed cultures. Supernatant lactate dehydrogenase and cleaved caspase 3/7 increased significantly at the highest DA concentration but not at lower DA concentrations. Increased keratin 5 ubiquitination occurred after DA exposure but resolved by day 3. Damage to airway basal cells persisted at day 3 in the presence of MG132.

CONCLUSIONS

Diacetyl exposure results in airway basal cell injury with keratin 5 ubiquitination and decreased p63 expression. The ubiquitin-proteasome-pathway partially mediates airway basal cell repair after acute DA exposure.

Flavorings-Related Lung Disease: A Brief Review and New Mechanistic Data

Flavorings-related lung disease is a potentially disabling and sometimes fatal lung disease of workers making or using flavorings. First identified almost 20 years ago in microwave popcorn workers exposed to butter-flavoring vapors, flavorings-related lung disease remains a concern today. In some cases, workers develop bronchiolitis obliterans, a severe form of fixed airways disease. Affected workers have been reported in microwave popcorn, flavorings, and coffee production workplaces. Volatile α-dicarbonyl compounds, particularly diacetyl (2,3-butanedione) and 2,3-pentanedione, are implicated in the etiology. Published studies on diacetyl and 2,3-pentanedione document their ability to cause airway epithelial necrosis, damage biological molecules, and perturb protein homeostasis. With chronic exposure in rats, they produce airway fibrosis resembling bronchiolitis obliterans. To add to this knowledge, we recently evaluated airway toxicity of the 3-carbon α-dicarbonyl compound, methylglyoxal. Methylglyoxal inhalation causes epithelial necrosis at even lower concentrations than diacetyl. In addition, we investigated airway toxicity of mixtures of diacetyl, acetoin, and acetic acid, common volatiles in butter flavoring. At ratios comparable to workplace scenarios, the mixtures or diacetyl alone, but not acetic acid or acetoin, cause airway epithelial necrosis. These new findings add to existing data to implicate α-dicarbonyl compounds in airway injury and flavorings-related lung disease.

Mechanisms of soft and hard electrophile toxicities

Electron-deficient chemicals (electrophiles) react with compounds that have one or more unshared valence electron pairs (nucleophiles). The resulting covalent reactions between electrophiles and nucleophiles (e.g., Michael addition, SN2 reactions) are important, not only to Organic Chemistry, but also to the fields of Molecular Biology and Toxicology. Specifically, covalent bond formation is the operational basis of many critically important cellular processes; e.g., enzyme function, neurotransmitter release, and membrane-vesicle fusion. Given this context it is understandable that these reactions are also relevant to Toxicology, since a significant number of xenobiotic chemicals are toxic electrophiles that can react with endogenous nucleophilic residues. Therefore, the purpose of this Review is to discuss electrophile-nucleophile chemistry as it pertains to cell injury and resulting organ toxicity. Our discussion will involve an introduction to the Hard and Soft, Acids and Bases (HSAB) theory of Pearson. The HSAB concept provides a framework for calculation of quantum chemical parameters that classify the electrophile and nucleophile covalent components according to their respective electronic nature (softness/hardness) and reactivity (electrophilicity/nucleophilicity). The calculated quantum indices in conjunction with corroborative in vivo, in chemico (cell free) and in vitro research can offer an illuminating approach to mechanistic discovery. Accordingly, we will provide examples that demonstrate how this approach has been used to discern mechanisms and sites of electrophile action.

Small molecule diketone flavorants diacetyl and 2,3-pentanedione promote neurotoxicity but inhibit amyloid β aggregation

We investigated the effects of the small molecule flavorants diacetyl, 2,3-pentanedione and acetoin on neuronal cell viability and β amyloid aggregation and morphology. Two neuroblastoma cell lines, SH-SY5Y and Neuro 2a (N2a) were exposed to diacetyl, 2,3-pentanedione and acetoin, while Thioflavin T fluorescence kinetics and transmission electron microscopy were used to assess effects on Aβ_1-42 fibril and aggregate formation and morphology respectively. Diacetyl was intrinsically toxic to both SH-SY5Y and N2a cells, with time and concentration-dependent reductions in cell viability occurring over 24 h and 48 h incubation periods. 2.3-Pentanedione evoked a similar concentration-dependent loss of cell viability in N2a cells at 48 h, but exhibited lessened toxicity in SH-SY5Y cells over 24 h, and minimal loss of cell viability by 48 h. Diacetyl inhibited Aβ_1-42 aggregation kinetics, reduced aggregate and fibril density and rendered Aβ_1-42 into amorphous small aggregates. 2,3-Pentanedione also reduced overall aggregate formation, but to a lesser extent than diacetyl and retaining the presence of a meshwork of Aβ_1-42 aggregates and fibrils. Acetoin was innocuous to neuronal cells and did not alter Aβ_1-42 fibril density or morphology. These findings highlight the intrinsic neurotoxicity of small molecule diketone flavorants. While providing further insight into their molecular interactions with amyloidogenic proteins, the neurotoxicity of such flavorants is a significant finding and warrants further investigation.

Carbonyl reductases from Daphnia are regulated by redox cycling compounds

Oxidative stress is a major source of reactive carbonyl compounds that can damage cellular macromolecules, leading to so-called carbonyl stress. Aside from endogenously formed carbonyls, including highly reactive short-chain aldehydes and diketones, air pollutants derived from diesel exhaust like 9,10-phenanthrenequinone (PQ) can amplify oxidative stress by redox cycling, causing tissue damage. Carbonyl reductases (CRs), which are inducible in response to ROS, represent a fundamental enzymatic defense mechanism against oxidative stress. While commonly two carbonyl reductases (CBR1 and CBR3) are found in mammalian genomes, invertebrate model organisms like Drosophila melanogaster express no CR but a functional homolog to human CBR1, termed sniffer. The microcrustacean Daphnia is an ideal model organism to investigate the function of CRs because of its unique equipment with even four copies of the CR gene (CR1, CR2, CR3, CR4) in addition to one sniffer gene. Cloning and catalytic characterization of two carbonyl reductases CR1 and CR3 from D. magna and D. pulex arenata revealed that both proteins reductively metabolize aromatic dicarbonyls (e.g., menadione, PQ) and aliphatic α-diketones (e.g., 2,3-hexanedione), while sugar-derived aldehydes (methylglyoxal, glyoxal) and lipid peroxidation products such as acrolein and butanal were poor substrates, indicating no physiological function in the metabolism of short-chain aldehydes. Treatment of D. magna with redox cyclers like menadione and the pesticide paraquat led to an upregulation of CR1 and CR3 mRNA, suggesting a role in oxidative stress defense. Further studies are needed to investigate their potential to serve as novel biomarkers for oxidative stress in Daphnia.

Mechanisms of toxicity and biomarkers of flavoring and flavor enhancing chemicals in emerging tobacco and non-tobacco products

Tobacco products containing flavorings, such as electronic nicotine delivery devices (ENDS) or e-cigarettes, cigars/cigarillos, waterpipes, and heat-not-burn devices (iQOS) are continuously evolving. In addition to increasing the exposure of teenagers and adults to nicotine containing flavoring products and flavoring enhancers, chances of nicotine addiction through chronic use and abuse also increase. These flavorings are believed to be safe for ingestion, but little information is available about their effects on the lungs. In this review, we have discussed the in vitro and in vivo data on toxicity of flavoring chemicals in lung cells. We have further discussed the common flavoring agents, such as diacetyl and menthol, currently available detection methods, and the toxicological mechanisms associated with oxidative stress, inflammation, mucociliary clearance, and DNA damage in cells, mice, and humans. Finally, we present potential biomarkers that could be utilized for future risk assessment. This review provides crucial parameters important for evaluation of risk associated with flavoring agents and flavoring enhancers used in tobacco products and ENDS. Future studies can be designed to address the potential toxicity of inhaled flavorings and their biomarkers in users as well as in chronic exposure studies.