Photosensitized co-generation of nitric oxide and singlet oxygen Enhanced toxicity against ovarian cancer cells.

Near micromolar concentrations of nitric oxide (NO) induce tumor cells death. However, an appropriate NO load has to be delivered selectively to the tumor site in order to avoid NO loss and secondary NO-induced effects. The encapsulation of millimolar concentrations of a NO source and an appropriate trigger of NO release within phospatidylcholine-based liposomes should provide an efficient tool for the selective release of the needed NO payload. In this work we report the photosensitized generation of singlet oxygen and NO from folate-targeted PEGylated liposomes, containing AlPcS4 as the sensitizer and S-nitrosoglutathione (GSNO), in millimolar amounts, as the NO source. Amounts of singlet oxygen detected outside the liposome when using PEGylated liposomes are near 200 % larger when GSNO is present inside the liposomes as compared to its absence. These liposomes, conjugated to folate, were found to enhance the photosensitized cytotoxicity to A2780CP20 ovarian cancer cells as compared to liposomes containing the sensitizer but no GSNO (30 % as compared to 70 % cell viability) under the conditions of this work. Fluorescense of AlPcS4 was observed inside cells incubated with folate-conjugated liposomes but not with liposomes without folate. The photosensitized activity enhancement by GSNO increased when light fluence or liposome concentration were increased. The majority of ovarian cancer patients are initially diagnosed with disseminated intra-abdominal disease (stages III-IV) and have a 5-year survival of less than 20%. This work suggests a novel ovarian cancer nodules treatment via the use of tumor-targeted liposome nanoparticles with the capability of generating simultaneously reactive oxygen and nitrogen species upon illumination with near-infrared light.

Synthesis and Evaluation of Folate-Conjugated Phenanthraquinones for Tumor-Targeted Oxidative Chemotherapy.

Almost all cells are easily killed by exposure to potent oxidants. Indeed, major pathogen defense mechanisms in both animal and plant kingdoms involve production of an oxidative burst, where host defense cells show an invading pathogen with reactive oxygen species (ROS). Although cancer cells can be similarly killed by ROS, development of oxidant-producing chemotherapies has been limited by their inherent nonspecificity and potential toxicity to healthy cells. In this paper, we describe the targeting of an ROS-generating molecule selectively to tumor cells using folate as the tumor-targeting ligand. For this purpose, we exploit the ability of 9,10-phenanthraquinone (PHQ) to enhance the continuous generation of H2O2 in the presence of ascorbic acid to establish a constitutive source of ROS within the tumor mass. We report here that incubation of folate receptor-expressing KB cells in culture with folate-PHQ plus ascorbate results in the death of the cancer cells with an IC50 of ~10 nM (folate-PHQ). We also demonstrate that a cleavable spacer linking folate to PHQ is significantly inferior to a noncleavable spacer, in contrast to most other folate-targeted therapeutic agents. Unfortunately, no evidence for folate-PHQ mediated tumor regression in murine tumor models is obtained, suggesting that unanticipated impediments to generation of cytotoxic quantities of ROS in vivo are encountered. Possible mechanisms and potential solutions to these unanticipated results are offered.

Identification of the first inhibitor of the GBP1:PIM1 interaction. Implications for the development of a new class of anticancer agents against paclitaxel resistant cancer cells.

Class III ß-tubulin plays a prominent role in the development of drug resistance to paclitaxel by allowing the incorporation of the GBP1 GTPase into microtubules. Once in the cytoskeleton, GBP1 binds to prosurvival kinases such as PIM1 and initiates a signaling pathway that induces resistance to paclitaxel. Therefore, the inhibition of the GBP1:PIM1 interaction could potentially revert resistance to paclitaxel. A panel of 44 4-azapodophyllotoxin derivatives was screened in the NCI-60 cell panel. The result is that 31 are active and the comparative analysis demonstrated specific activity in paclitaxel-resistant cells. Using surface plasmon resonance, we were able to prove that NSC756093 is a potent in vitro inhibitor of the GBP1:PIM1 interaction and that this property is maintained in vivo in ovarian cancer cells resistant to paclitaxel. Through bioinformatics, molecular modeling, and mutagenesis studies, we identified the putative NSC756093 binding site at the interface between the helical and the LG domain of GBP1. According to our results by binding to this site, the NSC756093 compound is able to stabilize a conformation of GBP1 not suitable for binding to PIM1.

Metal-independent reduction of hydrogen peroxide by semiquinones.

The quinones 1,4-naphthoquinone (NQ), tetramethyl-1,4-benzoquinone (DQ), 2-methyl-1,4-naphthoquinone (MNQ), 2,3-dimethoxy-5-methyl-1,4-benzoquinone (UBQ-0), 2,6-dimethylbenzoquinone (DMBQ), 2,6-dimethoxybenzoquinone (DMOBQ), and 9,10-phenanthraquinone (PHQ) enhance the rate of H2O2 reduction by ascorbate, under anaerobic conditions, as detected from the amount of methane produced after hydroxyl radical reaction with dimethyl sulfoxide. The amount of methane produced increases with an increase in the quinone one-electron reduction potential. The most active quinone in this series, PHQ, is only 14% less active than the classic Fenton reagent cation, Fe(2+), at the same concentration. Since PHQ is a common toxin present in diesel combustion smoke, the possibility that PHQ-mediated catalysis of hydroxyl radical formation is similar to that of Fe(2+) adds another important pathway to the modes in which PHQ can execute its toxicity. Because quinones are known to enhance the antitumor activity of ascorbate and because ascorbate enhances the formation of H2O2 in tissues, the quinone-mediated reduction of H2O2 should be relevant to this type of antitumor activity, especially under hypoxic conditions.

Roles of hydrophilicities and hydrophobicities of dye and sacrificial electron donor on the photochemical pathway.

Relative rates of the photosensitized production of singlet oxygen ((1)O(2)) and of superoxide (O(2) (•-)) were determined using different couples of dyes and sacrificial electron donors (SEDs) of either high or low hydrophobicities. Such rates were also measured in the absence and presence of single unilamellar vesicles (SUVs) with 9DMPC:1DMPA mol ratio composition. The dyes aluminum phthalocyanine tetrasulfonate (AlPcS(4)) and pheophorbide-a (PHEO) were used as hydrophilic and hydrophobic photosensitizers, respectively. Xanthine (X) and glutathione (GSH) were used as hydrophobic and hydrophilic SEDs, respectively. The presence of SUVs in the aqueous sample produces the physical separation or encounter of SEDs and photosensitizers according to their membrane binding constants. When both the SED and the photosensitizer are localized within the same phase, a strong decrease in the rate of (1)O(2) formation, united to a strong increase in the rate of O(2) (•-) formation, is observed, relative to when both of these species are localized in different phases. The lipid phase is always present in the biological milieu. Thus, the use of a hydrophobic couple of both dye and SED (as in the case of X and PHEO), as well as a hydrophilic couple of both dye and SED (as in the case of GSH and AlPcS4), should strongly favor the Type I mechanism over the Type II. Since only a small number of hydroxyl radicals are needed to initiate a chain reaction of phospholipid peroxidation, the latter could be more toxic to the tumor tissue than peroxidation by a much higher concentration of singlet oxygen molecules.

N-hydroxyethyl-4-aza-didehydropodophyllotoxin derivatives as potential antitumor agents.

Three different series of N-hydroxyethyl aza-podophyllotoxin derivatives containing (i) a five-membered methylenedioxy ring (7a-f), (ii) a five-membered ring with no heteroatom (8a-f) or (iii) a six membered ethylenedioxy ring (9a-f) as ring A were synthesized using a convenient one-pot multi-component reaction. Further variation on ring E was done by decorating it with methoxy and hydroxy groups at different positions. Calculation of logP values of these compounds indicates them to be better soluble than corresponding non-hydroxy derivatives. These novel aza-podophyllotoxin derivatives were screened for their cytostatic and cytotoxic activities on National Cancer Institute's 60 human tumor cell lines to study the structure activity relationship. The overall anticancer activity of these compounds was in the order of 8a-f>9a-f>7a-f. Furthermore, the compounds having 3'-methoxy and 3',4',5'-trimethoxy substitution at ring E were the most active within the series. The cytotoxicity of all the active compounds was low, while their antiproliferative (or cytostatic) activity was high, providing a wide therapeutic window for their potential application as anticancer drugs.

Quenching enhancement of the singlet excited state of pheophorbide-a by DNA in the presence of the quinone carboquone.

Changes in the emission fluorescence intensity of pheophorbide-a (PHEO) in the presence of carboquone (CARBOQ) were used to obtain the association constant, the number of CARBOQ molecules interacting with PHEO, and the fluorescence quantum yield of the complex. Excitation spectra of mixtures of PHEO and CARBOQ in ethanol (EtOH) show an unresolved doublet in the red-most excitation band of PHEO, indicating the formation of a loose ground-state complex. The 1:1 CARBOQ-PHEO complex shows a higher fluorescence quantum yield in EtOH (0.41 ± 0.02) than in buffer solution (0.089 ± 0.002), which is also higher than that of the PHEO monomer (0.28). Quenching of the PHEO fluorescence by DNA nucleosides and double-stranded oligonucleotides was also observed and the bimolecular quenching rate constants were determined. The quenching rate constant increase as the oxidation potential of the DNA nucleoside increases. Larger quenching constants were obtained in the presence of CARBOQ suggesting that CARBOQ enhances DNA photo-oxidation, presumably by inhibiting the back-electron-transfer reaction from the photoreduced PHEO to the oxidized base. Thus, the enhanced DNA-base photosensitized oxidation by PHEO in the presence of CARBOQ may be related to the large extent by which this quinone covalently binds to DNA, as previously reported.

Role of quinones in the ascorbate reduction rates of S-nitrosoglutathione.

Quinones are one of the largest classes of antitumor agents approved for clinical use, and several antitumor quinones are in various stages of clinical and preclinical development. Many of these are metabolites of, or are, environmental toxins. Because of their chemical structure they are known to enhance electron transfer processes such as ascorbate oxidation and NO reduction. The paraquinones 2,6-dimethyl-1,4-benzoquinone (DMBQ), 1,4-benzoquinone, methyl-1,4-benzoquinone, 2,6-dimethoxy-1,4-benzoquinone, 2-hydroxymethyl-6-methoxy-1,4-benzoquinone, trimethyl-1,4-benzoquinone, tetramethyl-1,4-benzoquinone, and 2,3-dimethoxy-5-methyl-1,4-benzoquinone; the paranaphthoquinones 1,4-naphthoquinone, menadione, 1,4-naphthoquinone-2-sulfonate, 2-ethylsulfanyl-3-methyl-1,4-naphthoquinone and juglone; and phenanthraquinone (PHQ) all enhance the anaerobic rate of ascorbate reduction of GSNO to produce NO and GSH. Rates of this reaction were much larger for p-benzoquinones and PHQ than for p-naphthoquinone derivatives with similar one-electron redox potentials. The quinone DMBQ also enhances the rate of NO production from S-nitrosylated bovine serum albumin upon ascorbate reduction. Density functional theory calculations suggest that stronger interactions between p-benzo- or phenanthrasemiquinones and GSNO than between p-naphthosemiquinones and GSNO are the major causes of these differences. Thus, quinones, and especially p-quinones and PHQ, could act as enhancers of NO release from GSNO in biomedical systems in the presence of ascorbate. Because quinones are exogenous toxins that could enter the human body via a chemotherapeutic application or as an environmental contaminant, they could boost the release of NO from S-nitrosothiol storages in the body in the presence of ascorbate and thus enhance the responses elicited by a sudden increase in NO levels.

Synthesis of Novel Functionalized 4-Aza-2,3-Didehydropodophyllotoxin Derivatives with Potential Antitumor Activity.

Novel arylamino alcohols were synthesized and these alcohols were used to prepare 12 novel N-(2-hydroxy-ethyl)-2,3-didehydroazapodophyllotoxins, in one step, by simple reflux in ethanol. Isolated yields in the range of 50-70% were obtained.

Quinone-enhanced reduction of nitric oxide by xanthine/xanthine oxidase.

The quinones 1,4-naphthoquinone, methyl-1,4-naphthoquinone, tetramethyl-1,4-benzoquinone, 2,3-dimethoxy-5-methyl-1,4-benzoquinone, 2,6-dimethylbenzoquinone, 2,6-dimethoxybenzoquinone, and 9,10-phenanthraquinone enhance the rate of nitric oxide reduction by xanthine/xanthine oxidase in nitrogen-saturated phosphate buffer (pH 7.4). Maximum initial rates of NO reduction (V(max)) and the amount of nitrous oxide produced after 5 min of reaction increase with quinone one- and two-electron redox potentials measured in acetonitrile. One of the most active quinones of those studied is 9,10-phenanthraquinone with a V(max) value 10 times larger than that corresponding to the absence of quinone, under the conditions of this work. Because NO production is enhanced under hypoxia and under certain pathological conditions, the observations obtained in this work are very relevant to such conditions.

Quinone-enhanced sonochemical production of nitric oxide from s-nitrosoglutathione.

Sonolysis at 75 kHz of argon- and air-saturated aqueous solutions at pH 7.4 containing s-nitrosogluthathione (GSNO) enhances the production rate of nitric oxide (NO). The quinones, anthraquinone-2-sulfonate (AQ2S) and anthraquinone-2,7-disulfonate (AQ27S) further enhance the NO production over that produced in quinone-depleted sonicated solutions. In contrast, the hydrophobic quinones juglone (JQ) and 1,4-naphthoquinone (NQ) inhibit ultrasound-induced NO detection as compared to quinone-depleted solutions. Larger sonolytical decomposition of the hydrophobic quinones NQ and JQ, as compared to AQ2S and AQ27S, is detected which correlates with a larger production of pyrolysis-derived carbon-centered radicals. Reaction of those radicals with NO could explain NQ and JQ inhibition. This work suggests that sulfonated quinones could be used to enhance NO release from GSNO in tissues undergoing ultrasound irradiation.

Role of the nitro functionality in the DNA binding of 3-nitro-10-methylbenzothiazolo[3,2-a]quinolinium chloride.

Interest in DNA binding drugs has increased in recent years due to their importance in the treatment of genome-related diseases, like cancer. A new family of water-soluble DNA binding compounds, the benzothiazolo[3,2- a]quinolinium chlorides (BQCls), is studied here as potential candidates for chemical treatment of solid tumor cells that may encounter low-oxygen environments, a condition known as hypoxia. These compounds are good DNA intercalators; however, no studies have been made of these compounds under hypoxic conditions. This work demonstrates the importance of the nitro-functionality in the DNA binding of 3-nitro-10-methylbenzothiazolo[3,2- a]quinolinium chloride (NBQ-91), which possesses nitro-functionality, and 10-methylbenzothiazolo[3,2- a]quinolinium chloride (BQ-106), which does not. Both NBQ-91 and BQ-106 have similar noncovalent binding affinity toward DNA. Dialysis experiments show that NBQ-91 binds DNA under N2-saturated conditions with increasing concentrations of reducing agent, presumably due to reduction of the nitro-functionality. Conversely, because of the lack of nitro-functionality, the presence of a reducing agent had no effect on BQ-106 binding to DNA under both aerobic and N2-saturated conditions. Clonogenic assays were performed to determine the quinolinium chloride cytotoxicities under both aerobic (95% air and 5% CO2) and hypoxic (80% N2 and 20% CO2) conditions. The calculated ratios of cellular toxicity under aerobic to hypoxic conditions caused by the same concentration of test agent (CTR values) show greater levels of cell death under hypoxia than under aerobic conditions for mitomycin C (MC) (CTR = 0.7 at 1 microM) and NBQ-91 (CTR = 0.4 at 200 microM) than for BQ-106 (CTR = 1.0 at 200 microM), which agreed with the previously reported data for MC and confirmed the importance of nitro-functionality for reactivity under hypoxic conditions. There was no correlation between noncovalent binding affinity constants and their cytotoxicity under hypoxic conditions. Adduct formation between the NBQ-91 and 2'-dG was also assessed by reacting 2'-dG or DNA with NBQ-91 and BQ-106 under N2-saturated conditions in the presence of hypoxanthine and xanthine oxidase (HX/XO). DNA covalent adduct formation was analyzed by two techniques: LC-ESI-MS and Sephadex size exclusion chromatography. LC-ESI-MS results clearly indicate the formation of a prominent molecular ion at masses of 266.0 and 530.58 Da, corresponding to the [M + H](+2) and [M](+) molecular ions of the monitored 2'-dG-NBQ-91 adduct. Results from the Sephadex size exclusion chromatography support these findings because the NBQ-91 elution percentage increases in the presence of HX/XO due to the reduction of the nitro-functionality, which results in covalent binding to DNA. This study reports evidence of the DNA binding capacity of this bioreductive drug. The preferential N2-saturated over aerobic conditions for DNA binding makes NBQ-91 a potential bioreductive compound for hypoxic cell killing.

Photosensitized oxidation of hypoxanthine and xanthine by aluminum phthalocyanine tetrasulfonate. Role of the alkylating quinone 2,5-dichloro-diaziridinyl-1,4-benzoquinone.

Photoirradiation of nitrogen-saturated aqueous solutions containing aluminum phthalocyanine tetrasulfonate (AlPcS4) at 675 nm in the presence of 2,5-dichloro-diaziridinyl-1,4-benzoquinone (AZDClQ) and hypoxanthine (HX) produces the oxidized HX derivatives, xanthine (X) and uric acid (UA). Concentrations of the AZDClQ semiquinone, X and UA increase at the expense of HX with an increase in irradiation time. Almost negligible decomposition of HX, as well as very low amounts of X, are detected if photolysis occurs under identical conditions but in the absence of AZDClQ. Addition of calf-thymus DNA produces quinone-DNA covalent adducts after photolysis of anaerobic samples containing quinone, DNA and AlPcS4, in the presence or absence of HX and at pH 5.5. However, larger amounts of quinone-DNA adducts are detected if HX is present. The results presented here could have applications in the photodynamic treatment of hypoxic tissues such as solid tumors, under conditions of high HX concentration, where Type-I pathways could be more important than singlet oxygen generation.

Thiols oxidation and covalent binding of BSA by cyclolignanic quinones are enhanced by the magnesium cation.

A novel cyclolignanic quinone, 7-acetyl-3',4'-didemethoxy-3',4'-dioxopodophyllotoxin (CLQ), inhibits topoisomerase II (TOPO II) activity. The extent of this inhibition was greater than that produced by the etoposide quinone (EQ) or etoposide. Glutathione (GSH) reduces EQ and CLQ to their corresponding semiquinones under anaerobic conditions. The latter were detected by EPR spectroscopy in the presence of MgCl(2) but not in its absence. Semiquinone EPR spectra change with quinone/GSH mol ratio, suggesting covalent binding of GSH to the quinones. Quinone-GSH covalent adducts were isolated and identified by ESI-MS. These orthoquinones also react with nucleophilic groups from BSA to bind covalently under anaerobic conditions. BSA thiol consumption and covalent binding by these quinones are enhanced by MgCl(2). Complex formation between the parent quinones and Mg(+2) was also observed. Density functional calculations predict the observed blue-shifts in the absorption spectra peaks and large decreases in the partial negative charge of electrophilic carbons at the quinone ring when the quinones are complexed to Mg(+2). These observations suggest a possible role of Mg(+2) chelation by these quinones in increasing TOPO II thiol and/or amino/imino reactivity with these orthoquinones.

Ortho-quinone-enhanced ascorbate oxidation. Combined roles of lipid charge and the magnesium cation.

Quinones are widely distributed compounds in nature. Of these, ortho-quinones are found to be involved in the pathogenic mechanism of Parkinson's disease, in oxidative deaminations to free-radical redox reactions, and as intermediates in the pathways implicated in the carcinogenicity of 2,3- and 3,4-catechol estrogens. Addition of MgCl(2) to solutions of the hydrophobic ortho-quinones, 1,10-phenanthroquinone (PHQ) and beta-lapachone (LQ) enhances ascorbate oxidation in the absence or presence of large unilamellar vesicles (LUVs) of the neutral lipid dimyristoylphos-phatidylcholine (DMPC), although initial rates of ascorbate oxidation are smaller in the presence of lipid as compared to its absence. Addition of this salt to solutions of the para-quinone 1,4-naphthoquinone (NQ) did not affect the ascorbate rate of oxidation in the absence or presence of DMPC. Addition of MgCl(2) to semiquinone solutions of PHQ or LQ in the presence or absence of DMPC increases semiquinone stability, as detected from the semiquinone disproportionation equilibrium displacement to semiquinone formation. Furthermore, MgCl(2) increases the partition of the ortho-semiquinones into the aqueous phase, although no such effect is observed for the semiquinone of NQ. For all the quinones under study, smaller rates of ascorbate oxidation and of semiquinone equilibrium concentration occur in the presence of negatively charged LUVs composed of an equimolar mixture of DMPC and dimyristoylphosphatidic acid DMPA. Ascorbate oxidation rate enhancements correlate with an increase in semiquinone concentration with addition of MgCl(2), in the absence or presence of neutral lipid. This observation favors the proposition that ascorbate oxidation rate increases are caused by semiquinone thermodynamic stabilization. Thus, the ascorbate oxidation rate enhancement by MgCl(2) in solutions containing hydrophobic ortho-quinones is still possible in systems with hydrophobic environments analogous to that of DMPC.

Comparison of the nucleic acid covalent binding capacity of two nitro-substituted benzazolo[3,2-a]quinolinium salts upon enzymatic reduction.

The DNA binding capacity of two nitro-substituted benzazolo[3,2-a]quinolinium chlorides (NBQs), NBQ-38 and NBQ-95, was evaluated upon their enzymatic reduction with hypoxanthine (HX)/xanthine oxidase (XO) under anaerobic conditions. In the presence of 2'-deoxyguanosine (2'-dG) or calf thymus DNA, covalent-addition products were monitored. Reactions of each NBQ with 2'-dG or DNA differed in the NBQ to HX molar ratio. Control reactions, one without HX/OX and another under aerobic conditions, were also analyzed. Adducts were isolated and characterized by high performance liquid chromatography (HPLC) and electrospray ionization-mass spectrometry (ESI-MS). Authentic samples of the reduced forms of these NBQs, identified as ABQ-38 and ABQ-95, were synthesized as standards to monitor bioreduction processes. HPLC analysis showed that the yield of formation of an unknown product (possibly, 2'-dG-NHBQ-38 adduct) from the reaction of NBQ-38 with 2'-dG and DNA was proportional to the HX to NBQ-38 molar ratio. ESI-MS analysis of the DNA hydrolysates showed evidence of an adduct formed upon bioreduction of NBQ-38 by the ions detection at m/z 528.3 and 454.8, consistent with chemical structures of a 2'-dG-NHBQ-38 adduct and a fragment ion. DNA adducts were not observed with NBQ-95, although the corresponding bioreduction product ABQ-95 was detected by ESI-MS. This study provides mechanistic information of these bioreductively-activated pro-drugs with potential therapeutic applications.

Sonochemically induced covalent binding of calf thymus DNA by aziridinylquinones.

Sonolysis of argon- or oxygen-containing samples in the presence of calf thymus DNA and the diaziridinylquinones 2,5-bis-aziridin-1-yl-3,6-dichloro-1,4-benzoquinone (AZClQ) and 2,5-bis(carboethoxyamino)-3,6-diaziridinyl-1,4-benzoquinone (AZQ) produced quinone-DNA covalent adducts at pH 5.5 and to a much lesser extent at pH 7.4. The corresponding semiquinone derivatives are detected using EPR spectroscopy after sonolysis of argon-saturated solutions at pH 7.4. The amount of covalent adducts decreases with addition of SOD, indicating a role of superoxide in this process. Addition of oxygen to the purging gas decreased but did not eliminate this covalent adduct. Thus this work suggests a possible synergism between bioreductive quinones and ultrasound in antitumor therapies based on alkylating quinone-DNA adduct formation with potential applications to both hypoxic and normally oxygenated conditions.

Oxidation of tea extracts and tea catechins by molecular oxygen.

Tea polyphenols (PP) are known as potent antioxidants. At the same time, PP have been repeatedly reported to oxidize by molecular oxygen with the formation of active forms of oxygen. In this work, the Clark electrode technique was applied to study the kinetics of the autoxidation of tea extracts and individual tea PP as well as model PP, catechol, gallic acid, and pyrogallol. Aqueous extracts of both green and black teas were found to undergo extensive autoxidation under physiological conditions. The addition of superoxide dismutase (SOD) and milk resulted in a significant decrease in the rate of oxidation. Studied individually, PP were found to autoxidize at a rate, which increased with pH, proportional to PP concentration and nearly proportional to oxygen concentration. The collected data were used for the extrapolation/interpolation of the starting rates of oxidation to the standard conditions (at pH 7.40, 100 microM PP, 200 microM O2). PP oxidizability is basically determined by that of the key PP fragment (pyrogallol > gallate > catechol). Meta-OH groups do not contribute to the oxidation even at pH 13.0. Similar to tea brew, the oxidation of individual PP was inhibited by milk and SOD addition, with catechol being the only exception (the oxidation of catechol was accelerated when SOD was added). Comparison of the autoxidation of PP (o-hydroquinones) with that of p-hydroquinones (Roginsky, V.; Barsukova, T. K. J. Chem. Soc., Perkin Trans. 2 2000, 1575-1582) displays the dramatic difference both in the oxidizability and in the kinetic regularities. The difference in the kinetics has been suggested to be due to the difference in the initiation of the chain process. Whereas for p-hydroquinones the oxidation is initiated by the reaction between hydroquinone and a corresponding quinone, the oxidation of o-hydroquinones is likely started by direct interaction between substrate and molecular oxygen. As the second process is much slower, this may explain the relatively low oxidizability of PP as compared to p-hydroquinones.

Quinone-enhanced ascorbate reduction of nitric oxide: role of quinone redox potential.

The quinones 1,4-naphthoquinone (NQ), methyl-1,4-naphthoquinone (MNQ), trimethyl-1,4-benzoquinone (TMQ) and 2,3-dimethoxy-5-methyl-1,4-benzoquinone (UQ-0) enhance the rate of nitric oxide (NO) reduction by ascorbate in nitrogen-saturated phosphate buffer (pH 7.4). The observed rate constants for this reaction were determined to be 16+/-2,215+/-6,290+/-14 and 462+/-18 M-1 s-1, for MNQ, TMQ, NQ and UQ-0, respectively. These rate constants increase with an increase in quinone one-electron redox potential at neutral pH, E1(7). Since NO production is enhanced under hypoxia and under certain pathological conditions, the observations obtained in this work are very relevant to such conditions.

Alkaline-earth cations enhance ortho-quinone-catalyzed ascorbate oxidation.

Ortho-quinones 1,10-phenanthroquinone and beta-lapachone but not para-quinones naphthazarin (NZQ) and 1,4-naphthoquinone enhance ascorbate oxidation in the presence of MgCl(2) and CaCl(2) at constant ionic strength. Alkaline-earth cation chelation is observed for the ortho-semiquinones but not for the para-semiquinones, while no interaction between these quinones (with the exception of NZQ) or ascorbate and these salts was detected, suggesting that semiquinone-metal complexes are responsible for the catalytic action on ascorbate oxidation of these metal salts in the presence of these ortho-quinones. Thus, redox cycling efficiency of the quinones under study here, in the presence of ascorbate, depends not only on the quinone redox potential but also on the semiquinone ability to chelate alkaline-earth cations.