ES936 stimulates DNA synthesis in HeLa cells independently on NAD(P)H:quinone oxidoreductase 1 inhibition,through a mechanism involving p38 MAPK

The indolequinone ES936 (5-methoxy-1,2-dimethyl-3-[(4-nitrophenol)methyl]-indole-4,7-dione) is a potent mechanism-based inhibitor of NAD(P)H:quinone oxidoreductase 1 (NQO1). Here, we report that ES936 significantly stimulated thymidine incorporation in sparse cultures of human adenocarcinoma HeLa cells, but was without effect in dense cultures. Stimulation of DNA synthesis was not related with a DNA repair response because an increase in thymidine incorporation was not observed in cells treated with 2,5 bis-[1-aziridyl]-1,4 benzoquinone, a well-established antitumor quinone that causes DNA damage. Conversely, it was related with an increase of cell growth. NQO1 inhibition was not involved in ES936 stimulation of DNA synthesis, because the same response was observed in cells where NQO1 expression had been knocked down by small interfering RNA. Stimulation of DNA synthesis was reverted by treatment with ambroxol, a SOD mimetic, and by pyruvate, an efficient peroxide scavenger, supporting the involvement of alterations in cellular redox state. Pharmacological inhibition of p38 with either SB203580 or PD169316 completely abolished ES936-stimulated DNA synthesis, indicating the requirement of p38 activity. This is the first report that demonstrates the existence of an ES936-sensitive system which is separate from NQO1, modulating the redox state and cell growth in HeLa cells through a p38-dependent mechanism. Our results show that the effect ES936 exerts on DNA synthesis may be either positive or negative depending on the cellular context and growth conditions.     

Indolequinone inhibitors of NRH:quinone oxidoreductase 2. Characterization of the mechanism of inhibition in both cell-free and cellular systems

We describe a series of indolequinones as efficient mechanism-based inhibitors of NRH:quinone oxidoreductase 2 (NQO2) for use either in cellular or cell-free systems. Compounds were designed to be reduced in the active site of the enzyme leading to loss of a substituted phenol leaving group and generation of a reactive iminium electrophile. Inhibition of NQO2 activity was assessed in both cell-free systems and the human leukemia K562 cell line. Inhibition of recombinant human NQO2 by the indolequinones was NRH-dependent, with kinetic parameters characteristic of mechanism-based inhibition and partition ratios as low as 2.0. Indolequinones inhibited NQO2 activity in K562 cells at nanomolar concentrations that did not inhibit NQO1 and were nontoxic to cells. Computation-based molecular modeling simulations demonstrated favorable conformations of indolequinones positioned directly above and in parallel with the isoalloxazine ring of FAD, and mass spectrometry extended our previous finding of adduction of the FAD in the active site of NQO2 by an indolequinone-derived iminium electrophile to the wider series of indolequinone inhibitors. Modeling combined with biochemical testing identified key structural parameters for effective inhibition, including a 5-aminoalkylamino side chain. Hydrogen bonding of the terminal amine nitrogen in the aminoalkylamino side chain was found to be critical for the correct orientation of the inhibitors in the active site. These indolequinones were irreversible inhibitors and were found to be at least 1 order of magnitude more potent than any previously documented competitive inhibitors of NQO2 and represent the first mechanism-based inhibitors of NQO2 to be characterized in cellular systems.     

Development of indolequinone mechanism-based inhibitors of NAD(P)H:quinone oxidoreductase 1 (NQO1): NQO1 inhibition and growth inhibitory activity in human pancreatic MIA PaCa-2 cancer cells

NAD(P)H:quinone oxidoreductase 1 (NQO1) is currently an emerging target in pancreatic cancer. In this report, we describe a series of indolequinones, based on 5-methoxy-1,2-dimethyl-3-[(4-nitrophenoxy)methyl]indole-4,7-dione (ES936), and evaluate NQO1 inhibition and growth inhibitory activity in the human pancreatic MIA PaCa-2 tumor cell line. The indolequinones with 4-nitrophenoxy, 4-pyridinyloxy, and acetoxy substituents at the (indol-3-yl)methyl position were NADH-dependent inhibitors of recombinant human NQO1, indicative of mechanism-based inhibition. However, those with hydroxy and phenoxy substituents were poor inhibitors of NQO1 enzyme activity, due to attenuated elimination of the leaving group. The ability of this series of indolequinones to inhibit recombinant human NQO1 correlated with NQO1 inhibition in MIA PaCa-2 cells. The examination of indolequinone interactions in complex with NQO1 from computational-based molecular docking simulations supported the observed biochemical data with respect to NQO1 inhibition. The design of both NQO1-inhibitory and noninhibitory indolequinone analogues allowed us to test the hypothesis that NQO1 inhibition was required for growth inhibitory activity in MIA PaCa-2 cells. ES936 and its 6-methoxy analogue were potent inhibitors of NQO1 activity and cell proliferation; however, the 4-pyridinyloxy and acetoxy compounds were also potent inhibitors of NQO1 activity but relatively poor inhibitors of cell proliferation. In addition, the phenoxy compounds, which were not inhibitors of NQO1 enzymatic activity, demonstrated potent growth inhibition. These data demonstrate that NQO1 inhibitory activity can be dissociated from growth inhibitory activity and suggest additional or alternative targets to NQO1 that are responsible for the growth inhibitory activity of this series of indolequinones in human pancreatic cancer.     

Inactivation of CMY-2 beta-lactamase by tazobactam: initial mass spectroscopic characterization.

The CMY-2 beta-lactamase, a plasmid determined class C cephalosporinase, was shown to be susceptible to inhibition by tazobactam (K(i)=40 microM). The reaction product(s) of CMY-2 beta-lactamase with the beta-lactamase inhibitor tazobactam were analyzed by electrospray ionization/mass spectrometry (ESI/MS) to characterize the prominent intermediates of the inactivation pathway. The ESI/MS determined mass of CMY-2 beta-lactamase was 39851+/-3 Da. After inactivating CMY-2 beta-lactamase with excess tazobactam, a single species, M(r)=39931+/-3.0, was detected. Comparison of the peptide maps from tryptic digestion of the native enzyme and the inactivated beta-lactamase followed by LC/MS identified two 22 amino acid peptides containing the active site Ser64 modified by a fragment of tazobactam. These two peptides were increased in mass by 70 and 88 Da, respectively. UV difference spectra following inactivation revealed the presence of a new species with a 302 nm lambda(max). Based upon the increase in molecular mass of the tazobactam inactivated CMY-2 beta-lactamase, we propose that during the inactivation of this beta-lactamase by tazobactam an imine is formed. Tautomerization forms the spectrally observed enamine. Hydrolysis generates the covalently attached malonyl semialdehyde, its hydrate, or an enol. This work provides information on the mass of a stable enzyme intermediate of a class C beta-lactamase inactivated by tazobactam and, for the first time, unequivocal evidence that a cross-linked species is not required for apparent inactivation.     

Identification of a metabolite of atrazine, N-ethyl-6-methoxy-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine, upon incubation with rat liver microsomes

Atrazine is an herbicide which has shown potential antimalarial effects both in vitro and in vivo in rats. In order to study the metabolism of atrazine in rat livers, we developed a sensitive LC/MS/MS method for the identification of atrazine and several of its metabolites. Using this method, we identified one previously unreported metabolite with a mass of 211 Da in addition to two known metabolites. This new metabolite was confirmed to be N-ethyl-6-methoxy-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine, also known as atraton, by comparison of the LC/MS/MS mass spectra and the retention time to those of a commercial standard.     

Antibacterial peptides in stimulated human granulocytes: characterization of ubiquitinated histone H1A.

Antibacterial peptides were isolated from human peripheral granulocytes of a healthy donor who had been treated with granulocyte-colony stimulating factor (G-CSF) and cortisol. Peptides were solubilized in acidified chloroform/methanol, and partitioned in chloroform/methanol/water. Water- soluble polypeptides were separated by cation-exchange and reversed-phase chromatography. Several previously characterized antibacterial polypeptides were identified; defensins 1-3, defensin 4, lysozyme, eosinophil cationic protein, and calgranulin A. In addition, several histone fragments were isolated and exhibited activity against the Gram- positive bacterium Bacillus megaterium strain Bm11. These fragments included two C-terminal fragments of histone H1A, three C-terminal fragments of histone H1D, one fragment of histone H1B, and two fragments of histone H4. The molecular masses of both histone H1A fragments, as determined by electrospray (ES) MS, were 270 Da higher than those calculated from their amino acid sequences. The two histone H1A fragments corresponded to Lys152-Lys222 (7527 +/- 1 Da) and Lys167-Lys222 (6023 +/- 1 Da). Tandem MS (MS/MS) of the 7.5 kDa and 6.0 kDa fragments indicated that the post-translational modification is on Lys222, the epsilon-amino group of which was conjugated with the alpha-carboxyl group of the tripeptide Arg-Gly-Gly. This finding was substantiated by digestion of the 7.5-kDa polypeptide with trypsin and analysis of the resulting peptides by ES MS and MS/MS. The tripeptide Arg-Gly-Gly corresponded uniquely to the three C-terminal residues of ubiquitin, demonstrating the presence of ubiquitinated histone H1A.     

Stem cell factor is localized in, released from, and cleaved by human mast cells.

Stem cell factor (SCF) is the most important cytokine regulating human mast cell growth and functions. The immunogold technique showed SCF in the secretory granules of skin mast cells and in lung parenchymal mast cells (HLMC). Immunoreactive SCF (iSCF) was detected in cell lysates of HLMC, but not in basophils; iSCF and histamine were detected in supernatants of HLMC 3 min after challenge with anti-FcepsilonRI or anti-IgE, and iSCF in supernatants rapidly declined after 30 min, whereas histamine remained unchanged for 120 min. HPLC and electrospray mass spectrometry (ES/MS) analysis of recombinant human SCF1-166 (18,656. 9 +/- 0.9 Da) treated with chymase showed a polypeptide of 17,977.1 +/- 0.6 Da and a minor component of 697.4 +/- 0.1 Da generated by specific cleavage at Phe159. SCF1-166 and SCF1-159 similarly activated HLMC, potentiated anti-IgE-induced activation of these cells, and stimulated HLMC chemotaxis. SCF159-166 had no effect on mast cells. Western blot analysis of supernatants of anti-IgE-activated HLMC incubated with recombinant human SCF1-166 showed that SCF1-166 was rapidly cleaved to SCF1-159 and SCF1-144. Experiments with supernatants of anti-IgE-activated HLMC incubated with SCF1-166 yielded similar results. In conclusion, SCF is stored in mast cell secretory granules and is immunologically released by human mast cells. SCF1-166 is rapidly and specifically cleaved to SCF1-159 by chymase, which retains its biological effect on mast cells. SCF is also cleaved by other proteases to several SCF species whose possible biological activities remain to be established.     

The mechanism-based inactivation of p450 2B4 by tert-butyl 1-methyl-2-propynyl ether: structural determination of the adducts to the p450 heme

tert-Butyl 1-methyl-2-propynyl ether (tBMP) was analyzed for its ability to act as a mechanism-based inactivator of p450 2B4. tBMP inactivated p450 2B4 in a time-, concentration-, and NADPH-dependent manner. Losses in activity occurred with concurrent losses in the reduced CO spectrum and native p450 heme; however, there was a greater loss in activity than could be accounted for by reduced CO spectra or native heme loss. LC/MS analysis demonstrated that the losses in native heme were accompanied by the appearance of two modified hemes with m/z values of 705Da, consistent with tBMP adducted hemes. Both adducts had identical fragmentation patterns when analyzed by LC/MS/MS. The spectra were consistent with a tBMP molecule and an oxygen atom attached to iron-depleted heme. Proton NMR studies suggest that the two modified hemes in p450 2B1 are N-alkylated on pyrrole rings A and D.     

Structure-function analysis of alpha-helix H4 using PSE-4 as a model enzyme representative of class A beta-lactamases

We extracted maximum information for structure-function analysis of the PSE-4 class A beta-lactamase by random replacement mutagenesis of three contiguous codons in the H4 alpha-helix at amino acid positions Ala125, Thr126, Met127, Thr128 and Thr129. These positions were predicted to interact with suicide mechanism-based inhibitors when examining the PSE-4 three-dimensional model. Structure-function studies on positions 125-129 indicated that in PSE-4 these amino acids have a role distinct from those in TEM-1, in tolerating substitutions at Ala125 and being invariant at Met127. The importance of Met127 was suspected to be implicated in a structural role in maintaining the integrity of the H4 alpha-helix structure together, thus maintaining the important Ser130-Asp131-Asn132 motif positioned towards the active site. At the structural level, the H4 region was analyzed using energy minimization of the H4 regions of the PSE-4 YAM mutant and compared with wild-type PSE-4. The Tyr 125 of the mutant YAM formed an edge to face pi-pi interaction with Phe 124 which also interacts with the Trp 210 with the same interactions. Antibiotic susceptibilities showed that amino acid changes in the the H4 alpha-helix region of PSE-4 are particularly sensitive to mechanism based-inhibitors. However, kinetic analysis of PSE-4 showed that the two suicide inhibitors belonging to the penicillanic acid sulfone class, sulbactam and tazobactam, were less affected by changes in the H4 alpha-helix region than clavulanic acid, an inhibitor of the oxypenam class. The analysis of H4 alpha-helix in PSE-4 suggests its importance in interactions with the three clinically useful inhibitors and in general to all class A enzymes.     

cDNA cloning and 1.75 A crystal structure determination of PPL2, an endochitinase and N-acetylglucosamine-binding hemagglutinin from Parkia platycephala seeds

Parkia platycephala lectin 2 was purified from Parkia platycephala (Leguminosae, Mimosoideae) seeds by affinity chromatography and RP-HPLC. Equilibrium sedimentation and MS showed that Parkia platycephala lectin 2 is a nonglycosylated monomeric protein of molecular mass 29 407+/-15 Da, which contains six cysteine residues engaged in the formation of three intramolecular disulfide bonds. Parkia platycephala lectin 2 agglutinated rabbit erythrocytes, and this activity was specifically inhibited by N-acetylglucosamine. In addition, Parkia platycephala lectin 2 hydrolyzed beta(1-4) glycosidic bonds linking 2-acetoamido-2-deoxy-beta-D-glucopyranose units in chitin. The full-length amino acid sequence of Parkia platycephala lectin 2, determined by N-terminal sequencing and cDNA cloning, and its three-dimensional structure, established by X-ray crystallography at 1.75 A resolution, showed that Parkia platycephala lectin 2 is homologous to endochitinases of the glycosyl hydrolase family 18, which share the (betaalpha)8 barrel topology harboring the catalytic residues Asp125, Glu127, and Tyr182.     

Modification of Escherichia coli thymidylate synthase at tyrosine-94 by 5-imidazolylpropynyl-2'-deoxyuridine 5'-monophosphate

Evidence is presented that 5-imidazolylpropynyl-2'-deoxyuridine 5'-monophosphate (IP-dUMP) is a mechanism-based, irreversible inactivator of Escherichia coli thymidylate synthase (TS), which covalently modifies Tyr94 at the active site of the enzyme. The inactivation of TS was time and concentration dependent and did not require the folate cofactor. Due to the rapidity of the inactivation process, accurate kinetic parameters could be determined only in the presence of saturating concentrations (1000K(M)) of the competing substrate, dUMP. Under these conditions, a K(I) of 0.36 +/- 0.09 microM and an inactivation rate constant (k(inact)) of 0.53 +/- 0.15 min(-1) were obtained from Kitz-Wilson plots. Electrospray ionization-mass spectrometry (ESI-MS) determined a 412 amu mass increase of TS after inhibition by IP-dUMP with no mass difference being detected for the TS mutants Tyr94Phe or Cys146Ala, thus indicating the importance of these residues for complex formation. The change in WT-TS mass was consistent with covalent modification by IP-dUMP, which was confirmed by proteolytic digestion of the modified protein followed by ESI-MS. By these means, a 43-residue trypsin peptide (residues 54-96), a 16-residue endoAspN peptide (residues 89-104), and an 8-residue endoAspN/endoLysC peptide (residues 89-96), each containing the IP-dUMP adduct, were observed. MS/MS analysis of the IP-dUMP-endoAspN peptide identified a modified 3-residue daughter ion, YGK (residues 94-96). A mechanistic scheme requiring the participation of Cys146 is proposed for the covalent modification of IP-dUMP by Tyr94, which, unlike an earlier proposal [Kalman, T. I., Nie, Z., and Kamat, A. (2001) Nucleosides Nucleotides Nucleic Acids 20, 869-871], does not require the release of imidazole for the activation of the inhibitor.     

NAD(P)H:Quinone Oxidoreductase 1 (NQO1) Localizes to the Mitotic Spindle in Human Cells

NAD(P)H:quinone oxidoreductase 1 (NQO1) is an FAD containing quinone reductase that catalyzes the 2-electron reduction of a broad range of quinones. The 2-electron reduction of quinones to hydroquinones by NQO1 is believed to be a detoxification process since this reaction bypasses the formation of the highly reactive semiquinone. NQO1 is expressed at high levels in normal epithelium, endothelium and adipocytes as well as in many human solid tumors. In addition to its function as a quinone reductase NQO1 has been shown to reduce superoxide and regulate the 20 S proteasomal degradation of proteins including p53. Biochemical studies have indicated that NQO1 is primarily located in the cytosol, however, lower levels of NQO1 have also been found in the nucleus. In these studies we demonstrate using immunocytochemistry and confocal imaging that NQO1 was found associated with mitotic spindles in cells undergoing division. The association of NQO1 with the mitotic spindles was observed in many different human cell lines including nontransformed cells (astrocytes, HUVEC) immortalized cell lines (HBMEC, 16HBE) and cancer (pancreatic adenocarcinoma, BXPC3). Confocal analysis of double-labeling experiments demonstrated co-localization of NQO1with alpha-tubulin in mitotic spindles. In studies with BxPc-3 human pancreatic cancer cells the association of NQO1 with mitotic spindles appeared to be unchanged in the presence of NQO1 inhibitors ES936 or dicoumarol suggesting that NQO1 can associate with the mitotic spindle and still retain catalytic activity. Analysis of archival human squamous lung carcinoma tissue immunostained for NQO1 demonstrated positive staining for NQO1 in the spindles of mitotic cells. The purpose of this study is to demonstrate for the first time the association of the quinone reductase NQO1 with the mitotic spindle in human cells.     

Quantification of dinor, dihydro metabolites of F2-isoprostanes in urine by liquid chromatography/tandem mass spectrometry

The F2-isoprostanes (F2-IsoP) are a series of prostaglandin (PG)-F2-like compounds that are produced by free-radical-mediated oxidation of arachidonic acid. One F2-IsoP with potent biological activity is 15-F2t-IsoP and increased levels of 15-F(2t)-IsoP have been measured in several diseases. The major urinary metabolite of 15-F2t-IsoP (8-iso-PGF(2alpha)) is 2,3-dinor-5,6-dihydro-15-F2t-IsoP (15-F2t-IsoP-M). Previously, we developed a stable isotope dilution gas chromatography/negative chemical ionization/mass spectrometry (MS) assay for 15-F2t-IsoP-M, which, while highly sensitive, required time-consuming derivatization and thin-layer chromatography purification. We now report the development of a more rapid high-performance liquid chromatography method coupled to electrospray ionization-tandem mass spectrometry (LC/MS/MS) to analyze all of the dinor,dihydro metabolites of the F2-IsoP isomers (F2-IsoP-M). The precision of this assay was +/-5.0% and the accuracy 80%. The assay remained linear over a range of 1-100 ng injected onto the LC column. Levels of F2-IsoP-M determined by the LC/MS/MS assay method significantly correlated with levels of 15-F2t-IsoP-M determined by the GC/MS assay (R = 0.77y = 67.2x-0.5). The levels of F2-IsoP-M detected in spot urines from 40 normal subjects were 38.1+/-19.1 ng/mg creatinine (mean+/-SD). This method provides an accurate and rapid assay to assess oxidative status in vivo.     

Mechanism of inhibition of the class A beta -lactamases PC1 and TEM-1 by tazobactam. Observation of reaction products by electrospray ionization mass spectrometry

The reactions of class A beta-lactamases PC1 and TEM-1 with tazobactam (TZB), a potent penicillanic sulfone inhibitor for class A beta-lactamases, were studied using electrospray ionization mass spectrometry (ESI/MS). Following inactivation of the beta-lactamases by TZB, new abundant high mass components were observed including three with molecular masses of 52, 70, and 88 Da greater than PC1 and TEM-1, respectively, and a component with a molecular mass of 300 Da greater than PC1. In addition, three TZB reaction products with molecular masses of 248, 264, and 280 Da were observed. High performance liquid chromatography (HPLC)/ESI/MS analysis of the TZB-PC1 adduct digested with Glu-C revealed three new components with masses 52, 70, and 88 Da greater than that of the peptide composed of amino acid residues 58-82 and one new component with a mass 70 Da greater than that of the peptide composed of amino acid residues 125-141. HPLC/ESI/MS/MS analysis of the two digested peptides whose masses increased by 70 Da indicated that Ser-70 and Ser-130 were the most likely TZB-modified amino acid residues. Based on these data, a mechanism for the inactivation of the class A beta-lactamases by TZB is proposed. In this scheme, initial acylation of Ser-70 by TZB and opening of the lactam ring are followed by one of several different events: (1) the rapid decomposition of TZB with loss of the enamine moiety to form the propiolylated enzyme, (2) an intramolecular nucleophilic displacement of the imine or enamine moiety by Ser-130 to form a cross-linked vinyl ether, and (3) hydrolysis of the imine or enamines to form a Ser-70-linked aldehyde.     

Significance of SIMS microscopy for the radioiodine detection in animal and human thyroid tissue.

We defined the SIMS conditions for radioiodine detection in animal and man thyroid follicles, in tissue sections (3 microns) chemically fixed and resin embedded. Two radioisotopes were tested: 125I and 129I, of high (14 mCi 125I micrograms-1) and low specific activity (1.07 10(-6) mCi 129I micrograms-1). In animal study, Wistar rats fed a normal iodine diet (10 micrograms 127I day-1) were injected ip 24 h before sacrifice either with 125I (7 10(-3) micrograms) or with 129I at a dose identical to iodine diet (10 micrograms) or 3 times higher (30 micrograms). No SIMS signal of 125I was obtained in vivo due to its too low concentration, while radioiodine distribution was evidenced with both doses of 129I. Local concentration of previously stored 127I in follicular lumen was not modified, when compared to control (4.14 +/- 0.03 micrograms/mg, m +/- SE), by 125I or 129I at a dose of 10 micrograms, but was nearly doubled with 129I at a dose of 30 micrograms, proof of a pharmacological effect on thyroid iodine regulation. In human study 129I was excluded due to its long half-life (1.6 10(7) years), and 125I was tested only in vitro on two surgical specimens of normal perinodular thyroid tissue maintained in mini-organ culture for 48 h in presence of 100 microCi/ml of 125I. The 125I was detectable, its concentration was 1,000-fold higher than that of 127I (1.5 +/- 0.004 micrograms/mg). For both in vivo and in vitro studies, a positive correlation exists between newly organified radioiodine (125I or 129I) and previously stored iodine (127I).(ABSTRACT TRUNCATED AT 250 WORDS)     

Accurate quantification of basal plasma levels of 3-nitrotyrosine and 3-nitrotyrosinoalbumin by gas chromatography-tandem mass spectrometry

Measurement of 3-nitro-L-tyrosine (NO(2)Tyr) and protein-related 3-nitro-L-tyrosine in human plasma is associated with numerous methodological problems which result in highly divergent basal plasma levels often ranging within two orders of magnitude. Recently, we have described an interference-free GC-tandem MS-based method for NO(2)Tyr which yielded the lowest basal plasma NO(2)Tyr levels reported thus far. This method was extended to quantify protein-associated 3-nitrotyrosine and in particular 3-nitrotyrosinated albumin (NO(2)TyrALB) in human plasma. NO(2)TyrALB and albumin (ALB) were extracted from plasma by affinity column extraction and digested enzymatically at neutral pH. 3-Nitro- L-[2H(3)]tyrosine was used as internal standard. In plasma of 18 healthy young volunteers the molar ratio of NO(2)TyrALB to albumin-derived tyrosine (TyrALB), i.e. NO(2)TyrALB/TyrALB, was determined to be 1.55+/-0.54x1:10(6) (mean+/-SD). The plasma concentration of NO(2)TyrALB was estimated as 24+/-4 nM. The NO(2)Tyr plasma levels in these volunteers were determined to be 0.73+/-0.53 nM. In the same volunteers, NO(2)TyrALB/TyrALB, NO(2)TyrALB and NO(2)Tyr were measured 15 days later and the corresponding values were determined to be 1.25+/-0.58x1:10(6), 25+/-6 nM and 0.69+/-0.16 nM. For comparison, NO(2)Tyr and NO(2)TyrALB were measured in six plasma samples from healthy volunteers by GC-MS and GC-tandem MS. Different values were found for NO(2)Tyr, i.e. 5.4+/-2.8 versus 2.7+/-1.5 nM, and comparable values for NO(2)TyrALB/TyrALB, i.e. 0.5+/-0.2x1:10(6) versus 0.4+/-0.1x1:10(6), by these methods. The ratio of the values measured by GC-MS to those measured by GC-tandem MS were 2.9+/-3.1 for NO(2)Tyr and 1.2+/-0.2 for NO(2)TyrALB/TyrALB. The present GC-tandem MS method provides accurate values of NO(2)Tyr and NO(2)TyrALB in human plasma.     

Prooxidant cytotoxicity of chromate in mammalian cells: the opposite roles of DT-diaphorase and glutathione reductase

The geno- and cytotoxicity of chromate, an important environmental pollutant, is partly attributed to the flavoenzyme-catalyzed reduction with the concomitant formation of reactive oxygen species. The aim of this work was to characterize the role of NAD(P)H:quinone oxidoreductase (NQO1, DT-diaphorase, EC 1.6.99.2) and glutathione reductase (GR, EC 1.6.4.2) in the mammalian cell cytotoxicity of chromate, which was evidenced controversially so far. The chromate reductase activity of NQO1 was higher than that of GR, but lower than that of lipoamide dehydrogenase (EC 1.6.4.3), ferredoxin:NADP+ reductase (EC 1.18.1.2), and NADPH: cytochrome P-450 reductase (EC 1.6.2.4). The reduction of chromate by NQO1 was accompanied by the formation of reactive oxygen species. The concentration of chromate for 50% survival of bovine leukemia virus-transformed lamb kidney fibroblasts (line FLK) during a 24-h incubation was (22 +/- 4) microM. The cytotoxicity was partly prevented by desferrioxamine, the antioxidant N,N'-diphenyl-p-phenylene diamine and by an inhibitor of NQO1, dicumarol, and potentiated by 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU), which inactivates GR. The NADPH-dependent chromate reduction by digitonin-permeabilized FLK cells was partly inhibited by dicumarol and not affected by BCNU. Taken together, these data indicate that the oxidative stress-type cytotoxicity of chromate in FLK cells may be partly attributed to its reduction by NQO1, but not by GR. The effect of BCNU on the chromate cytotoxicity may indicate that the general antioxidant action of reduced glutathione is more important than its prooxidant activities arising from the reactions with chromate.     

Evidence for NQO2-mediated reduction of the carcinogenic estrogen ortho-quinones

The physiological function of NAD(P)H:quinone oxidoreductase (NQO1, DT-diaphorase) is to detoxify potentially reactive quinones by direct transfer of two electrons. A similar detoxification role has not been established for its homologue NRH:quinone oxidoreductase 2 (NQO2). Estrogen quinones, including estradiol(E(2))-3,4-Q, generated by estrogen metabolism, are thought to be responsible for estrogen-initiated carcinogenesis. In this investigation, we have shown for the first time that NQO2 catalyzes the reduction of electrophilic estrogen quinones and thereby may act as a detoxification enzyme. ESI and MALDI mass spectrometric binding studies involving E(2)-3,4-Q with NQO2 clearly support the formation of an enzyme-substrate physical complex. The problem of spontaneous reduction of substrate by cofactor, benzyldihydronicotinamide riboside (BNAH), was successfully overcome by taking advantage of the ping-pong mechanism of NQO2 catalysis. The involvement of the enzyme in the reduction of E(2)-3,4-Q was further supported by addition of the inhibitor quercetin to the assay mixture. NQO2 is a newly discovered binding site (MT3) of melatonin. However, addition of melatonin to the assay mixture did not affect the catalytic activity of NQO2. Preliminary kinetic studies show that NQO2 is faster in reducing estrogen quinones than its homologue NQO1. Both UV and liquid chromatography-tandem mass spectrometry assays unequivocally corroborate the reduction of estrogen ortho-quinones by NQO2, indicating that it could be a novel target for prevention of breast cancer initiation.