Metabolic activation of the antitumor drug 5-(Aziridin-1-yl)-2,4-dinitrobenzamide (CB1954) by NO synthases

Nitric oxide synthases (NOSs) are flavohemeproteins that catalyze the oxidation of L-arginine to L-citrulline with formation of the signaling molecule nitric oxide (NO). In addition to their fundamental role in NO biosynthesis, NOSs are also involved in the formation of reactive oxygen and nitrogen species (RONS) and in the interactions with some drugs. 5-(Aziridin-1-yl)-2,4-dinitrobenzamide (CB1954) is a dinitroaromatic compound tested as an antitumor prodrug that requires reduction to the 2- and 4-hydroxylamines to be cytotoxic. Here, we studied the interaction of neuronal, inducible, and endothelial NOSs with CB1954. Our results showed that the three purified recombinant NOSs selectively reduced the 4-nitro group of CB1954 to the corresponding 4-hydroxylamine with minimal 2-nitroreduction. Little further two-electron reduction of the hydroxylamines to the corresponding 2- and 4-amines was observed. The reduction of CB1954 catalyzed by the neuronal NOS (nNOS) was inhibited by O 2 and a flavin/NADPH binding inhibitor, diphenyliodonium (DPI), but insensitive to the addition of the heme ligands imidazole and carbon monoxide and of l-arginine analogues. This reduction proceeded with intermediate formation of a nitro-anion free radical observed by EPR. Involvement of the reductase domain of nNOS in the reduction of CB1954 was confirmed by the ability of the isolated reductase domain of nNOS to catalyze the reaction and by the stimulating effect of Ca (2+)/calmodulin on the accumulation of 4- and 2-hydroxylamines. The recombinant inducible and endothelial NOS isoforms reduced CB1954 with lower activity but higher selectivity for the cytotoxic 4-hydroxylamine compared with nNOS. Finally, CB1954 did not modify the formation of l-citrulline and RONS catalyzed by nNOS. Our results show that all three NOS isoforms are involved in the nitroreduction of CB1954, with predominant formation of the cytotoxic 4-hydroxylamine derivative. This nitroreduction could be of interest for the selective activation of prodrugs by NOSs overexpressed in tumor cells.

Unusual regioselectivity and active site topology of human cytochrome P450 2J2

The oxidation of six derivatives of terfenadone by recombinant human CYP2J2 (CYP = cytochrome P450) was studied by high-performance liquid chromatography coupled to mass spectrometry (MS) using tandem MS techniques and by 1H NMR spectroscopy. CYP2J2 exhibited a surprising regioselectivity in favor of the hydroxylation of the substrate terminal chain at the weakly reactive homobenzylic position. In contrast, hydroxylation of the same substrates by CYP3A4 mainly occurred on the most chemically reactive sites of the substrates (N-oxidation and benzylic hydroxylation). A 3D homology model of CYP2J2 was constructed using recently published structures of CYP2A6, CYP2B4, CYP2C8, CYP2C9, and CYP2D6 as templates. In contrast with other CYP2 structures, it revealed an active site cavity with a severely restricted access of substrates to the heme through a narrow hydrophobic channel. Dynamic docking of terfenadone derivatives in the CYP2J2 active site allowed one to interpret the unexpected regioselectivity of the hydroxylation of these substrates by CYP2J2, which is mainly based on this restricted access to the iron. The structural features that have been found to be important for recognition of substrates or inhibitors by CYP2J2 were also interpreted on the basis of CYP2J2-substrate interactions in this model.

Great adaptability of the heme-cysteinate monooxygenases family to very diverse substrates and sophisticated reactions

Heme-cysteinate proteins, such as cytochromes P 450( CYPs ) and nitric oxide synthases (NOSs), catalyze the monooxygenation of a huge number of substrates with very diverse structures. The ability of CYPs to oxidize a myriad of xenobiotics, in order to facilitate their elimination, plays a key role in the adaptation of aerobic organisms to their always changing chemical environment. Moreover, some members of the CYP superfamily and the NOSs are involved in the biosynthesis of key biological endogenous molecules, such as estrogens or NO, through the catalysis of highly sophisticated and regulated reactions. How can proteins using the same catalytic heme-cysteinate cofactor and mechanism of dioxygen activation oxidize such diverse and always changing substrates and catalyze different, sometimes very sophisticated reactions? Recent data on the first X-ray structures of mammalian cytochrome P 450-substrate complexes and on the mechanism of NO-synthases has permitted an understanding of this"double adaptation" of heme-cysteinate monooxygenases towards very diverse substrates and different reactions. These data show that cytochromes P 450 involved in the metabolism of xenobiotics are able to oxidize very different substrates by offering a great choice of very diverse and malleable active sites. They also show that heme-cysteinate monooxygenases are able to catalyze special, sophisticated reactions, such as the selective oxidation of L-arginine to NO, by using supplementary cofactors adapted for the required catalysis.

Analogies and surprising differences between recombinant nitric oxide synthase-like proteins from Staphylococcus aureus and Bacillus anthracis in their interactions with l-arginine analogs and iron ligands

Genome sequencing has recently shown the presence of genes coding for NO-synthase (NOS)-like proteins in bacteria. The roles of these proteins remain unclear. The interactions of a series of l-arginine (l-arg) analogs and iron ligands with two recombinant NOS-like proteins from Staphylococcus aureus (saNOS) and Bacillus anthracis (baNOS) have been studied by UV-visible spectroscopy. SaNOS and baNOS in their ferric native state, as well as their complexes with l-arg analogs and with various ligands, exhibit spectral characteristics highly similar to the corresponding complexes of heme-thiolate proteins such as cytochromes P450 and NOSs. However, saNOS greatly differs from baNOS at the level of three main properties: (i) native saNOS mainly exists under an hexacoordinated low-spin ferric state whereas native baNOS is mainly high-spin, (ii) the addition of tetrahydrobiopterin (H4B) or H4B analogs leads to an increase of the affinity of l-arg for saNOS but not for baNOS, and (iii) saNOS Fe(II), contrary to baNOS, binds relatively bulky ligands such as nitrosoalkanes and tert-butylisocyanide. Thus, saNOS exhibits properties very similar to those of the oxygenase domain of inducible NOS (iNOS(oxy)) not containing H4B, as expected for a NOSoxy-like protein that does not contain H4B. By contrast, the properties of baNOS which look like those of H4B-containing iNOS(oxy) are unexpected for a NOS-like protein not containing H4B. The origin of these surprising properties of baNOS remains to be determined.

Differential Effects of Alkyl- and Arylguanidines on the Stability and Reactivity of Inducible NOS Heme−Dioxygen Complexes

NO-Synthases are heme proteins that catalyze the oxidation of L-arginine into NO and L-citrulline. Some non-amino acid alkylguanidines may serve as substrates of inducible NOS (iNOS), while no NO* production is obtained from arylguanidines. All studied guanidines induce uncoupling between electrons transferred from the reductase domain and those required for NO formation. This uncoupling becomes critical with arylguanidines, leading to the exclusive formation of superoxide anion O2*- as well as hydrogen peroxide H2O2. To understand these different behaviors, we have conducted rapid scanning stopped-flow experiments with dihydrobiopterin (BH2) and tetrahydrobiopterin (BH4) to study, respectively, the (i) autoxidation and (ii) activation processes of heme ferrous-O2 complexes (Fe(II)O2) in the presence of eight alkyl- and arylguanidines. The Fe(II)O2 complex is more easily autooxidized by alkylguanidines (10-fold) and arylguanidines (100-fold) compared to L-arginine. In the presence of alkylguanidines and BH4, the oxygen-activation kinetics are very similar to those observed with L-arginine. Conversely, in the presence of arylguanidines, no Fe(II)O2 intermediate is detected. To understand such variations in reactivity and stability of Fe(II)O2 complex, we have characterized the effects of alkyl- and arylguanidines on Fe(II)O2 structure using the Fe(II)CO complex as a mimic. Resonance Raman and FTIR spectroscopies show that the two classes of guanidine derivatives induce different polar effects on Fe(II)CO environment. Our data suggest that the structure of the substituted guanidine can modulate the stability and the reactivity of heme-dioxygen complexes. We thus propose differential mechanisms for the electron- and proton-transfer steps in the NOS-dependent, oxygen-activation process, contingent upon whether alkyl- or arylguanidines are bound.

Design and synthesis of selective, high-affinity inhibitors of human cytochrome P450 2J2

The active site topology, substrate specificity, and biological roles of the human cytochrome P450 CYP2J2, which is mainly expressed in the cardiovascular system, are poorly known even though recent data suggest that it could be a novel biomarker and potential target for therapy of human cancer. This paper reports a first series of high-affinity, selective CYP2J2 inhibitors that are related to terfenadine, with K(i) values as low as 160nM, that should be useful tools to determine the biological roles of CYP2J2.

Reactivity of the heme-dioxygen complex of the inducible nitric oxide synthase in the presence of alternative substrates

Single turnover reactions of the inducible nitric oxide synthase oxygenase domain (iNOSoxy) in the presence of several non alpha-amino acid N-hydroxyguanidines and guanidines were studied by stopped-flow visible spectroscopy, and compared with reactions using the native substrates L-arginine (L-arg) or N(omega)-hydroxy-L-arginine (NOHA). In experiments containing dihydrobiopterin, a catalytically incompetent pterin, and each of the studied substrates, L-arg, butylguanidine (BuGua), para-fluorophenylguanidine (FPhGua), NOHA, N-butyl- and N-(para-fluorophenyl)-N'-hydroxyguanidines (BuNOHG and FPhNOHG), the formation of a iron(II) heme-dioxygen intermediate (Fe(II)O2) was always observed. The Fe(II)O2 species then decayed to iron(III) iNOSoxy at rates that were dependent on the nature of the substrate. Identical reactions containing the catalytically competent cofactor tetrahydrobiopterin (BH4), iNOSoxy and the three N-hydroxyguanidines, all exhibited an initial formation of an Fe(II)O2 species that was successively converted to an Fe(III)NO complex and eventually to high-spin iron(III) iNOSoxy. The formation and decay kinetics of the Fe(III)NO complex did not vary greatly as a function of the N-hydroxyguanidine structure, but the formation of Fe(III)NO was substoichiometric in the cases of BuNOHG and FPhNOHG. Reactions between BH4-containing iNOSoxy and BuGua exhibited kinetics similar to those of the corresponding reaction with L-arginine, with formation of an Fe(II)O2 intermediate that was directly converted to high-spin iron(III) iNOSoxy. In contrast, no Fe(II)O2 intermediate was observed in the reaction of BH4-containing iNOSoxy and FPhGua. Multi-turnover reaction of iNOS with FPhGua did not lead to formation of NO or to hydroxylation of the substrate, contrary to reactions with BuGua or L-arg. Our results reveal how different structural and chemical properties of NOS substrate analogues can impact on the kinetics and reactivity of the Fe(II)O2 intermediate, and support an important role for substrate pKa during NOS oxygen activation.

Reduction of ferric haemoproteins by tetrahydropterins: a kinetic study

We previously showed that one-electron transfer from tetrahydropterins to iron porphyrins is a very general reaction, with formation of an intermediate cation radical similar to the one detected in NO synthase. As a model reaction, the rates of reduction of eight haemoproteins by diMePH4 (6,7-dimethyltetrahydropterin) have been studied and correlated with their one-electron reduction potentials, E(m) (Fe(III)/Fe(II)). On the basis of kinetic data analyses, a bimolecular collisional mechanism is proposed for the electron transfer from diMePH4 to ferrihaemoproteins. Haemoproteins with reduction potentials below -160 mV were shown not to be reduced by diMePH4 to the corresponding ferrohaemoproteins. For haemoproteins with reduction potentials more positive than -160 mV, such as chloroperoxidase, cytochrome b5, methaemoglobin and cytochrome c, there was a good correlation between the second-order reduction rate constant and the redox potential, E(m) (Fe(III)/Fe(II)): [formula: see text]. The rate of reduction of cytochrome c by BH4 [(6R)-5,6,7,8-tetrahydrobiopterin] was determined to be similar to that of the reduction of cytochrome c by diMePH4. These results confirm the role of tetrahydropterins as one-electron donors to Fe(III) porphyrins.

Synthesis and evaluation of new omega-borono-alpha-amino acids as rat liver arginase inhibitors

Recent studies have demonstrated that arginase plays important roles in pathologies such as asthma or erectile dysfunctions. We have synthesized new omega-borono-alpha-amino acids that are analogues of the previously known arginase inhibitors S-(2-boronoethyl)-l-cysteine (BEC) and 2-amino-6-boronohexanoic acid (ABH) and evaluated them as inhibitors of purified rat liver arginase (RLA). In addition to the distance between the B(OH)(2) and the alpha-amino acid functions, the position of the sulfur atom in the side chain also appears as a key determinant for the interaction with the active site of RLA. Furthermore, substitution of the alkyl side chain of BEC by methyl groups and conformational restriction of ABH by incorporation of its side chain in a phenyl ring led to inactive compounds. These results suggest that subtle interactions govern the affinity of inhibitors for the active site of RLA.

Nomega-hydroxy-L-arginine homologues and hydroxylamine as nitric oxide-dependent vasorelaxant agents

Endothelium-independent relaxant activities of N(omega)-hydroxy-L-arginine (L-NOHA) homologues and hydroxylamine, a possible intermediate in nitric oxide (NO) formation, were examined in rat aortic rings. Addition of one -CH(2)- group to the -(CH(2))(x)- chain between the alpha-amino acid and the hydroxyguanidine group (x=4) almost abolished-while deletion of one or two -CH(2)- (x=1 or 2) enhanced-the relaxant activity of L-NOHA homologues. N(omega)-hydroxy-nor-L-arginine- (x=2) and hydroxylamine-induced relaxations were blunted by a NO scavenger and by inhibitors of the guanylyl cyclase pathway, but not by NO synthase or cytochrome P(450) inhibitors (except 7-ethoxyresorufin). However, aortic NO formation was detected (using electron paramagnetic resonance) in the presence of concentrations of these compounds higher than those producing relaxation. These findings support the view that endothelium-independent vasorelaxations induced by both L-NOHA homologues with a required chain length x</=3 and hydroxylamine are mediated by NO-dependent activation of guanylyl cyclase, through a 7-ethoxyresorufin-inhibited mechanism.

First evidence that cytochrome P450 may catalyze both S-oxidation and epoxidation of thiophene derivatives

Oxidation of 2-phenylthiophene (2PT) by rat liver microsomes, in the presence of NADPH and glutathione (GSH), led to three kinds of metabolites whose structures were established by 1H NMR and mass spectrometry. The first ones were 2PT-S-oxide dimers formed by Diels-Alder type dimerization of 2PT-S-oxide, while the second ones were GSH adducts derived from the 1,4-Michaël-type addition of GSH to 2PT-S-oxide. The third metabolites were GSH adducts resulting from a nucleophilic attack of GSH to the 4,5-epoxide of 2PT. Oxidation of 2PT by recombinant, human cytochrome P4501A1, in the presence of NADPH and GSH, also led to these three kinds of metabolites. These results provide the first evidence that cytochrome P450 may catalyze the oxidation of thiophene compounds with the simultaneous formation of two reactive intermediates, a thiophene-S-oxide and a thiophene epoxide.

Relationship between the structure of guanidines and N-hydroxyguanidines, their binding to inducible nitric oxide synthase (iNOS) and their iNOS-catalysed oxidation to NO

The binding of several alkyl- and aryl-guanidines and N-hydroxyguanidines to the oxygenase domain of inducible NO-synthase (iNOS(oxy)) was studied by UV/Vis difference spectroscopy. In a very general manner, monosubstituted guanidines exhibited affinities for iNOS(oxy) that were very close to those of the corresponding N-hydroxyguanidines. The highest affinities were observed for the natural substrates, L-arginine and N(omega)-hydroxy-L-arginine (K(d) at the microm level). The deletion of either the CO2H or the NH2 function of their amino acid moiety led to dramatic decreases in the affinity. However, alkylguanidines with a relatively small alkyl chain exhibited interesting affinities, the best being observed for a butyl chain (K(d) =20 microM). Arylguanidines also bound to iNOS(oxy), however, with lower affinities (K(d) > 250 microm). Many N-alkyl- and N-aryl-N'-hydroxyguanidines are oxidized by iNOS with formation of NO, whereas only few alkylguanidines led to significant production of NO under identical conditions, and all the arylguanidines tested to date were unable to lead to the production of NO. The k(cat) values of NO production from the oxidation by iNOS of the studied N-hydroxyguanidines were found to vary independently of their affinity for the protein. The k(cat) values determined for the two-step oxidation of alkylguanidines to NO were not clearly related to the K(d) of these substrates toward iNOS(oxy). However, there is a qualitative relationship between these k(cat) values and the apparent rate constants of dissociation of the complex between iNOS(oxy) and the corresponding N-alkyl-N'-hydroxyguanidine (k(off) (app)) that were determined by stopped-flow UV/Vis spectroscopy. These data indicate that a key factor for efficient oxidation of a guanidine by iNOS to NO is the ability of the corresponding N-hydroxyguanidine to bind to the active site without being too rapidly released before its further oxidation. This explains why 4,4,4-trifluorobutylguanidine is so far the best non-alpha-amino acid guanidine substrate of iNOS with formation of NO, because the k(off) (app) of the corresponding N-hydroxyguanidine is particularly low. This suggests that the rational design of guanidines as new NO donors upon in situ oxidation by NOSs should take into account both thermodynamic and kinetic characteristics of the interaction of the protein not only with the guanidine but also with the corresponding N-hydroxyguanidine.

Importance of valine 567 in substrate recognition and oxidation by neuronal nitric oxide synthase

Nitric oxide (NO) is synthesised by a two-step oxidation of -arginine (L-Arg) in the active site of nitric oxide synthase (NOS) with formation of an intermediate, N omega-hydroxy-L-Arg (NOHA). Crystal structures of NOSs have shown the importance of an active-site Val567 residue (numbered for rat neuronal NOS, nNOS) interacting with non-amino acid substrates. To investigate the role of this Val residue in substrate recognition and NO-formation activity by nNOS, we generated and purified four Val567 mutants of nNOS, Val567Leu, Val567Phe, Val567Arg and Val567Glu. We characterized these proteins and tested their ability to generate NO from the oxidation of natural substrates L-Arg and NOHA, and from N-hydroxyguanidines previously identified as alternative substrates for nNOS. The Val567Leu mutant displayed lower NO formation activities than the wild type (WT) in the presence of all tested compounds. Surprisingly, the Val567Phe mutant formed low amounts of NO only from NOHA. These two mutants displayed lower affinity for L-Arg and NOHA than the WT protein. Val576Glu and Val567Arg mutants were much less stable and did not lead to any formation of NO. These results suggest that Val567 is an important residue for preserving the integrity of the active site, for substrate binding, and subsequently for NO-formation in nNOS.

Analysis of human cytochrome P450 2C8 substrate specificity using a substrate pharmacophore and site-directed mutants

The structural determinants of substrate specificity of human liver cytochrome P450 2C8 (CYP2C8) were investigated using site-directed mutants chosen on the basis of a preliminary substrate pharmacophore and a three-dimensional (3D) model. Analysis of the structural features common to CYP2C8 substrates exhibiting a micromolar K(m) led to a substrate pharmacophore in which the site of oxidation by CYP2C8 is 12.9, 8.6, 4.4, and 3.9 A from features that could establish ionic or hydrogen bonds, and hydrophobic interactions with protein amino acid residues. Comparison of this pharmacophore with a 3D model of CYP2C8 constructed using the X-ray structure of CYP2C5 suggested potential CYP2C8 amino acid residues that could be involved in substrate recognition. Twenty CYP2C8 site-directed mutants were constructed and expressed in yeast to compare their catalytic activities using five CYP2C8 substrates that exhibit different structures and sizes [paclitaxel, fluvastatin, retinoic acid, a sulfaphenazole derivative (DMZ), and diclofenac]. Mutation of arginine 241 had marked effects on the hydroxylation of anionic substrates of CYP2C8 such as retinoic acid and fluvastatin. Serine 100 appears to be involved in hydrogen bonding interactions with a polar site of the CYP2C8 substrate pharmacophore, as shown by the 3-4-fold increase in the K(m) of paclitaxel and DMZ hydroxylation after the S100A mutation. Residues 114, 201, and 205 are predicted to be in close contact with substrates, and their mutations lead either to favorable hydrophobic interactions or to steric clashes with substrates. For instance, the S114F mutant was unable to catalyze the 6alpha-hydroxylation of paclitaxel. The S114F and F205A mutants were the best catalysts for retinoic acid and paclitaxel (or fluvastatin) hydroxylation, respectively, with k(cat)/K(m) values 5 and 2.1 (or 2.4) times higher, respectively, than those found for CYP2C8. Preliminary experiments of docking of the substrate into the experimentally determined X-ray structure of substrate-free CYP2C8, which became available quite recently [Schoch, G. A., et al. (2004) J. Biol. Chem. 279, 9497], were consistent with key roles for S100, S114, and F205 residues in substrate binding. The results suggest that the effects of mutation of arginine 241 on anionic substrate hydroxylation could be indirect and result from alterations of the packing of helix G with helix B'.

JunD reduces tumor angiogenesis by protecting cells from oxidative stress

Reactive oxygen species (ROS) are implicated in the pathophysiology of various diseases, including cancer. In this study, we show that JunD, a member of the AP-1 family of transcription factors, reduces tumor angiogenesis by limiting Ras-mediated production of ROS. Using junD-deficient cells, we demonstrate that JunD regulates genes involved in antioxidant defense, H2O2 production, and angiogenesis. The accumulation of H2O2 in junD-/- cells decreases the availability of FeII and reduces the activity of HIF prolyl hydroxylases (PHDs) that target hypoxia-inducible factors-alpha (HIFalpha) for degradation. Subsequently, HIF-alpha proteins accumulate and enhance the transcription of VEGF-A, a potent proangiogenic factor. Our study uncovers the mechanism by which JunD protects cells from oxidative stress and exerts an antiangiogenic effect. Furthermore, we provide new insights into the regulation of PHD activity, allowing immediate reactive adaptation to changes in O2 or iron levels in the cell.

Alternative nitric oxide-producing substrates for NO synthases

Nitric oxide (NO) is a key inter- and intracellular molecule involved in the maintenance of vascular tone, neuronal signaling, and host response to infection. The biosynthesis of NO in mammals involves a two-step oxidation of L-arginine (L-Arg) to citrulline and NO catalyzed by a particular class of heme-thiolate proteins, called NO-synthases (NOSs). The NOSs successively catalyze the Nomega-hydroxylation of the guanidine group of L-Arg with formation of Nomega-hydroxy-L-arginine (NOHA) and the oxidative cleavage of the CN(OH) bond of NOHA with formation of citrulline and NO. During the last decade, a great number of compounds bearing a CNH or CNOH function have been synthesized and studied as possible NO-producing substrates of recombinant NOSs. This includes derivatives of L-Arg and NOHA, N-alkyl (or aryl) guanidines, N,N'- or N,N-disubstituted guanidines, N-alkyl (or aryl) N'-hydroxyguanidines, N- (or O-) disubstituted N'-hydroxyguanidines, as well as amidoximes, ketoximes, and aldoximes. However, only those involving the NHC(NH2)=NH (or NOH) moiety have led to a significant formation of NO. All the N-monosubstituted N'-hydroxyguanidines that are well recognized by the NOS active site lead to NO with catalytic efficiences (kcat/Km) up to 50% of that of NOHA. This is the case of many N-aryl and N-alkyl N'-hydroxyguanidines, provided that the aryl or alkyl substituent is small enough to be accommodated by a NOS hydrophobic site located in close proximity of the NOS "guanidine binding site." As far as N-substituted guanidines are concerned, few compounds bearing a small alkyl group have been found to act as NO-producing substrates. The kcat value found for the best compound may reach 55% of the kcat of L-Arg oxidation. However, the best catalytic efficiency (kcat/Km) that was obtained with N-(4,4,4-trifluorobutyl) guanidine is only 100-fold lower than that of L-Arg. In a general manner, NOS II is a better catalyst that NOS I and III for the oxidation of exogenous guanidines and N-hydroxyguanidines to NO. This is particularly true for guanidines as the ones acting as substrates for NOS II have been found to be almost inactive for NOS I and NOS III. Thus, a good NO-producing guanidine substrate for the two latter isozymes remains to be found.