Natural and synthetic quinones and their reduction by the quinone reductase enzyme NQO1: from synthetic organic chemistry to compounds with anticancer potential

Injectable and Sprayable Fluorescent Nanoprobe for Rapid Real-Time Detection of Human Colorectal Tumors

The development of minimally invasive surgery has greatly advanced precision tumor surgery, but sometime suffers from restricted visualization of the surgical field, especially during the removal of abdominal tumors. A 3-D inspection of tumors could be achieved by intravenously injecting tumor-selective fluorescent probes, whereas most of which are unable to instantly distinguish tumors via in situ spraying, which is urgently needed in the process of surgery in a convenient manner. In this study, this work has designed an injectable and sprayable fluorescent nanoprobe, termed Poly-g-BAT, to realize rapid tumor imaging in freshly dissected human colorectal tumors and animal models. Mechanistically, the incorporation of γ-glutamyl group facilitates the rapid internalization of Poly-g-BAT, and these internalized nanoprobes can be subsequently activated by intracellular NAD(P)H: quinone oxidoreductase-1 to release near-infrared fluorophores. As a result, Poly-g-BAT can achieve a superior tumor-to-normal ratio (TNR) up to 12.3 and enable a fast visualization (3 min after in situ spraying) of tumor boundaries in the xenograft tumor models, Apcmin/+ mice models and fresh human tumor tissues. In addition, Poly-g-BAT is capable of identifying minimal premalignant lesions via intravenous injection.

Manganese-stimulated redox cycling of dopamine derivatives: Implications for manganism

Manganism, the condition caused by chronic exposure to high levels of manganese, selectively targets the dopamine-rich basal ganglia causing a movement disorder with symptoms similar to Parkinson's disease. While the basis for this specific targeting is unknown, we hypothesize that it may involve complexation of Mn by dopamine derivatives. At micromolar concentrations, MnCl2 accelerates the two-equivalent redox cycling of a dopamine-derived benzothiazine (dopathiazine) by an order of magnitude. In the process, O2 is reduced to superoxide and hydrogen peroxide. This effect is unique to Mn and is not shared by Fe, Cu, Zn, Co, Ca or Mg. Notably, the effect of Mn requires the presence of inorganic phosphate, suggesting that phosphate may stabilize a Mn/catecholate complex, which reacts readily with O2. This or similar endogenous dopamine derivatives may exacerbate Mn-dependent oxidative stress accounting for the neurological selectivity of manganism.

Characterization of a small molecule inhibitor of disulfide reductases that induces oxidative stress and lethality in lung cancer cells

Phenotype-based screening can identify small molecules that elicit a desired cellular response, but additional approaches are required to characterize their targets and mechanisms of action. Here, we show that a compound termed LCS3, which selectively impairs the growth of human lung adenocarcinoma (LUAD) cells, induces oxidative stress. To identify the target that mediates this effect, we use thermal proteome profiling (TPP) and uncover the disulfide reductases GSR and TXNRD1 as targets. We confirm through enzymatic assays that LCS3 inhibits disulfide reductase activity through a reversible, uncompetitive mechanism. Further, we demonstrate that LCS3-sensitive LUAD cells are sensitive to the synergistic inhibition of glutathione and thioredoxin pathways. Lastly, a genome-wide CRISPR knockout screen identifies NQO1 loss as a mechanism of LCS3 resistance. This work highlights the ability of TPP to uncover targets of small molecules identified by high-throughput screens and demonstrates the potential therapeutic utility of inhibiting disulfide reductases in LUAD.

Probing altered enzyme activity in the biochemical characterization of cancer

Enzymes have evolved to catalyze their precise reactions at the necessary rates, locations, and time to facilitate our development, to respond to a variety of insults and challenges, and to maintain a healthy, balanced state. Enzymes achieve this extraordinary feat through their unique kinetic parameters, myriad regulatory strategies, and their sensitivity to their surroundings, including substrate concentration and pH. The Cancer Genome Atlas (TCGA) highlights the extraordinary number of ways in which the finely tuned activities of enzymes can be disrupted, contributing to cancer development and progression often due to somatic and/or inherited genetic alterations. Rather than being limited to the domain of enzymologists, kinetic constants such as k_cat, K_m, and k_cat/K_m are highly informative parameters that can impact a cancer patient in tangible ways—these parameters can be used to sort tumor driver mutations from passenger mutations, to establish the pathways that cancer cells rely on to drive patients’ tumors, to evaluate the selectivity and efficacy of anti-cancer drugs, to identify mechanisms of resistance to treatment, and more. In this review, we will discuss how changes in enzyme activity, primarily through somatic mutation, can lead to altered kinetic parameters, new activities, or changes in conformation and oligomerization. We will also address how changes in the tumor microenvironment can affect enzymatic activity, and briefly describe how enzymology, when combined with additional powerful tools, and can provide us with tremendous insight into the chemical and molecular mechanisms of cancer.

Structure and Function of a Dual Reductase-Dehydratase Enzyme System Involved in p-Terphenyl Biosynthesis

We report the identification of the ter gene cluster responsible for the formation of the p-terphenyl derivatives terfestatins B and C and echoside B from the Appalachian Streptomyces strain RM-5-8. We characterize the function of TerB/C, catalysts that work together as a dual enzyme system in the biosynthesis of natural terphenyls. TerB acts as a reductase and TerC as a dehydratase to enable the conversion of polyporic acid to a terphenyl triol intermediate. X-ray crystallography of the apo and substrate-bound forms for both enzymes provides additional mechanistic insights. Validation of the TerC structural model via mutagenesis highlights a critical role of arginine 143 and aspartate 173 in catalysis. Cumulatively, this work highlights a set of enzymes acting in harmony to control and direct reactive intermediates and advances fundamental understanding of the previously unresolved early steps in terphenyl biosynthesis.

Benzo[f]indole-4,9-dione Derivatives Effectively Inhibit the Growth of Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is a subtype of breast cancer with poor clinical outcome, and currently no effective targeted therapies are available. Indole compounds have been shown to have potential antitumor activity against various cancer cells. In the present study, we found that new four benzo[f]indole-4,9-dione derivatives reduce TNBC cell viability by reactive oxygen species (ROS) accumulation stress in vitro. Further analyses showed that LACBio1, LACBio2, LACBio3 and LACBio4 exert cytotoxic effects on MDA-MB 231 cancer cell line by inducing the intrinsic apoptosis pathway, activating caspase 9 and Bax/Bcl-2 pathway in vitro. These results provide evidence that these new four benzo[f]indole-4,9-dione derivatives could be potential therapeutic agents against TNBC by promoting ROS stress-mediated apoptosis through intrinsic-pathway caspase activation.

Design, synthesis, and biological evaluation of 3-(1-benzotriazole)-nor-β-lapachones as NQO1-directed antitumor agents

A series of novel 3-(1-benzotriazole)-nor-β-lapachones 5a-5l were synthesized as the NQO1-targeted anticancer agents. Most of these compounds displayed good antiproliferative activity against the breast cancer MCF-7, lung cancer A549 and hepatocellular carcinoma HepG2 cells in agreements with their NQO1 activity. Among them, compound 5k was identified as a favorable NQO1 substrate. It could activate the ROS production in a NQO1-dependent manner, arrest tumor cell cycle at G0/G1 phase, promote tumor cell apoptosis, and decrease the mitochondrial membrane potential. In HepG2 xenograft models, 5k significantly suppressed the tumor growth with no influences on animal body weights. Therefore, 5k could be a good lead for further anticancer drug developments.

Strategies for the Synthesis of Mono- and Bis-Thionaphthoquinones

The subclass of compounds that have the nucleus 1,4-naphthoquinone is the most diverse of the class of quinones, which have a large number of substances and several that have useful applications ranging from medicinal chemistry to application in materials with special properties. The introduction of one or two substituents with the sulfur heteroatom in the naphthoquinone nucleus generates products containing alkyl and aryl groups that amplify certain biological properties against bacteria, viruses and fungi. There are several methods of preparing these compounds, mainly from low molecular weight naphthoquinones with two electrophilic sites capable of reacting with sulfides generating diversity and new classes of compounds, including new sulfur heterocycles and sulfur heterocycles fused with naphthoquinones. These compounds have been shown to be bioactive against several biological targets. This review will describe the methods of their synthesis and, when applicable, their biological activities.

Coenzyme Q10 Analogues: Benefits and Challenges for Therapeutics

Coenzyme Q₁₀ (CoQ₁₀ or ubiquinone) is a mobile proton and electron carrier of the mitochondrial respiratory chain with antioxidant properties widely used as an antiaging health supplement and to relieve the symptoms of many pathological conditions associated with mitochondrial dysfunction. Even though the hegemony of CoQ₁₀ in the context of antioxidant-based treatments is undeniable, the future primacy of this quinone is hindered by the promising features of its numerous analogues. Despite the unimpeachable performance of CoQ₁₀ therapies, problems associated with their administration and intraorganismal delivery has led clinicians and scientists to search for alternative derivative molecules. Over the past few years, a wide variety of CoQ₁₀ analogues with improved properties have been developed. These analogues conserve the antioxidant features of CoQ₁₀ but present upgraded characteristics such as water solubility or enhanced mitochondrial accumulation. Moreover, recent studies have proven that some of these analogues might even outperform CoQ₁₀ in the treatment of certain specific diseases. The aim of this review is to provide detailed information about these Coenzyme Q₁₀ analogues, as well as their functionality and medical applications.

Perfluoro effect on the electronic excited states of para-benzoquinone revealed by experiment and theory

We report a comprehensive study on the electronic excited states of tetrafluoro-1,4-benzoquinone, through high-resolution vacuum ultraviolet photoabsorption spectroscopy and time-dependent density functional theory calculations performed within the nuclear ensemble approach. Absolute cross section values were experimentally determined in the 3.8-10.8 eV energy range. The present experimental results represent the highest resolution data yet reported for this molecule and reveal previously unresolved spectral structures. The interpretation of the results was made in close comparison with the available data for para-benzoquinone [Jones et al., J. Chem. Phys., 2017, 146, 184303]. While the dominant absorption features for both molecules arise from analogous π* ← π transitions, some remarkable differences have been identified. The perfluoro effect manifests in different ways: shifts in band positions and cross sections, appearance of features associated with excitations to σCF* orbitals, and spectrum broadening by quenching of either vibrational or Rydberg progressions. The level of agreement between experiment and theory is very satisfactory, yet that required the inclusion of nuclear quantum effects in the calculations. We have also discussed the role of temperature on the absorption spectrum, as well as the involvement of core-excited resonances in promoting dissociative electron attachment reactions in the 3-5 eV range.

Enamides and dienamides in phosphoric acid-catalysed enantioselective cycloadditions for the synthesis of chiral amines

This feature article describes how enamides and dienamides can participate in chiral phosphoric acid catalyzed enantioselective cycloadditions to prepare a wide range of cyclic amines.

Emodin inhibited NADPH-quinone reductase via competitive mode of inhibition and induced cytotoxicity in rat primary hepatocytes

Consumption of Cassia occidentalis (CO) seeds, a ubiquitously distributed weed plant, is responsible for a pathological condition known as hepato-myo-encephalopathy (HME). The toxicity of CO seeds is largely attributed to the presence of anthraquinones. Here, we report that Emodin, a CO anthraquinone, inhibits the enzymatic activity of NADPH-Quinone reductase, which is an intracellular enzyme fundamentally involved in the detoxification of quinone containing compounds. Emodin binds to the active site of the enzyme and acts as a competitive inhibitor with respect to 2, 6-Dichlorophenolindophenol, a known substrate of NADPH-Quinone reductase. Moreover, our in-vitro study further revealed that Emodin was cytotoxic to primary rat hepatocytes.

A novel multifunctional 2-nitroimidazole-based bioreductive linker and its application in hypoxia-activated prodrugs

Tumor hypoxia has been widely explored over the years as a diagnostic and therapeutic marker. Herein, we designed, optimized and synthesized a new multifunctional bioreductive linker (12) containing an alkynyl group (potential click chemistry fragment); the linker is based on 2-nitroimidazole which was expected to simultaneously overcome the drawbacks of hypoxia-activated prodrugs (poor selectivity and unsatisfactory water solubility). Furthermore, a hypoxia-activated, water-soluble SN-38 prodrug was obtained, and it was stable under physiological conditions and was rapidly released as an active drug under hypoxic conditions.

Oxidative Phosphorylation—an Update on a New, Essential Target Space for Drug Discovery in Mycobacterium tuberculosis

New drugs with new mechanisms of action are urgently required to tackle the global tuberculosis epidemic. Following the FDA-approval of the ATP synthase inhibitor bedaquiline (Sirturo®), energy metabolism has become the subject of intense focus as a novel pathway to exploit for tuberculosis drug development. This enthusiasm stems from the fact that oxidative phosphorylation (OxPhos) and the maintenance of the transmembrane electrochemical gradient are essential for the viability of replicating and non-replicating Mycobacterium tuberculosis (M. tb), the etiological agent of human tuberculosis (TB). Therefore, new drugs targeting this pathway have the potential to shorten TB treatment, which is one of the major goals of TB drug discovery. This review summarises the latest and key findings regarding the OxPhos pathway in M. tb and provides an overview of the inhibitors targeting various components. We also discuss the potential of new regimens containing these inhibitors, the flexibility of this pathway and, consequently, the complexity in targeting it. Lastly, we discuss opportunities and future directions of this drug target space.

Enantioselective Redox-Divergent Chiral Phosphoric Acid Catalyzed Quinone Diels-Alder Reactions

An efficient enantioselective construction of tetrahydronaphthalene-1,4-diones as well as dihydronaphthalene-1,4-diols by a chiral phosphoric acid catalyzed quinone Diels-Alder reaction with dienecarbamates is reported. The nature of the protecting group on the diene is key to the success of achieving high enantioselectivity. The divergent "redox" selectivity is controlled by using an adequate amount of quinones. Reversible redox switching without erosion of enantioselectivity was possible from individual redox isomers.

Cytosolic NQO1 Enzyme-Activated Near-Infrared Fluorescence Imaging and Photodynamic Therapy with Polymeric Vesicles

The utilization of enzymes as a triggering module could endow responsive polymeric nanostructures with selectivity in a site-specific manner. On the basis of the fact that endogenous NAD(P)H:quinone oxidoreductase isozyme 1 (NQO1) is overexpressed in many types of tumors, we report on the fabrication of photosensitizer-conjugated polymeric vesicles, exhibiting synergistic NQO1-triggered turn-on of both near-infrared (NIR) fluorescence emission and a photodynamic therapy (PDT) module. For vesicles self-assembled from amphiphilic block copolymers containing quinone trimethyl lock-capped self-immolative side linkages and quinone-bridged photosensitizers (coumarin and Nile blue) in the hydrophobic block, both fluorescence emission and PDT potency are initially in the "off" state due to "double quenching" effects, that is, dye-aggregation-caused quenching and quinone-rendered PET (photoinduced electron transfer) quenching. After internalization into NQO1-positive vesicles, the cytosolic NQO1 enzyme triggers self-immolative cleavage of quinone linkages and fluorogenic release of conjugated photosensitizers, leading to NIR fluorescence emission turn-on and activated PDT. This process is accompanied by the transformation of vesicles into cross-linked micelles with hydrophilic cores and smaller sizes and triggered dual drug release, which could be directly monitored by enhanced magnetic resonance (MR) imaging for vesicles conjugated with a DOTA(Gd) complex in the hydrophobic bilayer. We further demonstrate that the above strategy could be successfully applied for activated NIR fluorescence imaging and tissue-specific PDT under both cellular and in vivo conditions.

Rapanone, a naturally occurring benzoquinone, inhibits mitochondrial respiration and induces HepG2 cell death

Rapanone is a natural occurring benzoquinone with several biological effects including unclear cytotoxic mechanisms. Here we addressed if mitochondria are involved in the cytotoxicity of rapanone towards cancer cells by employing hepatic carcinoma (HepG2) cells and isolated rat liver mitochondria. In the HepG2, rapanone (20-40 μM) induced a concentration-dependent mitochondrial membrane potential dissipation, ATP depletion, hydrogen peroxide generation and, phosphatidyl serine externalization; the latter being indicative of apoptosis induction. Rapanone toxicity towards primary rats hepatocytes (IC₅₀ = 35.58 ± 1.50 μM) was lower than that found for HepG2 cells (IC₅₀ = 27.89 ± 0.75 μM). Loading of isolated mitochondria with rapanone (5-20 μM) caused a concentration-dependent inhibition of phosphorylating and uncoupled respirations supported by complex I (glutamate and malate) or the complex II (succinate) substrates, being the latter eliminated by complex IV substrate (TMPD/ascorbate). Rapanone also dissipated mitochondrial membrane potential, depleted ATP content, released Ca²⁺ from Ca²⁺-loaded mitochondria, increased ROS generation, cytochrome c release and membrane fluidity. Further analysis demonstrated that rapanone prevented the cytochrome c reduction in the presence of decylbenzilquinol, identifying complex III as the site of its inhibitory action. Computational docking results of rapanone to cytochrome bc1 (Cyt bc1) complex from the human sources found spontaneous thermodynamic processes for the quinone-Q_o and Q_i binding interactions, supporting the experimental in vitro assays. Collectively, these observations suggest that rapanone impairs mitochondrial respiration by inhibiting electron transport chain at Complex III and promotes mitochondrial dysfunction. This property is potentially involved in rapanone toxicity on cancer cells.

Incorporating Morpholine and Oxetane into Benzimidazolequinone Antitumor Agents: The Discovery of 1,4,6,9-Tetramethoxyphenazine from Hydrogen Peroxide and Hydroiodic Acid-Mediated Oxidative Cyclizations

The reactivity of hydrogen peroxide and catalytic hydroiodic acid toward 3,6-dimethoxy-2-(cycloamino)anilines is tunable to give ring-fused benzimidazoles or 1,4,6,9-tetramethoxyphenazine in high yield. Mechanisms via a detected nitroso-intermediate are proposed for oxidative cyclization and the unexpected intermolecular displacement of the oxazine. An aqueous solution of molecular iodine is capable of the same transformations. Oxidative demethylation gave targeted benzimidazolequinones, including without cleavage of the incorporated oxetane.

Synthesis of quinones with highlighted biological applications: A critical update on the strategies towards bioactive compounds with emphasis on lapachones

Naphthoquinones are of key importance in organic synthesis and medicinal chemistry. In the last few years, various synthetic routes have been developed to prepare bioactive compounds derived or based on lapachones. In this sense, this review is mainly focused on the synthetic aspects and strategies used for the design of these compounds on the basis of their biological activities for the development of drugs against the neglected diseases leishmaniases and Chagas disease and also cancer. Three strategies used to develop bioactive quinones are discussed and categorized: (i) C-ring modification, (ii) redox centre modification and (iii) A-ring modification. Framed within these strategies for the development of naphthoquinoidal compounds against T. cruzi. Leishmania and cancer, reactions including copper-catalyzed azide-alkyne cycloaddition (click chemistry), palladium-catalysed cross couplings, C-H activation reactions, Ullmann couplings and heterocyclisations reported up to July 2019 will be discussed. The aim of derivatisation is the generation of novel molecules that can potentially inhibit cellular organelles/processes, generate reactive oxygen species and increase lipophilicity to enhance penetration through the plasma membrane. Modified lapachones have emerged as promising prototypes for the development of drugs against leishmaniases, Chagas disease and cancer.

Boric Acid Activation of eIF2α and Nrf2 Is PERK Dependent: a Mechanism that Explains How Boron Prevents DNA Damage and Enhances Antioxidant Status

Boron is abundant in vegetables, nuts, legumes, and fruit and intake is associated with reduced risk of cancer and DNA damage and increased antioxidant status. Blood boric acid (BA) levels are approximately 10 μM BA in men at the mean US boron intake. Treatment of DU-145 human prostate cancer cells with 10 μM BA stimulates phosphorylation of elongation initiation factor 2α (eIF2α) at Ser51 leading to activation of the eIF2α/ATF4 pathway which activates the DNA damage-inducible protein GADD34. In the present study, we used MEF WT and MEF PERK (±) cells to test the hypothesis that BA-activated eIF2α phosphorylation requires protein kinase RNA-like endoplasmic reticulum kinase (PERK) and activates Nrf2 and the antioxidant response element (ARE). BA (10 μM) increased phosphorylation of eIF2α Ser51 in MEF WT cells at 1 h, but not in MEF Perk -/- cells exposed for as long as 6 h. GCN2 kinase-dependent phosphorylation of eIF2α Ser51 was activated in MEF PERK -/- cells by amino acid starvation. Nrf2 phosphorylation is PERK dependent and when activated is translocated from the cytoplasm to the nucleus where it acts as a transcription factor for ARE. DU-145 cells were treated with 10 μM BA and Nrf2 measured by immunofluorescence. Cytoplasmic Nrf2 was translocated to the nucleus at 1.5-2 h in DU-145 and MEF WT cells, but not MEF PERK -/- cells. Real-time PCR was used to measure mRNA levels of three ARE genes (HMOX-1, NQO1, and GCLC). Treatment with 10 μM BA increased the mRNA levels of all three genes at 1-4 h in DU-145 cells and HMOX1 and GCLC in MEF WT cells. These results extend the known boric acid signaling pathway to ARE-regulated genes. The BA signaling pathway can be expressed using the schematic [BA + cADPR → cADPR-BA → [[ER]_i Ca²⁺↓] → 3 pathways: PERK/eIF2αP → pathways ATF4 and Nrf2; and [[ER]_i Ca²⁺↓] → ER stress → ATF6 pathway. This signaling pathway provides a framework that links many of the molecular changes that underpin the biological effects of boron intake.

Hypoxia-targeted drug delivery

Hypoxia is a state of low oxygen tension found in numerous solid tumours. It is typically associated with abnormal vasculature, which results in a reduced supply of oxygen and nutrients, as well as impaired delivery of drugs. The hypoxic nature of tumours often leads to the development of localized heterogeneous environments characterized by variable oxygen concentrations, relatively low pH, and increased levels of reactive oxygen species (ROS). The hypoxic heterogeneity promotes tumour invasiveness, metastasis, angiogenesis, and an increase in multidrug-resistant proteins. These factors decrease the therapeutic efficacy of anticancer drugs and can provide a barrier to advancing drug leads beyond the early stages of preclinical development. This review highlights various hypoxia-targeted and activated design strategies for the formulation of drugs or prodrugs and their mechanism of action for tumour diagnosis and treatment.

Tryptophol and derivatives: natural occurrence and applications to the synthesis of bioactive compounds

This report presents some fundamental aspects related to the natural occurrence and bioactivity of tryptophol as well as the synthesis of tryptophols and their utilization for the preparation of naturally occurring alkaloid metabolites embedding the indole system.

Cyclic Changes in Active Site Polarization and Dynamics Drive the 'Ping-pong' Kinetics in NRH:Quinone Oxidoreductase 2: An Insight from QM/MM Simulations

Quinone reductases belong to the family of flavin-dependent oxidoreductases. With the redox active cofactor, flavin adenine dinucleotide, quinone reductases are known to utilize a 'ping-pong' kinetic mechanism during catalysis in which a hydride is bounced back and forth between flavin and its two substrates. However, the continuation of this catalytic cycle requires product displacement steps, where the product of one redox half-cycle is displaced by the substrate of the next half-cycle. Using improved hybrid quantum mechanical/molecular mechanical simulations, both the catalytic hydride transfer and the product displacement reactions were studied in NRH:quinone oxidoreductase 2. Initially, the self-consistent charge-density functional tight binding theory was used to describe flavin ring and the substrate atoms, while embedded in the molecular mechanically-treated solvated active site. Then, for each step of the catalytic cycle, a further improvement of energetics was made using density functional theory-based corrections. The present study showcases an integrated interplay of solvation, protonation, and protein matrix-induced polarization as the driving force behind the thermodynamic wheel of the 'ping-pong' kinetics. Reported here is the first-principles model of the 'ping-pong' kinetics that portrays how cyclic changes in the active site polarization and dynamics govern the oscillatory hydride transfer and product displacement in this enzyme.

Biophysical and functional perturbation analyses at cancer-associated P187 and K240 sites of the multifunctional NADP(H):quinone oxidoreductase 1

Once whole-genome sequencing has reached the clinical practice, a main challenge ahead is the high-throughput and accurate prediction of the pathogenicity of genetic variants. However, current prediction tools do not consider explicitly a well-known property of disease-causing mutations: their ability to affect multiple functional sites distant in the protein structure. Here we carried out an extensive biophysical characterization of fourteen mutant variants at two cancer-associated sites of the enzyme NQO1, a paradigm of multi-functional protein. We showed that the magnitude of destabilizing effects, their molecular origins (structural vs. dynamic) and their efficient propagation through the protein structure gradually led to functional perturbations at different sites. Modulation of these structural perturbations also led to switches between molecular phenotypes. Our work supports that experimental and computational perturbation analyses would improve our understanding of the molecular basis of many loss-of-function genetic diseases as well as our ability to accurately predict the pathogenicity of genetic variants in a high-throughput fashion.

An NAD(P)H:Quinone Oxidoreductase 1 Responsive and Self-Immolative Prodrug of 5-Fluorouracil for Safe and Effective Cancer Therapy

Tripartite prodrug 1, composed of an NAD(P)H:quinone oxidoreductase 1 (NQO1)-responsive trigger group, a self-immolative linker, and the active drug 5-fluorouracil (5-FU), was designed and synthesized for site-specific cancer therapy. Upon bioreductive activation by NQO1, 1 can release the parent drug 5-FU specifically in NQO1-overexpressing cancer cells. This prodrug exerts comparable antitumor activity and a more favorable safety profile compared with 5-FU both in vitro and in vivo.

Apoptosis inducing factor (AIF) mediates lethal redox stress induced by menadione

Mitochondrial apoptosis inducing factor (AIF) is a redox-active enzyme that participates to the biogenesis/maintenance of complex I of the respiratory chain, yet also contributes to catabolic reactions in the context of regulated cell death when AIF translocates to the cytosol and to the nucleus. Here we explore the contribution of AIF to cell death induced by menadione (2-methyl-1,4-naphtoquinone; also called vitamin K3) in conditions in which this pro-oxidant does not cause the mitochondrial release of AIF, yet causes caspase-independent cell killing. Depletion of AIF from human cancer cells reduced the cytotoxicity of menadione. This cytoprotective effect was accompanied by the maintenance of high levels of reduced glutathione (GSH), which are normally depleted by menadione. In addition, AIF depletion reduced the arylation of cellular proteins induced by menadione. This menadione-triggered arylation, which can be measured by a fluorescence assay, is completely suppressed by addition of exogenous glutathione or N-acetyl cysteine. Complex I inhibition by Rotenone did not mimic the cytoprotective action of AIF depletion. Altogether, these results are compatible with the hypothesis that mitochondrion-sessile AIF facilitates lethal redox cycling of menadione, thereby precipitating protein arylation and glutathione depletion.

Thermodynamics of cooperative binding of FAD to human NQO1: Implications to understanding cofactor-dependent function and stability of the flavoproteome

The stability of human flavoproteins strongly depends on flavin levels, although the structural and energetic basis of this relationship is poorly understood. Here, we report an in-depth analysis on the thermodynamics of FAD binding to one of the most representative examples of such relationship, NAD(P)H:quinone oxidoreductase 1 (NQO1). NQO1 is a dimeric enzyme that tightly binds FAD, which triggers large structural changes upon binding. A common cancer-associated polymorphism (P187S) severely compromises FAD binding. We show that FAD binding is described well by a thermodynamic model explicitly incorporating binding cooperativity when applied to different sets of calorimetric analyses and NQO1 variants, thus providing insight on the effects in vitro and in cells of cancer-associated P187S, its suppressor mutation H80R and the role of NQO1 C-terminal domain to modulate binding cooperativity and energetics. Furthermore, we show that FAD binding to NQO1 is very sensitive to physiologically relevant environmental conditions, such as the presence of phosphate buffer and salts. Overall, our results contribute to understanding at the molecular level the link between NQO1 stability and fluctuations of FAD levels intracellularly, and supports the notion that FAD binding energetics and cooperativity are fundamentally linked with the dynamic nature of apo-NQO1 conformational ensemble.

Combined molecular modelling and 3D-QSAR study for understanding the inhibition of NQO1 by heterocyclic quinone derivatives

A combination of three-dimensional quantitative structure-activity relationship (3D-QSAR), and molecular modelling methods were used to understand the potent inhibitory NAD(P)H:quinone oxidoreductase 1 (NQO1) activity of a set of 52 heterocyclic quinones. Molecular docking results indicated that some favourable interactions of key amino acid residues at the binding site of NQO1 with these quinones would be responsible for an improvement of the NQO1 activity of these compounds. The main interactions involved are hydrogen bond of the amino group of residue Tyr128, π-stacking interactions with Phe106 and Phe178, and electrostatic interactions with flavin adenine dinucleotide (FADH) cofactor. Three models were prepared by 3D-QSAR analysis. The models derived from Model I and Model III, shown leave-one-out cross-validation correlation coefficients (q²_LOO ) of .75 and .73 as well as conventional correlation coefficients (R² ) of .93 and .95, respectively. In addition, the external predictive abilities of these models were evaluated using a test set, producing the predicted correlation coefficients (r²_pred ) of .76 and .74, respectively. The good concordance between the docking results and 3D-QSAR contour maps provides helpful information about a rational modification of new molecules based in quinone scaffold, in order to design more potent NQO1 inhibitors, which would exhibit highly potent antitumor activity.

Flavoenzyme-mediated reduction reactions and antitumor activity of nitrogen-containing tetracyclic ortho-quinone compounds and their nitrated derivatives

Nitrogen-based tetracyclic ortho-quinones (naphtho[1'2':4.5]imidazo[1,2-a]pyridine-5,6-diones, NPDOs) and their nitro-substituted derivatives (nitro-(P)NPDOs) were obtained by condensation of substituted 2,3-dichloro-1,4-naphthoquinones with 2-amino-pyridine and -pyrimidine and nitration at an elevated temperature. The structural features of the compounds as well as their global and regional electrophilic potency were characterized by means of DFT computation. The compounds were highly reactive substrates of single- and two-electron (hydride) - transferring P-450R (CPR; EC 1.6.2.4) and NQO-1 (DTD; EC 1.6.99.2), respectively, concomitantly producing reactive oxygen species. Their catalytic efficiency defined in terms of the apparent second-order rate constant (k_cat/K_{M (Q)}) values in P-450R- and NQO-1-mediated reactions varied in the range of 3-6 × 10⁷ M^-1 s^-1 and 1.6-7.4 × 10⁸ M^-1 s^-1, respectively. The cytotoxic activities of the compounds on tumor cell lines followed the concentration-dependent manner exhibiting relatively high cytotoxic potency against breast cancer MCF-7, with CL₅₀ values of 0.08-2.02 µM L^-1 and lower potency against lung cancer A-549 (CL₅₀ = 0.28-7.66 µM L^-1). 3-nitro-pyrimidino-NPDO quinone was the most active compound against MCF-7 with CL₅₀ of 0.08 ± 0.01 µM L^-1 (0.02 µg mL^-1)) which was followed by 3-nitro-NPDO with CL₅₀ of 0.12 ± 0.03 µM L^-1 (0.035 µg mL^-1)) and 0.28 ± 0.08 µM L^-1 (0.08 µg mL^-1) on A-549 and MCF-7 cells, respectively, while 1- and 4-nitro-quinoidals produced the least cytotoxic effects. Tumor cells quantified by AO/EB staining showed that the cell death induced by the compounds occurs primarily through apoptosis.

Resonances in low-energy electron scattering from para-benzoquinone

Shape and core-excited resonances ofpara-benzoquinone identified and characterized in electron scatteringR-matrix calculations.

Formation and Biological Targets of Quinones: Cytotoxic versus Cytoprotective Effects

Quinones represent a class of toxicological intermediates, which can create a variety of hazardous effects in vivo including, acute cytotoxicity, immunotoxicity, and carcinogenesis. In contrast, quinones can induce cytoprotection through the induction of detoxification enzymes, anti-inflammatory activities, and modification of redox status. The mechanisms by which quinones cause these effects can be quite complex. The various biological targets of quinones depend on their rate and site of formation and their reactivity. Quinones are formed through a variety of mechanisms from simple oxidation of catechols/hydroquinones catalyzed by a variety of oxidative enzymes and metal ions to more complex mechanisms involving initial P450-catalyzed hydroxylation reactions followed by two-electron oxidation. Quinones are Michael acceptors, and modification of cellular processes could occur through alkylation of crucial cellular proteins and/or DNA. Alternatively, quinones are highly redox active molecules which can redox cycle with their semiquinone radical anions leading to the formation of reactive oxygen species (ROS) including superoxide, hydrogen peroxide, and ultimately the hydroxyl radical. Production of ROS can alter redox balance within cells through the formation of oxidized cellular macromolecules including lipids, proteins, and DNA. This perspective explores the varied biological targets of quinones including GSH, NADPH, protein sulfhydryls [heat shock proteins, P450s, cyclooxygenase-2 (COX-2), glutathione S-transferase (GST), NAD(P)H:quinone oxidoreductase 1, (NQO1), kelch-like ECH-associated protein 1 (Keap1), IκB kinase (IKK), and arylhydrocarbon receptor (AhR)], and DNA. The evidence strongly suggests that the numerous mechanisms of quinone modulations (i.e., alkylation versus oxidative stress) can be correlated with the known pathology/cytoprotection of the parent compound(s) that is best described by an inverse U-shaped dose-response curve.

Insight into the kinetics and thermodynamics of the hydride transfer reactions between quinones and lumiflavin: a density functional theory study

The kinetics and equilibrium of the hydride transfer reaction between lumiflavin and a number of substituted quinones was studied using density functional theory. The impact of electron withdrawing/donating substituents on the redox potentials of quinones was studied. In addition, the role of these substituents on the kinetics of the hydride transfer reaction with lumiflavin was investigated in detail under the transition state (TS) theory assumption. The hydride transfer reactions were found to be more favorable for an electron-withdrawing substituent. The activation barrier exhibited a quadratic relationship with the driving force of these reactions as derived under the formalism of modified Marcus theory. The present study found a significant extent of electron delocalization in the TS that is stabilized by enhanced electrostatic, polarization, and exchange interactions. Analysis of geometry, bond-orders, and energetics revealed a predominant parallel (Leffler-Hammond) effect on the TS. Closer scrutiny reveals that electron-withdrawing substituents, although located on the acceptor ring, reduce the N-H bond order of the donor fragment in the precursor complex. Carried out in the gas-phase, this is the first ever report of a theoretical study of flavin's hydride transfer reactions with quinones, providing an unfiltered view of the electronic effect on the nuclear reorganization of donor-acceptor complexes.