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

Applications for nitric oxide in halting proliferation of tumor cells

Tumor resistance to cytotoxic therapeutics coupled with dose-limiting toxicity is a serious hurdle in the field of medical oncology. In the face of this obstacle, nitric oxide has emerged as a powerful adjuvant for the hypersensitization of tumors to more traditional chemo- and radio-therapeutics. Furthermore, emerging evidence indicates that nitric oxide donors have the potential to function independently in the clinical management of cancer. Herein, we discuss the role of nitric oxide in cancer and the potential for nitric oxide donors to support conventional therapeutics.

Mechanism and kinetics of inducible nitric oxide synthase auto-S-nitrosation and inactivation

Nitric oxide (NO), the product of the nitric oxide synthase (NOS) reaction, was previously shown to result in S-nitrosation of the NOS Zn(2+)-tetrathiolate and inactivation of the enzyme. To probe the potential physiological significance of NOS S-nitrosation, we determined the inactivation time scale of the inducible NOS isoform (iNOS) and found it directly correlates with an increase in the level of iNOS S-nitrosation. A kinetic model of NOS inactivation in which arginine is treated as a suicide substrate was developed. In this model, NO synthesized at the heme cofactor is partitioned between release into solution (NO release pathway) and NOS S-nitrosation followed by NOS inactivation (inactivation pathway). Experimentally determined progress curves of NO formation were fit to the model. The NO release pathway was perturbed through addition of the NO traps oxymyoglobin (MbO(2)) and β2 H-NOX, which yielded partition ratios between NO release and inactivation of ~100 at 4 μM MbO(2) and ~22000 at saturating trap concentrations. The results suggest that a portion of the NO synthesized at the heme cofactor reacts with the Zn(2+)-tetrathiolate without being released into solution. Perturbation of the inactivation pathway through addition of the reducing agent GSH or TCEP resulted in a concentration-dependent decrease in the level of iNOS S-nitrosation that directly correlated with protection from iNOS inactivation. iNOS inactivation was most responsive to physiological concentrations of GSH with an apparent K(m) value of 13 mM. NOS turnover that leads to NOS S-nitrosation might be a mechanism for controlling NOS activity, and NOS S-nitrosation could play a role in the physiological generation of nitrosothiols.

Cryoradiolysis and cryospectroscopy for studies of heme-oxygen intermediates in cytochromes p450

Cryogenic radiolytic reduction is one of the most straightforward and convenient methods of generation and stabilization of reactive iron-oxygen intermediates for mechanistic studies in chemistry and biochemistry. The method is based on one-electron reduction of the precursor complex in frozen solution via exposure to the ionizing radiation at cryogenic temperatures. Such approach allows for accumulation of the fleeting reactive complexes which otherwise could not be generated at sufficient amount for structural and mechanistic studies. Application of this method allowed for characterizing of peroxo-ferric and hydroperoxo-ferric intermediates, which are common for the oxygen activation mechanism in cytochromes P450, heme oxygenases, and nitric oxide synthases, as well as for the peroxide metabolism by peroxidases and catalases.

Synthesis of N-(methoxycarbonylthienylmethyl)thioureas and evaluation of their interaction with inducible and neuronal nitric oxide synthase

Two isomeric N-(methoxycarbonylthienylmethyl)thioureas were synthesised by a sequence of radical bromination of methylthiophenecarboxylic esters, substitution with trifluoroacetamide anion, deprotection, formation of the corresponding isothiocyanates and addition of ammonia. The interaction of these new thiophene-based thioureas with inducible and neuronal nitric oxide synthase was evaluauted. These novel thienylmethylthioureas stimulated the activity of inducible Nitric Oxide Synthase (iNOS).

4,4-Difluorinated analogues of l-arginine and N(G)-hydroxy-l-arginine as mechanistic probes for nitric oxide synthase

4,4-Difluoro-l-arginine and 4,4-difluoro-N(G)-hydroxy-l-arginine were synthesized and shown to be substrates for the inducible isoform of nitric oxide synthase (iNOS). Binding of both fluorinated analogues to the NOS active site was also investigated using a spectral binding assay employing a heme domain construct of the inducible NOS isoform (iNOS(heme)). 4,4-Difluoro-N(G)-hydroxy-arginine was found to bind at the NOS active site in a unique manner consistent with a model involving ligation of the Fe(III) heme center by the oxygen atom of the N(G)-hydroxy moiety.

N(G)-aminoguanidines from primary amines and the preparation of nitric oxide synthase inhibitors

A concise, general, and high-yielding method for the preparation of N(G)-aminoguanidines from primary amines is reported. Using available and readily prepared materials, primary amines are converted to protected N(G)-aminoguanidines in a one-pot procedure. The method has been successfully applied to a number of examples including the syntheses of four nitric oxide synthase (NOS) inhibitors. The inhibitors prepared were investigated as competitive inhibitors and as mechanistic inactivators of the inducible isoform of NOS (iNOS). In addition, one of the four inhibitors prepared, N(G)-amino-N(G)-2,2,2-trifluoroethyl-L-arginine 19, displays the unique ability to both inhibit NO formation and prevent NADPH consumption by iNOS without irreversible inactivation of the enzyme.

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.

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.