Cytochrome c/H2O2-mediated one electron oxidation of carcinogenic N-fluorenylacetohydroxamic acids to nitroxyl free radicals

The oxidation of carcinogenic hydroxamic acids, N-hydroxy-N-2-fluorenylacetamide (N-OH-2-FAA) and N-hydroxy-N-3-fluorenylacetamide (N-OH-3-FAA) catalyzed by horseradish peroxidase (HRP) or cytochrome c in the presence of H2O2 was investigated. HRP/H2O2 was a more efficient system in oxidation of both hydroxamic acids and the standard substrate, guaiacol, then cytochrome c/H2O2. Peroxidative activity of cytochrome c was shown after incubation with Triton X-100 and H2O2 for 20 min at room temperature in 0.05 M phosphate buffer (pH 7.5) or in 0.1 M sodium acetate (pH 6.0) without Triton X-100. Both hydroxamic acids were oxidized to nitroxyl free radicals as shown by electron spin resonance (ESR) spectroscopy. These radicals dismutated to equimolar amounts of 2- or 3-nitrosofluorene and acetate esters of the corresponding hydroxamic acids as shown by thin layer chromatography and spectrophotometric analysis of the products. In addition, large amounts of the N-fluorenylamides were generated in the reactions with cytochrome c/H2O2 system. Of the products, only 2- or 3-nitrosofluorene per se or when generated from the oxidation of the hydroxamic acids, interacted with lecithin (1 mg/ml) to yield ESR signals of the immobilized nitroxyl free radicals. In contrast to HRP/H2O2 system, in which the initial velocity of the radical formation was too fast to measure and the maximal concentrations of the nitroxyl free radicals of both hydroxamic acids were similar, in the cytochrome c/H2O2 system the nitroxyl free radical of N-OH-2-FAA formed at a 6-fold faster rate and accumulated at a 2-fold higher concentration than the radical of N-OH-3-FAA. In both enzyme systems, the persistence of the signal and the length of time before it had decreased to one half its maximum were several-fold longer for the nitroxyl free radical of N-OH-3-FAA than for that of N-OH-2-FAA. These data showed that these nitroxyl free radicals differed in their kinetic properties. One electron oxidation of N-OH-3-FAA by HRP/H2O2 system and of both isomeric hydroxamic acids by cytochrome c/H2O2 system are reported for the first time in this work and may be considered an activation reaction in carcinogenesis by these compounds.     

Estimation of free radical formation by beta-ray irradiation in rat liver

In vivo free radical reactions in rat liver as a result of exposure to low-dose beta-radiation was evaluated with electron paramagnetic resonance (EPR) spectroscopy by monitoring the reduction of the nitroxyl spin probe after intravenous administration. The EPR signal intensity of a nitroxyl probe as a function of time in bile flow was monitored by cannulating the bile duct through the cavity of an X-band EPR spectrometer. The results show that the rate of nitroxyl signal loss was higher in rats whose livers were exposed to beta-rays compared to unexposed rats. However, the rate of signal loss was lower in animals whose organs were exposed to air by opening the abdominal cavity. In vitro experiments also showed that the nitroxyl EPR signal loss was greater in an atmosphere of nitrogen than in air. Results suggest that under low levels of tissue oxygen, exposure to beta-rays results in nitroxyl signal loss, which may be mediated by free radical dependent pathways. When tissue oxygen were higher, hydrogen peroxide mediated oxidation of hydroxylamine may predominate resulting in a signal loss of smaller magnitudes. This study shows possible evidence of reactive oxygen species formation by low-dose beta-ray irradiation in a living animal.     

Hydroxylamine is a vasorelaxant and a possible intermediate in the oxidative conversion of L-arginine to nitric oxide

Our objective was to determine whether hydroxylamine is a possible intermediate in the oxidative conversion of L-arginine to nitric oxide. Vasorelaxation by hydroxylamine is known to be mediated by nitric oxide. The vasorelaxant properties of hydroxylamine were examined using rat aortic rings and an isolated rat lung perfusion model. Hydroxylamine and acetylcholine were equally effective in relaxing norepinephrine-contracted intact aortic rings, whereas only hydroxylamine relaxed aortic rings with endothelium removed. This endothelium-independent vasorelaxation by hydroxylamine indicated that the hydroxylamine-converting enzyme is not localized solely within endothelial cells. Catalase, an enzyme known to oxidize hydroxylamine to nitric oxide, was present in homogenates of intact and endothelium-denuded rings. Cyanamide, another catalase substrate and a known precursor of nitroxyl (HNO), was not a vasorelaxant of aortic rings or of isolated, hypoxia-constricted lungs. These results suggest that free nitroxyl is not an intermediate in the oxidation of hydroxylamine to nitric oxide. An overall pathway for the oxidative conversion of L-arginine through an hydroxylamine intermediate to nitric oxide is proposed.     

A new theoretical approach to simulation of EPR spectra of spin-labels in protein and membrane systems

A certain model of nitroxyl radial movement is used to interpret EPR spectra of spin-labeled products. In the model proposed a fast reorientation of a spin label occurred due to the order parameter, and a slow motion of the carrier of a spin label, already with partially averaged magnetic parameters, is determined by the rotational correlation time of the macromolecule. By the example of different proteins we demonstrate how to get a fairly good coincidence of simulated EPR spectra with experimental ones within that simple model in solution and using an adsorbent. It is minimalistic in sense of involved motion parameters, and thus attempts to minimize the ambiguity of spectra interpretation. We show that TEMPO radical in lipid phase obeys two-motion model, undergoing fast anisotropic reorientation due to being incorporated into axial phase, and, is additionally affected by slow stochastic process characterized by effective nanosecond-scale correlation time, which can be attributed to domain structure of the membrane.     

Obligatory free radical intermediate in the oxidative activation of the carcinogen N-hydroxy-2-acetylaminofluorene

We have demonstrated that the nitroxyl free radical form of the carcinogen N-hydroxy-2-acetylaminofluorene (OH-AAF) is an obligatory intermediate in the cumene hydroperoxide-hematin-induced oxidative activation of this carcinogen into 2-nitrosofluorene and N-acetoxy-2-acetylaminofluorene. Both the rate of N-OH-2-acetylaminofluorene oxidation and the amount of its nitroxyl free radical were experimently observed as a function of reaction time. Rate equations were derived for a model in which the nitroxyl free radical form of OH-AAF was an obligatory intermediate in the reaction. Using this theory it was possible to compute one experimental variable, the rate of OH-AAF oxidation, utilizing the other experimental variable, the amount of nitroxyl free radical present at any time during the reaction. The theory also predicts a linear relationship between the rate of OH-AAF oxidation and the square of the free radical content; and this was found to be true experimentally. The dismutation rate of constant of the nitroxyl free radical of OH-AAF was found to be 2.7 · 10⁵ M^?1 · s^?1.     

Concentration dependence of nitroxyl spin probes in liposomal solution: electron spin resonance and overhauser-enhanced magnetic resonance studies

In this work, the detailed studies of electron spin resonance (ESR) and overhauser-enhanced magnetic resonance imaging (OMRI) were carried out for permeable nitroxyl spin probe, MC-PROXYL as a function of agent concentration in liposomal solution. In order to compare the impermeable nature of nitroxyl radical, the study was also carried out only at 2?mM concentration of carboxy-PROXYL. The ESR parameters were estimated using L-band and 300?MHz ESR spectrometers. The line width broadening was measured as a function of agent concentration in liposomal solution. The estimated rotational correlation time is proportional to the agent concentration, which indicates that less mobile nature of nitroxyl spin probe in liposomal solution. The partition parameter and permeability values indicate that the diffusion of nitroxyl spin probe distribution into the lipid phase is maximum at 2?mM concentration of MC-PROXYL. The dynamic nuclear polarization (DNP) parameters such as DNP factor, longitudinal relaxivity, saturation parameter, leakage factor and coupling factor were estimated for 2?mM MC-PROXYL in 400?mM liposomal dispersion. The spin lattice relaxation time was shortened in liposomal solution, which leads to the high relaxivity. Reduction in coupling factor is due to less interaction between the electron and nuclear spins, which causes the reduction in enhancement. The leakage factor increases with increasing agent concentration. The increase in DNP enhancement was significant up to 2?mM in liposomal solution. These results paves the way for choosing optimum agent concentration and OMRI scan parameters used in intra and extra membrane water by loading the liposome vesicles with a lipid permeable nitroxyl spin probes in OMRI experiments.     

Affinity chromatography of the Neurospora NADP-specific glutamate dehydrogenase, its mutational variants and hybrid hexamers.

The synthesis of an affinity adsorbent, 8-(6-aminohexyl)aminoadenosine 2'-phosphate-Sepharose 4B, is described. The assembly of the 2'-AMP ligand and the hexanediamide spacer arm was synthesized in free solution before its attachment to the Sepharose matrix. This adsorbent retarded the hexameric NADP-specific glutamate dehydrogenase of Neurospora crassa, showing a capacity for this enzyme similar to that of comparable coenzyme-analogue adsorbents for other dehydrogenases. The enzyme was eluted either at pH 6.8 in a concentration gradient of NADP+, or at pH 8.5 in the presence of NADP+ in concentration gradients of either dicarboxylates or NaCl. Anomalous effects of dicarboxylates in facilitating elution are discussed. 2'-AMP and its derivatives, 8-bromoadenosine 2'-phosphate and 8-(l-aminohexyl)aminoadenosine 2'-phosphate, which were used in the synthesis of the adsorbent, all acted as enzyme inhibitors competitive with NADP+. The chromatographic properties of the wild-type enzyme were compared with those of mutationally modified variants containing defined amino acid substitutions. This approach was used to assess the biospecificity of adsorption and elution and the contribution of non-specific binding. The adsorbent showed a low capacity for the enzyme from mutant am1 (Ser-336 replaced by Phe), a variant that has a localized defect in NADP binding, but an otherwise almost normal conformation, suggesting that non-specific interactions are at most weak. The enzyme from mutant am3, a variant modified in a conformational equilibrium, was fully retarded by the adsorbent, but showed a significantly earlier elution position than the wild-type enzyme. This is consistent with measurements in free solution that showed the am3 enzyme to have a higher Ki for 2'-AMP than the wild-type enzyme. The enzyme from mutant am19 was eluted as two distinct peaks at both pH 6.8 and 8.5. The adsorbent was used to separate hybrid hexamers constructed in vitro by a freeze-thaw procedure from pairs of purified variants. Several chromatographically distinct peaks of differing enzymological properties were purified from each hybridization mixture in quantities of up to a few milligrams, and represented distinct species of hybrid hexamers differing in subunit ratio.     

Spectral characteristics of the mechanism of oxidase activity of ceruloplasmin

The absorbance and EPR spectra of type 1 and 2 copper-binding centres which are present in ceruloplasmin (Cp) molecule were shown to disappear upon the reduction of the enzyme by ascorbate under anaerobic conditions. The fluorescence band attributed to type 3 Cu was altered concomitantly. The electron-accepting nitroxyl radical added to reduced Cp restored the absorbance, EPR and fluorescence spectra of the oxidase. Only type 1 and 3 copper ions, as judged by spectral changes, can be reduced by ascorbate and then reoxidized by the nitroxyl radical in the azide-treated Cp. The spectral properties of Cp provided by copper ions of different types change simultaneously and concordantly upon oxidation/reduction. This seems to be caused by cooperative interaction of these ions involved in the electron transfer from the donating substrate to the accepting molecule of the nitroxyl radical (in model studies of oxidase reaction) or oxygen (under natural conditions). The copper ions in the active centre of Cp constitute an intramolecular electron transport chain, which may, at least in vitro, function without one of its links.     

Noninvasive evaluation of in vivo free radical reactions catalyzed by iron using in vivo ESR spectroscopy.

The noninvasive, real time technique of in vivo electron spin resonance (ESR) spectroscopy was used to evaluate free radical reactions catalyzed by iron in living mice. The spectra and signal decay of a nitroxyl probe, carbamoyl-PROXYL, were observed in the upper abdomen of mice. The signal decay was significantly enhanced in mice subcutaneously loaded with ferric citrate (0.2 micromol/g body wt) and the enhancement was suppressed by pre-treatment with either desferrioxamine (DF) or the chain breaking antioxidant Trolox, but only slightly suppressed by the hydroxyl radical scavenger DMSO. To determine the catalytic form of iron, DF was administered at different times with respect to iron loading: before, simultaneously, and after 20 and 50 min. The effect of DF on signal decay, liver iron content, iron excretion, and lipid peroxidation (TBARs) depended on the time of the treatment. There was a good correlation between the signal decay, iron content, and lipid peroxidation, indicating that "chelatable iron" contributed to the enhanced signal decay. The nitroxyl probe also exhibited in vivo antioxidant activity, implying that the process responsible for the signal decay of the nitroxyl probe is involved in free radical oxidative stress reactions catalyzed by iron.     

Isolation of cathepsins D by affinity chromatography]

The paper deals with the isolation of cathepsin D from rat liver, chicken liver and bovine spleen by affinity chromatography. The synthesis of the adsorbent was performed using the competitive inhibitor of cathepsins D and pepsin pepstatin and activated Sepharose. Application of a pepstatin-Sepharose column allows obtaining a highly active and purified enzyme in a short period of time.     

Quantification of lipid alkyl radicals trapped with nitroxyl radical via HPLC with postcolumn thermal decomposition

Lipid alkyl radicals generated from polyunsaturated fatty acids via chemical or enzymatic H-abstraction have been a pathologically important target to quantify. In the present study, we established a novel method for the quantification of lipid alkyl radicals via nitroxyl radical spin-trapping. These labile lipid alkyl radicals were converted into nitroxyl radical-lipid alkyl radical adducts using 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrroline-N-oxyl (CmdeltaP) (a partition coefficient between octanol and water is approximately 3) as a spin-trapping agent. The resulting CmdeltaP-lipid alkyl radical adducts were determined by HPLC with postcolumn online thermal decomposition, in which the adducts were degraded into nitroxyl radicals by heating at 100 degrees C for 2 min. The resulting nitroxyl radicals were selectively and sensitively detected by electrochemical detection. With the present method, we, for the first time, determined the lipid alkyl radicals generated from linoleic acid, linolenic acid, and arachidonic acid via soybean lipoxygenase-1 or the radical initiator 2,2'-azobis(2,4-dimethyl-valeronitrile).     

Control of an affinity purification procedure using a thermal biosensor

Lactate dehydrogenase (LDH) was recovered from a solution by affinity binding to an N(6)-(6-aminohexyl)-AMP-Sepharose gel. An enzyme thermistor unit was employed to continously measure the activity of the unbound LDH. The enzyme activity signal from the enzyme thermistor was used in a PID controller to regulate the addition of AMP-Sepharose gel to the LDH solution. In another type of experiment, a desktop computer was utilized to control the addition of the adsorbent. Both systems worked satisfactorily, and enabled a rapid and accurate assessment of correct addition of adsorbent.     

In vitro cytotoxicity of nitroxyl radicals with respect to tumor and diploid human cells and estimation of their antiviral activity

Thirty nine water soluble nitroxyl radicals of various classes, belonging to piperidine, pyrrolidine and imidazolidine series were synthesized. Twenty seven of them were cytotoxic in vitro with respect to the tumor cell culture A431. The CC50 of the most active nitroxyl radicals with respect to cells SW480 and A431 was within 0.16-2.5 mM at the selectivity index of 3.91-7.81 in relation to cytotoxicity of the compounds for the cells of the normal L68 phenotype and tumor cells. The tests on the antiviral activity showed that 16 out of 22 nitroxyl radicals had antiviral activity in Vero cell culture with respect to the West Nile virus and Herpes simplex virus of type II respectively. The EC50 ranged within 0.09-3.45 mM. Some of the nitroxyl radicals had only antiviral activity, but a number of the compounds had both cytotoxic properties and antiviral activity.     

Interaction of spin-labeled methacyne analog with butyrylcholinesterase

Interaction between spin-labeled methacyne (I) and butyrylcholinesterase (BChE) was studied by ESR and enzyme kinetic methods. The compound (I) was shown to be a competitive reversible inhibitor, the value of Ki appeared to be 1.3 X 10(-5) M. Insertion of nitroxyl fragment in the methacyne molecule results in a two-fold increase of its inhibitory activity. The ESR spectrum of the enzyme-inhibitor complex was registered. This complex dissociates under the action of eserine, tetramethylammonium and hexamethonium. Scatchard plot reveals two different types of binding sites with Kdiss values 1.5 X 10(-5) M and 2.6 X 10(-4) M. One type of binding sites is identified as the enzyme active centre. The restricted motion of (I) in complex with BChE proves the assumption that the enzyme active centre is located in the split of macromolecule surface.     

Arginine conversion to nitroxide by tetrahydrobiopterin-free neuronal nitric-oxide synthase. Implications for mechanism

We studied catalysis by tetrahydrobiopterin (H4B)-free neuronal nitric-oxide synthase (nNOS) to understand how heme and H4B participate in nitric oxide (NO) synthesis. H4B-free nNOS catalyzed Arg oxidation to N(omega)-hydroxy-l-Arg (NOHA) and citrulline in both NADPH- and H(2)O(2)-driven reactions. Citrulline formation was time- and enzyme concentration-dependent but was uncoupled relative to NADPH oxidation, and generated nitrite and nitrate without forming NO. Similar results were observed when NOHA served as substrate. Steady-state and stopped-flow spectroscopy with the H4B-free enzyme revealed that a ferrous heme-NO complex built up after initiating catalysis in both NADPH- and H(2)O(2)-driven reactions, consistent with formation of nitroxyl as an immediate product. This differed from the H4B-replete enzyme, which formed a ferric heme-NO complex as an immediate product that could then release NO. We make the following conclusions. 1) H4B is not essential for Arg oxidation by nNOS, although it helps couple NADPH oxidation to product formation in both steps of NO synthesis. Thus, the NADPH- or H(2)O(2)-driven reactions form common heme-oxy species that can react with substrate in the presence or absence of H4B. 2) The sole essential role of H4B is to enable nNOS to generate NO instead of nitroxyl. On this basis we propose a new unified model for heme-dependent oxygen activation and H4B function in both steps of NO synthesis.     

Beta-glycosylamidine as a ligand for affinity chromatography tailored to the glycon substrate specificity of beta-glycosidases

An affinity adsorbent for beta-glycosidases has been prepared by using beta-glycosylamidine as a ligand. beta-Glucosylamidine and beta-galactosylamidine, highly potent and selective inhibitors of beta-glucosidases and beta-galactosidases, respectively, were immobilized by a novel one-pot procedure involving the addition of a beta-glycosylamine and 2-iminothiolane.HCl simultaneously to a matrix modified with maleimido groups via an appropriate spacer to give an affinity adsorbent for beta-glucosidases and beta-galactosidases, respectively. This one-pot procedure enables various beta-glycosylamidine ligands to be formed and immobilized conveniently according to the glycon substrate specificities of the enzymes. A crude enzyme extract from tea leaves (Camellia sinensis) and a beta-galactosidase from Penicillium multicolor were chromatographed directly on each affinity adsorbent to give a beta-glucosidase and a beta-galactosidase to apparent homogeneity in one step by eluting the column with glucose or by a gradient NaCl elution, respectively. The beta-glucosidase and beta-galactosidase were inhibited competitively by a soluble form of the corresponding beta-glycosylamidine ligand with an inhibition constant (K(i)) of 2.1 and 0.80 microM, respectively. Neither enzyme was bound to the adsorbent with a mismatched ligand, indicating that the binding of the glycosidases was of specific nature that corresponds to the glycon substrate specificity of the enzymes. The ease of preparation and the selective nature of the affinity adsorbent should promise a large-scale preparation of the affinity adsorbent for the purification and removal of specific glycosidases according to their glycon substrate specificities.     

Nitric oxide irreversibly inhibits cytochrome oxidase at low oxygen concentrations: evidence for inverse oxygen concentration-dependent peroxynitrite formation

The present study shows that nitric oxide (NO) irreversibly inhibits purified cytochrome oxidase in a reverse oxygen concentration-dependent manner. The inhibition is dramatically protected by a peroxynitrite scavenger, suggesting that peroxynitrite is formed from the reaction of NO with cytochrome oxidase at low oxygen concentration, and that peroxynitrite is involved in irreversible cytochrome oxidase inactivation. Production of nitroxyl anion or superoxide was tested as potential mechanisms underlying the conversion of NO to peroxynitrite. A nitroxyl anion scavenger potently protected the irreversible inhibition, whereas a superoxide dismutase did not provide protective effect, suggesting that the peroxynitrite was formed from nitroxyl anion rather than the reaction of NO with superoxide.     

Recovery of Acinetobacter radioresistens lipase by hydrophobic adsorption to n-hexadecane coated on nonwoven fabric

A simple and clean adsorption/desorption process was proposed for recovering Acinetobacter radioresistens lipase from fermentation broth. The adsorbent used was n-hexadecane coated on a hydrophobic nonwoven fabric (NWF). n-Hexadecane has a melting point of 16-18 degrees C, and its affinity for lipase decreases markedly from liquid to solid state. Accordingly, performing the adsorption and desorption above and below, respectively, the melting point would need no extraneous materials for separation. The adsorption isotherms at various temperatures were found to follow the Langmuir model. Simulation of the batch adsorption/desorption process showed that there exists an optimal amount of adsorbent for both concentration factor and enzyme recovery; the process is restrained by equilibrium. The performance of column adsorption/desorption could also be simulated using the adsorption isotherm, and it was shown that the concentration factor was proportional to the amount of adsorbent used. The benefits of this process include easy preparation of adsorbent, low operational cost, no extraneous materials needed, negligible enzyme denaturation, high efficiency, and simple process simulation.     

Exposure of Mn and FeSODs, but not Cu/ZnSOD, to NO leads to nitrosonium and nitroxyl ions generation which cause enzyme modification and inactivation: an in vitro study

The effect of NO treatment in vitro on structural and functional alterations of Cu/Zn, Mn, and Fe type of SODs was studied. Significant difference in response to NO of Cu/ZnSOD compared to the Mn and Fe types was demonstrated. Cu/ZnSOD was shown to be stable with respect to NO: even on prolonged exposure, NO produced negligible effect on its structure and activity. In contrast, both Mn and Fe types were found to be NO-sensitive: exposure to NO led to their fast and extensive inactivation, which was accompanied by extensive structural alterations, including (in some of the samples tested) the cleavage of enzyme polypeptide chains, presumably at His residues of the enzyme metal binding sites. The generation of nitrosonium (NO+) and nitroxyl (NO-) ions in NO treated Mn and FeSODs, which produce enzyme modifications and inactivation, was demonstrated. The physiological and biomedical significance of described findings is briefly discussed.     

An affinity adsorbent containing deoxyguanosine 5'-triphosphate linked to sepharose and its use for large scale preparation of ribonucleotide reductase of Lactobacillus leichmannii.

P3-(6-(N-Trifluoracetyl)aminohex-1-yl) deoxyguanosine triphosphate has been prepared by the reaction of N-trifluoroacetyl-6-aminohexanol 1-pyrophosphate with the imidazolide of dGMP and has been characterized. This compound and the corresponding free amine, obtained by removal of the protective trigluoroacetyl group, are activators of ribonucleotide reductase of Lactobacillus leichmannii. An affinity adsorbent for the reductase, prepared by reaction of the amine derivative with CNBr-activated Sepharose, contains dGTP covalently attached through the gamma-phosphate via a six-carbon chain to the matrix. The method of synthesis of the dGTP derivative is generally applicable to the synthesis of P3-(omega-aminoalk-1-yl)nucleoside triphosphate esters for the preparation of analogous affinity adsorbents. Ribonucleotide reductase can be rapidly purified to homogeneity, on a large scale, by use of dGTP-Sepharose and conditions for optimum recovery of the enzyme have been determined. The affinity of ribonucleotide reductase and other proteins for dGTP-Sepharose is increased by either raising the ionic strength or lowering the temperature of the eluent. Elution of the enzyme from the adsorbent can be achieved between pH 5.8 and 7.3, whereas at pH 5.3 the reductase is bound extremely tightly and cannot be recovered. Ribonucleotide reductase can be eluted from the adsorbent with dGTP or urea. Elution with urea is carried out at pH 6.3, where the enzyme is stable and maximum recovery is obtained. Affinity chromatography consistently produces ribonucleotide reductase of high specific activity (170-180 units/mg). In the presence of 0.1 to 1.2 M urea or hydroxyurea, the enzyme is inhibited, but allosteric activation is unchanged. No alteration in the structure or function of the reductase was detected when the enzyme was exposed to 2.0 M urea during elution from the affinity adsorbent, but exposure for longer periods causes some inactivation.