Reactions of para-substituted nitrosobenzenes with human hemoglobin

Crystal structural investigations of heme protein derivatives resulting from reactions of aryl- and alkylhydroxylamines with human hemoglobin

Phenylhydroxylamine (PhNHOH) and nitrosobenzene (PhNO) interact with human tetrameric hemoglobin (Hb) to form the nitrosobenzene adduct Hb(PhNO). These interactions also frequently lead to methemoglobin formation in red blood cells. We utilize UV-vis spectroscopy and X-ray crystallography to identify the primary and secondary products that form when PhNHOH and related alkylhydroxylamines (RNHOH; R = Me, t-Bu) react with human ferric Hb. We show that with MeNHOH, the primary product is Hb[α-FeIII(H2O)][β-FeII(MeNO)], in which nitrosomethane is bound to the β subunit but not the α subunit. Attempts to isolate a nitrosochloramphenicol (CAMNO) adduct resulted in our isolation of a Hb[α-FeII][β-FeII-cySOx]{CAMNO} product (cySOx = oxidized cysteine) in which CAMNO was located outside of the protein in the solvent region between the β2 and α2 subunits of the same tetramer. We also observed that the βcys93 residue had been oxidized. In the case of t-BuNHOH, we demonstrate that the isolated product is the β-hemichrome Hb[α-FeIII(H2O)][β-FeIII(His)2]{t-BuNHOH}, in which the β heme has slipped ∼4.4 Å towards the solvent exterior to accommodate the bis-His heme coordination. When PhNHOH is used, a similar β-hemichrome Hb[α-FeIII(H2O)][β-FeIII(His)2-cySOx]{PhNHOH} was obtained. Our results reveal, for the first time, the X-ray structural determination of a β-hemichrome in a human Hb derivative. Our UV-vis and X-ray crystal structural result reveal that although Hb(PhNO) and Hb(RNO) complexes may form as primary products, attempted isolation of these products by crystallization may result in the structural determination of their secondary products which may contain β-hemichromes en route to further protein degradation.

Insights into Nitrosoalkane Binding to Myoglobin Provided by Crystallography of Wild-Type and Distal Pocket Mutant Derivatives

Nitrosoalkanes (R-N═O; R = alkyl) are biological intermediates that form from the oxidative metabolism of various amine (RNH₂) drugs or from the reduction of nitroorganics (RNO₂). RNO compounds bind to and inhibit various heme proteins. However, structural information on the resulting Fe-RNO moieties remains limited. We report the preparation of ferrous wild-type and H64A sw Mb^II-RNO derivatives (λ_max 424 nm; R = Me, Et, Pr, iPr) from the reactions of Mb^III-H₂O with dithionite and nitroalkanes. The apparent extent of formation of the wt Mb derivatives followed the order MeNO > EtNO > PrNO > iPrNO, whereas the order was the opposite for the H64A derivatives. Ferricyanide oxidation of the Mb^II-RNO derivatives resulted in the formation of the ferric Mb^III-H₂O precursors with loss of the RNO ligands. X-ray crystal structures of the wt Mb^II-RNO derivatives at 1.76-2.0 Å resoln. revealed N-binding of RNO to Fe and the presence of H-bonding interactions between the nitroso O-atoms and distal pocket His64. The nitroso O-atoms pointed in the general direction of the protein exterior, and the hydrophobic R groups pointed toward the protein interior. X-ray crystal structures for the H64A mutant derivatives were determined at 1.74-1.80 Å resoln. An analysis of the distal pocket amino acid surface landscape provided an explanation for the differences in ligand orientations adopted by the EtNO and PrNO ligands in their wt and H64A structures. Our results provide a good baseline for the structural analysis of RNO binding to heme proteins possessing small distal pockets.

Insight into the preferential N-binding versus O-binding of nitrosoarenes to ferrous and ferric heme centers

Nitrosoarenes (ArNOs) are toxic metabolic intermediates that bind to heme proteins to inhibit their functions. Although much of their biological functions involve coordination to the Fe centers of hemes, the factors that determine N-binding or O-binding of these ArNOs have not been determined. We utilize X-ray crystallography and density functional theory (DFT) analyses of new representative ferrous and ferric ArNO compounds to provide the first theoretical insight into preferential N-binding versus O-binding of ArNOs to hemes. Our X-ray structural results favored N-binding of ArNO to ferrous heme centers, and O-binding to ferric hemes. Results of the DFT calculations rationalize this preferential binding on the basis of the energies of associated spin-states, and reveal that the dominant stabilization forces in the observed ferrous N-coordination and ferric O-coordination are dπ-pπ* and dσ-pπ*, respectively. Our results provide, for the first time, an explanation why in situ oxidation of the ferrous-ArNO compound to its ferric state results in the observed subsequent dissociation of the ligand.

Nephrotoxic Potential of Putative 3,5-Dichloroaniline (3,5-DCA) Metabolites and Biotransformation of 3,5-DCA in Isolated Kidney Cells from Fischer 344 Rats

The current study was designed to explore the in vitro nephrotoxic potential of four 3,5-dichloroaniline (3,5-DCA) metabolites (3,5-dichloroacetanilide, 3,5-DCAA; 3,5-dichlorophenylhydroxylamine, 3,5-DCPHA; 2-amino-4,6-dichlorophenol, 2-A-4,6-DCP; 3,5-dichloronitrobenzene, 3,5-DCNB) and to determine the renal metabolism of 3,5-DCA in vitro. In cytotoxicity testing, isolated kidney cells (IKC) from male Fischer 344 rats (~4 million/mL, 3 mL) were exposed to a metabolite (0–1.5 mM; up to 90 min) or vehicle. Of these metabolites, 3,5-DCPHA was the most potent nephrotoxicant, with 3,5-DCNB intermediate in nephrotoxic potential. 2-A-4,6-DCP and 3,5-DCAA were not cytotoxic. In separate experiments, 3,5-DCNB cytotoxicity was reduced by pretreating IKC with antioxidants and cytochrome P450, flavin monooxygenase and peroxidase inhibitors, while 3,5-DCPHA cytotoxicity was attenuated by two nucleophilic antioxidants (glutathione and N-acetyl-L-cysteine). Incubation of IKC with 3,5-DCA (0.5–1.0 mM, 90 min) produced only 3,5-DCAA and 3,5-DCNB as detectable metabolites. These data suggest that 3,5-DCNB and 3,5-DCPHA are potential nephrotoxic metabolites and may contribute to 3,5-DCA induced nephrotoxicity in vivo. In addition, the kidney can bioactivate 3,5-DCNB to toxic metabolites, and 3,5-DCPHA appears to generate reactive metabolites to contribute to 3,5-DCA nephrotoxicity. In vitro, N-oxidation of 3,5-DCA appears to be the primary mechanism of bioactivation of 3,5-DCA to nephrotoxic metabolites.

Role of Free Radicals and Biotransformation in Trichloronitrobenzene-Induced Nephrotoxicity In Vitro

This study determined the comparative nephrotoxic potential of four trichloronitrobenzenes (TCNBs) (2,3,4-; 2,4,5-; 2,4,6-; and 3,4,5-TCNB) and explored the effects of antioxidants and biotransformation inhibitors on TCNB-induced cytotoxicity in isolated renal cortical cells (IRCC) from male Fischer 344 rats. IRCC were incubated with a TCNB up to 1.0 mM for 15–120 min. Pretreatment with an antioxidant or cytochrome P450 (CYP), flavin monooxygenase (FMO), or peroxidase inhibitor was used in some experiments. Among the four TCNBs, the order of decreasing nephrotoxic potential was approximately 3,4,5- > 2,4,6- > 2,3,4- > 2,4,5-TCNB. The four TCNBs exhibited a similar profile of attenuation of cytotoxicity in response to antioxidant pretreatments. 2,3,4- and 3,4,5-TCNB cytotoxicity was attenuated by most of the biotransformation inhibitors tested, 2,4,5-TCNB cytotoxicity was only inhibited by isoniazid (CYP 2E1 inhibitor), and 2,4,6-TCNB-induced cytotoxicity was inhibited by one CYP inhibitor, one FMO inhibitor, and one peroxidase inhibitor. All of the CYP specific inhibitors tested offered some attenuation of 3,4,5-TCNB cytotoxicity. These results indicate that 3,4,5-TCNB is the most potent nephrotoxicant, free radicals play a role in the TCNB cytotoxicity, and the role of biotransformation in TCNB nephrotoxicity in vitro is variable and dependent on the position of the chloro groups.

Nitric Oxide Reactivity of a Cu(II) Complex of an Imidazole-Based Ligand: Aromatic C-Nitrosation Followed by the Formation of N-Nitrosohydroxylaminato Complex

A binuclear Cu(II) complex, 1, [Cu₂(L^-)₂(OAc)](OAc) of imidazole-based ligand LH {LH = 2-(bis(2-ethyl-5-methyl-1H-imidazol-4-yl)methyl)phenol} was synthesized and characterized spectroscopically and structurally. Addition of an equivalent amount of nitric oxide (NO) by a gastight syringe to the acetonitrile:methanol (5:1, v/v) solution of complex 1 at room temperature resulted in the reduction of Cu(II) center to Cu(I) with concomitant C-nitrosation of the ligand. Spectroscopic characterization of the resulting Cu(I) complex (1a) of the C-nitrosylated ligand, L' {L' = 2-(bis(2-ethyl-5-methyl-1H-imidazol-4-yl)methyl)-4-nitroso-phenol} has been done. The Cu(I) complex, 1a, further reacted with NO to result in the corresponding N-nitrosohydroxylaminato complex, 2, [Cu₂(L-ONNO)₂](OAc)₂ through the formation of a Cu(I)-nitrosyl intermediate. A small fraction of the nitrosyl intermediate decomposed to the corresponding Cu(II) complex 3, [Cu(L')₂], and N₂O in a parallel reaction.

The role of biotransformation and oxidative stress in 3,5-dichloroaniline (3,5-DCA) induced nephrotoxicity in isolated renal cortical cells from male Fischer 344 rats

Among the mono- and dichloroanilines, 3,5-dichloroaniline (3,5-DCA) is the most potent nephrotoxicant in vivo and in vitro. However, the role of renal biotransformation in 3,5-DCA induced nephrotoxicity is unknown. The current study was designed to determine the in vitro nephrotoxic potential of 3,5-DCA in isolated renal cortical cells (IRCC) obtained from male Fischer 344 rats, and the role of renal bioactivation and oxidative stress in 3,5-DCA nephrotoxicity. IRCC (∼ 4 million cells/ml) from male rats were exposed to 3,5-DCA (0-1.0mM) for up to 120 min. In IRCC, 3,5-DCA was cytotoxic at 1.0mM by 60 min as evidenced by the increased release of lactate dehydrogenase (LDH), but 120 min was required for 3,5-DCA 0.5mM to increase LDH release. In subsequent studies, IRCC were exposed to a pretreatment (antioxidant or enzyme inhibitor) prior to exposure to 3,5-DCA (1.0mM) for 90 min. Cytotoxicity induced by 3,5-DCA was attenuated by pretreatment with inhibitors of flavin-containing monooxygenase (FMO; methimazole, N-octylamine), cytochrome P450 (CYP; piperonyl butoxide, metyrapone), or peroxidase (indomethacin, mercaptosuccinate) enzymes. Use of more selective CYP inhibitors suggested that the CYP 2C family contributed to 3,5-DCA bioactivation. Antioxidants (glutathione, N-acetyl-l-cysteine, α-tocopherol, ascorbate, pyruvate) also attenuated 3,5-DCA nephrotoxicity, but oxidized glutathione levels and the oxidized/reduced glutathione ratios were not increased. These results indicate that 3,5-DCA may be activated via several renal enzyme systems to toxic metabolites, and that free radicals, but not oxidative stress, contribute to 3,5-DCA induced nephrotoxicity in vitro.

3,4,5-Trichloroaniline nephrotoxicity in vitro: potential role of free radicals and renal biotransformation

Chloroanilines are widely used in the manufacture of drugs, pesticides and industrial intermediates. Among the trichloroanilines, 3,4,5-trichloroaniline (TCA) is the most potent nephrotoxicant in vivo. The purpose of this study was to examine the nephrotoxic potential of TCA in vitro and to determine if renal biotransformation and/or free radicals contributed to TCA cytotoxicity using isolated renal cortical cells (IRCC) from male Fischer 344 rats as the animal model. IRCC (~4 million cells/mL; 3 mL) were incubated with TCA (0, 0.1, 0.25, 0.5 or 1.0 mM) for 60–120 min. In some experiments, IRCC were pretreated with an antioxidant or a cytochrome P450 (CYP), flavin monooxygenase (FMO), cyclooxygenase or peroxidase inhibitor prior to incubation with dimethyl sulfoxide (control) or TCA (0.5 mM) for 120 min. At 60 min, TCA did not induce cytotoxicity, but induced cytotoxicity as early as 90 min with 0.5 mM or higher TCA and at 120 min with 0.1 mM or higher TCA, as evidenced by increased lactate dehydrogenase (LDH) release. Pretreatment with the CYP inhibitor piperonyl butoxide, the cyclooxygenase inhibitor indomethacin or the peroxidase inhibitor mercaptosuccinate attenuated TCA cytotoxicity, while pretreatment with FMO inhibitors or the CYP inhibitor metyrapone had no effect on TCA nephrotoxicity. Pretreatment with an antioxidant (α-tocopherol, glutathione, ascorbate or N-acetyl-l-cysteine) also reduced or completely blocked TCA cytotoxicity. These results indicate that TCA is directly nephrotoxic to IRCC in a time and concentration dependent manner. Bioactivation of TCA to toxic metabolites by CYP, cyclooxygenase and/or peroxidase contributes to the mechanism of TCA nephrotoxicity. Lastly, free radicals play a role in TCA cytotoxicity, although the exact nature of the origin of these radicals remains to be determined.

The chemistry of nitroxyl-releasing compounds

Nitroxyl (HNO) demonstrates a diverse and unique biological profile compared to nitric oxide, a redox-related compound. Although numerous studies support the use of HNO as a therapeutic agent, the inherent chemical reactivity of HNO requires the use of donor molecules. Two general chemical strategies currently exist for HNO generation from nitrogen-containing molecules: (i) the disproportionation of hydroxylamine derivatives containing good leaving groups attached to the nitrogen atom and (ii) the decomposition of nitroso compounds (X-N=O, where X represents a good leaving group). This review summarizes the synthesis and structure, the HNO-releasing mechanisms, kinetics and by-product formation, and alternative reactions of six major groups of HNO donors: Angeli's salt, Piloty's acid and its derivatives, cyanamide, diazenium diolate-derived compounds, acyl nitroso compounds, and acyloxy nitroso compounds. A large body of work exists defining these six groups of HNO donors and the overall chemistry of each donor requires consideration in light of its ability to produce HNO. The increasing interest in HNO biology and the potential of HNO-based therapeutics presents exciting opportunities to further develop HNO donors as both research tools and potential treatments.

Binding of nitrobenzene to hepatic DNA and hemoglobin at low doses in mice

Nitrobenzene (NB) is a widely used industrial chemical, and is considered a hazardous air pollutant. Evidence has recently showed that nitrobenzene is an animal carcinogen. We investigated the binding of 14C-NB to hepatic DNA and Hb in mice at low doses using an ultrasensitive method of accelerator mass spectrometry (AMS). In a dose-response profile, NB-DNA and NB-Hb adduct levels increased with increasing administered doses from 0.1 microg/kg b.w. to 10 mg/kg b.w. with a good linearity in a log/log presentation. At 2 h after NB administration, NB-DNA adduct levels were about twofold greater than that of NB-Hb at all doses. In the time course study NB-DNA adduct levels reduced rapidly through an exponential decay profile, whereas NB-Hb adducts showed a different decay mode, declining rather slowly to low levels. Our findings on the genotoxicity of NB do furnish a significant evidence in support of the probable carcinogenic property of NB previously reported.

Cholinesterase status, pharmacokinetics and laboratory findings during obidoxime therapy in organophosphate poisoned patients

1 The effectiveness of oxime therapy in organophosphate poisoning is still a matter of debate. It appears, however, that the often cited ineffectiveness of oximes may be due to inappropriate dosing. By virtue of in vitro findings and theoretical considerations we concluded in the preceding paper that oximes should preferably be administered by continuous infusion following an initial bolus dose for as long as reactivation of inhibited acetylcholinesterase (AChE) can be expected. This conclusion has called for a clinical trial to evaluate such oxime therapy on the basis of objective parameters. 2 Before transfer to the intensive care unit (ICU), 5 patients received primary care by an emergency physician. In the ICU, atropine sulphate was administered i.v. upon demand according to the endpoints: no bronchorrhoea, dry mucous membranes, no axillary sweating, heart rate of about 100/min. Obidoxime (Toxogonin) was given as an i.v. bolus (250 mg) followed by continuous infusion of 750 mg/24 h. 3 Intoxication and therapy were monitored by determining erythrocyte AChE (eryAChE) activity, reactivatability of the patient's eryAChE ex vivo, plasma cholinesterase activity, the presence of AChE inhibiting compounds, as well as the concentrations of obidoxime and atropine in plasma. 4 Obidoxime was effective in life-threatening parathion poisoning, in particular when the dose absorbed was comparably low. In mega-dose poisoning, net reactivation was not achieved until several days after ingestion, when the concentration of active poison in plasma had declined. Reactivatability in vivo lasted for a longer period than expected from in vitro experiments. 5 Obidoxime was quite ineffective in oxydemetonmethyl poisoning, when the time elapsed between ingestion and oxime therapy was longer than 1 day. When obidoxime was administered shortly after ingestion (1 h) reactivation was nearly complete. 6 Obidoxime levels of 10-20 microM were achieved by our regimen, and atropine could rapidly be reduced to approx. 20 microM, as attained by continuous infusion of 1 mg atropine sulphate/h. Maintenance of the desired plasma levels was not critical even when renal function deteriorated. 7 Signs of transiently impaired liver function were observed in patients who showed transient multiorgan failure. In the present stage of knowledge, we feel it advisable to keep the plasma concentration of obidoxime at 10-20 microM, although the full reactivating potential of obidoxime will not then be exploited. Still, the reactivation rate, with an apparent half-time of some 3 min, is twice that estimated for a tenfold higher pralidoxime concentration.

Structural basis for the haemotoxicity of dapsone: the importance of the sulphonyl group

The structural basis of dapsone (4,4'-diaminodiphenyl sulphone) haemotoxicity has been determined by investigation of the in vitro bioactivation of a series of 4-substituted arylamines. In the presence of rat liver microsomes, dapsone (100 microM) was the most potent former of methaemoglobin in human erythrocytes (44.8 +/- 6.7%). Substitution of the sulphone group with sulphur (11.6 +/- 1.4% methaemoglobin), oxygen (4.5 +/- 1.1%), nitrogen (0.0 +/- 3.2%), carbon (13.6 +/- 0.8%) or a keto group (34.0 +/- 6.1%) resulted in a decrease in methaemoglobin formation. Only one compound, 4,4'-diaminodiphenylamine, generated significant (P < 0.001) amounts of methaemoglobin (25.6 +/- 2.5%) in the absence of NADPH. To assess further the role of the 4-substituent in methaemoglobinaemia, the toxicity of a series of 4-substituted aniline derivatives was also studied. Of the anilines studied, 4-nitroaniline caused the most methaemoglobin (36.5 +/- 8.0%), whilst aniline caused the least (0.3 +/- 0.5%). Overall, there was a significant correlation (r2 = 0.83) between the haemotoxicity and the Hammett constant, sigma(p), suggesting that it is the electron-withdrawing properties of the substituent that influence the methaemoglobin formation. In the presence of microsomes prepared from two human livers, dapsone was the most haemotoxic bis arylamine, whereas 4-iodoaniline was the most potent methaemoglobin former (60.6 and 73.6%) and aniline the least potent (1.1 and 2.4%). As a whole, these results indicate that the sulphonyl group, which is essential for the pharmacological activity of dapsone, is also largely responsible for the haemotoxicity seen with this drug.

Erythrocyte enzymes catalyze 1-nitropyrene and 3-nitrofluoranthene nitroreduction

Nitroarenes are environmental contaminants produced during incomplete combustion processes. Nitroreduction, the most important pathway of nitroarene toxification, occurs mainly in the liver and intestine. In the present study, we show that human red cells may also possess the metabolic competence to reduce 1-nitropyrene (NP) and 3-nitrofluoranthene (NF), the nitroarenes chosen as model compounds, to their corresponding amino derivatives, 1-aminopyrene (AP) and 3-aminofluoranthene (AF). The requirement of the cofactor couple NADH/FMN suggests that erythrocyte nitroreductase activity occurs via one electron transfer. The presence of oxygen strongly inhibited the haemolysate-catalyzed nitroarene reduction, whether measured as amine formation or nitroarene disappearance. Intermediate reactive species, that bind covalently to haemoglobin and/or other erythrocyte proteins, are formed during nitroreduction catalyzed by human haemolysate. In fact, the reduced metabolites AP and AF were released after mild acid hydrolysis of red cell proteins exposed to NP and NF, thus suggesting that sulphinamide adducts have been formed.

In vivo EPR studies of the metabolic fate of nitrosobenzene in the mouse

We report the first demonstration of EPR spectroscopy to study free radical reactions in live mice and excised muscle tissue resulting from the metabolism of nitrosobenzene. A broad three-line EPR spectrum (aN = 11.6 G) appeared in the buttocks region of a mouse place in an L-band loop gap resonator after intramuscular or intraperitoneal injection of 0.2 mmol/kg nitrosobenzene. The signal intensity reached a maximum at 20 to 30 min and remained constant well beyond 2 h. If muscle tissue was dosed with nitrosobenzene and excised within 5 min, a similar three-line X-band EPR spectrum was obtained which was preceded by the rapid growth and subsequent decay of an EPR spectrum identical with that of the phenylhydronitroxide radical, which was presumably generated from reactions between nitrosobenzene and reducing agents in the blood or tissue such as NADH or ascorbic acid. A model system containing nitrosobenzene and unsaturated fatty acids (olive oil or animal fat) yielded an identical three-line spectrum resulting from radical adducts of nitrosobenzene across the double bond. Overall, these results suggest that the most probable mechanism in vivo was nitrosobenzene covalently adding ("binding") to polyunsaturated fatty acid clusters in fat or membranes.

Study of procainamide hapten-specific antibodies in rabbits and humans

Procainamide (PA) is the drug most commonly associated with the induction of autoantibodies and drug-related lupus (DRL). While the majority of these patients express autoantibodies, antibodies to the parent drug and metabolites, PA-hydroxylamine (PAHA) or nitroso-PA (NOPA), have not been reported in humans. Hapten-carrier conjugates were prepared using human hemoglobin (HgB) or autologous rabbit erythrocytes with PAHA or NOPA. PA was conjugated to rabbit serum albumin (RSA) or egg albumin (OVA) via diazotization and condensation methods. Rabbits were immunized with hapten conjugates in Freund's adjuvant. These hapten-carrier compounds (5-10 micrograms/ml) were used as test antigens for antibodies in sera from the rabbits and 40 patients on chronic PA treatment. 10 SLE patients, 33 elderly and 20 young normal controls by ELISA. Type I and II collagens were also used as test antigens for human sera. Sera from rabbits immunized with the PA compounds had elevated IgG antibody values to PA, PAHA and NOPA, but no autoantibodies. Absorption of the rabbit sera with the PA compounds reduced the antibody levels; ssDNA and histones failed to inhibit the total binding values. Mean binding to PA-OVA was 0.95 +/- 0.41 for PA patients and 1.37 +/- 0.26 standard error of means (S.E.M.) in the SLE patients compared to 0.37 +/- 0.14 S.E.M. in the normal sera (P < or = 0.05); similar binding values to PAHA-HgB and NOPA-HgB were also observed. Sixty-eight percent of the PA patients had antibodies to type II collagen. Elevated binding values to PA compounds were inhibited by absorption of human sera with ssDNA or total histones; absorption with PA or PAHA had no significant effect. These findings suggest that sera from PA patients containing high titers of autoantibodies cross-react in vitro with unrelated antigens.

Effects of the phenacetin metabolite 4-nitrosophenetol on glycolysis and pentose phosphate pathway in human red cells

Human erythrocytes exposed to 4-nitrosophenetol showed marked alterations of their endogenous metabolism. Rapid ferrihemoglobin formation mediated by the NADPH-dependent enzymic cycling of the nitrosoarene ("Kiese cycle") and extensive GSSG production caused an immediate drain of G-6-P into the pentose phosphate pathway at maximal flow. Despite a 2.4-fold increase in glucose phosphorylation rate and a branching ratio of 97:3 between pentose phosphate pathway and Embden-Meyerhof pathway, the G-6-P supply was obviously insufficient to meet the immense NADPH demand. Thus, a significant recycling of pentose phosphate pathway-derived F-6-P was observed in the order of 65%. Comparison of NADPH regeneration and ferrihemoglobin formation indicates the "Kiese cycle" to be a minor mechanism in ferrihemoglobin production in the case of high 4-nitrosophenetol concentrations. Most probably, reactive intermediates of 4-nitrosophenetol other than N-hydroxy-4-phenetidine, i.e. bicyclic arylamines and glutathione S-conjugates are formed which produce ferrihemoglobin without involvement of NADPH. The experiments have shown that red cells are remarkable robust to tackle the massive oxidative stress as elicited by 4-nitrosophenetol. The immediate metabolic response of the pentose phosphate pathway allows rapid regeneration of reduced glutathione. Thereby, SH-containing enzymes are effectively protected and/or regenerated and hemolysis is kept minimal. Hence, red cells are favourably suited for clearing the blood from N-oxygenated arylamines before they can reach more sensitive target organs.

Hemoglobin binding of monocyclic aromatic amines: molecular dosimetry and quantitative structure activity relationships for the N-oxidation

Aromatic amines are important intermediates in industrial manufacturing. N-oxidation to the N-hydroxyarylamines is a key step determining the genotoxic properties of aromatic amines. N-hydroxyarylamines can form adducts with DNA, with tissue proteins and with the blood proteins albumin and hemoglobin in a dose-dependent manner. The determination of hemoglobin adducts is a useful tool for biomonitoring exposed populations. We have established the hemoglobin binding index (HBI) [(mmol compound/mol Hb)/(mmol compound/kg body wt)] of several aromatic amines in female Wistar rats. Including the values obtained by other researchers in the same rat strain, the logarithm of hemoglobin binding (log HBI) was plotted against the following parameters: the sum of the Hammett constants (sigma sigma = sigma p + sigma m), pKa, log P (octanol/water), the half wave oxidation potential (E1/2) and the electronic descriptors of the amines and their corresponding nitrenium ions obtained by semiempirical calculations (MNDO, AM1 and PM3), such as atomic charge densities, energies of the HOMO and LUMO and their coefficients, the C-N bond order, the dipole moments and the 'reaction enthalpy' [MNDOHF, AM1HF or PM3HF = Hf(nitrenium) - Hf(amine)]. The correlation coefficients were determined from the plots of all parameters against log HBI for all amines by means of linear regression analysis. The amines were classified into three groups: group 1, all para-substituted amines, group 2, all amines with halogens and group 3, all amines with alkyl groups. For the amines of group 1, log HBI correlates with sigma sigma, MNDOHF, E1/2, the pKa and the log P with r = 0.84, 0.71, 0.73, - 0.69 and 0.50, respectively. For the amines of group 2, log HBI correlates with pKa, sigma sigma, MNDOHF, E1/2 and log P with r = 0.81, -0.76, -0.55, -0.46 and -0.20, respectively. For the amines of group 3, log HBI correlates with the E1/2, PM3HF, sigma sigma, pKa and log P with r = 0.92, 0.89, 0.76, 0.19 and 0.12, respectively. The apparent Michaelis-Menten constants Km and Vmax of the N-acetyltransferase of liver cytosol were determined for several amines. Km and Vmax do not correlate with any of the electronic descriptors. Female Wistar rats were dosed with nitroarenes. Hemoglobin binding of nitroarenes correlates with the energy levels of the LUMO. This investigation determines for a large variety of aromatic amines the bioavailability of the N-hydroxyarylamine--the genotoxic metabolite--and the utility of electronic descriptors for prediction of the N-oxidation.

Cellular effects of some metabolic oxidation products pertinent to 4-ethoxyaniline

The toxicity of some metabolic products pertinent to 4-ethoxyaniline in isolated hepatocytes were investigated. The compounds investigated were 4-ethoxynitrosobenzene (1), 4-ethoxy-4'-nitrosodiphenylamine (2), 3,6-bis(4-ethoxy-phenylimino)-4-ethoxy-1,4-cyclohexadienylamine (3), 4-(4-ethoxyphenylimino)-2,3-dimethyl-2,5-cyclohexadiene-1-one (4) and 4-(4-ethoxyphenylimino)-2,6-dimethyl-2,5-cyclohexadiene-1-one (5). Of these, 1, 2 and 3 are oxidation products of 4-ethoxyaniline. Compounds 4 and 5 are dimethyl analogues of previously investigated oxidation product 4-(4-ethoxyphenylimino(-2,5-cyclohexadiene-1-one (NEPBQI). Among the investigated compounds, 1 and 2 were the most toxic towards isolated hepatocytes. In hepatocytes treated with compounds 1, 2 and 4, loss of cell viability was also accompanied by surface bleb formation. All compounds except 3 reacted with GSH resulting in depletion of cellular GSH. No formation of GSSG was observed, however. Thus, the GSH depletion was apparently due to conjugate formation rather than oxidation. No superoxide dismutase inhibitable reduction of acetylated cytochrome c was observed, thus none of the compounds undergoes measurable redox cycling.

N-hydroxy-N-arylacetamides. V. Differences in the mechanism of haemoglobin oxidation in vitro by N-hydroxy-4-chloroacetanilide and N-hydroxy-4-chloroaniline

1. Autoxidation of N-hydroxy-4-chloroaniline(I) in buffer pH 7.4 was rapid and yielded 4,4'-azoxybischlorobenzene, 4-chloronitrosobenzene, 4-chloronitrobenzene, and 4-chlorophenyl nitroxide. In contrast, autoxidation of N-hydroxy-4-chloroacetanilide(II) was very slow, since in ether and water 78 and 92%, respectively, had decomposed in six months. 2. Haemoglobin(HbO2)-catalysed autoxidation of (I) occurred at a molar ratio of haemoglobin-Fe2+ to (I) of less than 0.25 and was accompanied by ferrihaemoglobin(HbFe3+)-formation and oxygen consumption. Coupled oxidation of HbO2 with (I) occurred at a molar ratio of greater than 0.2 and was accompanied by liberation of oxygen and the formation of HbFe3+, haemoglobin-4-chloronitrosobenzene complex, HbO2, desoxyhaemoglobin, 4-chloronitrosobenzene, 4-chloronitrobenzene, 4-chloroaniline, 4,4'-azoxybischlorobenzene, and 4-chlorophenyl nitroxide. At an equimolar ratio of 10(-3) M haemoglobin-Fe2+ to (I), 96% HbO2 was converted into HbFe3+ (50%) and haemoglobin-4-chloronitrosobenzene complex in the initial fast phase of the reaction, but only 34% of the bound oxygen was liberated, the rest was sequentially reduced to water. (I) completely disappeared, and 4-chloronitrosobenzene was the major metabolite, mainly bound to haemoglobin. 3. Chemical oxidation of (II) by PbO2 in benzene produced acetyl 4-chlorophenyl nitroxide, whose spontaneous decomposition gave 38% 4-chloronitrosobenzene, 33% N-acetoxy-4-chloroacetanilide, 10% 4-chloroacetanilide, and 8% 4-chloronitrobenzene. Its spontaneous decomposition in water also followed second order kinetics, K = 350 l mol-1 sec-1 and yielded N-(2-acetylamino-5-chlorophenyl)-p-benzo-quinoneimine-N-oxide in addition. 4. In the coupled oxidation of 10(-3) M haemoglobin-Fe2+ with 10(-3) M (II), 75% HbFe3+ was formed after 1 h, but only one third of the equivalent of oxygen was released, and two thirds were reduced to water. Concentration of (II) decreased by 5% only, indicating that one mol of (II) had catalysed the oxidation of 15 equivalents of haemoglobin-Fe2+. The identity of the product pattern formed with HbO2 with that produced by chemical one-electron oxidation indicated that oxygen bound to haemoglobin also functions as an acceptor for electrons from (II) as from (I), but the different redox potentials can explain why the secondary aromatic nitroxide was catalytically active and the primary nitroxide was not.

Procainamide hydroxylamine lymphocyte toxicity--I. Evidence for participation by hemoglobin

A number of lines of evidence suggest that the lupus-like symptoms associated with procainamide therapy may be caused by products of metabolic N-oxidation. In the present study, the perfusion of the isolated rat liver with a hemoglobin-free solution containing procainamide (100 microM) resulted in the rapid appearance of the N-oxidation metabolite procainamide hydroxylamine in the perfusate. Addition of procainamide hydroxylamine in vitro to whole rat blood (1-40 microM) resulted in a concentration-dependent loss of proliferative response among mononuclear cells isolated from the treated blood and cultured with mitogens (phytohemagglutinin, PHA-P: concanavalin A, Con A; and pokeweed mitogen, PWM), as well as a loss of viability. Similar effects on lymphocyte mitogen responsiveness were observed when procainamide hydroxylamine (1-40 microM) was added to rat whole splenic cell populations. Carbon monoxide or ascorbic acid pretreatment inhibited the toxicity of procainamide hydroxylamine to lymphocytes in whole blood, but only carbon monoxide pretreatment inhibited procainamide hydroxylamine-induced methemoglobin formation. These observations are consistent with the participation of hemoglobin in a redox cycle with procainamide hydroxylamine, generating products which are primarily responsible for its cytotoxicity in blood.

Detoxication of N-oxygenated arylamines in erythrocytes. An overview

1. Reactive N-oxygenated arylamines, namely, N-hydroxyarylamines and nitrosoarenes, are toxic, mutagenic, carcinogenic and allergenic. 2. Erythrocytes are a very sensitive target for these compounds, but little is known of the detoxication capacity of these cells. 3. This overview considers the most important reactions of p-substituted N-oxygenated arylamines in red cells, namely, (i) ferrihaemoglobin formation by N-hydroxyarylamines with concomitant co-oxidation to nitrosoarenes; (ii) compartmentation of nitrosobenzenes by ligation to deoxyhaemoglobin, (iii) reactions of nitrosobenzene with glutathione, (iv) adduct formation of nitrosobenzenes with thiol groups of haemoglobin. 4. To predict the metabolic fate of N-oxygenated arylamines in red cells, the respective kinetic parameters of reactions (i) to (iv) have been determined, and indicate good linear free energy correlations (pH 7.4, 37 degrees C). These data may help to estimate the detoxication capacity of erythrocytes in vivo.

Chloramphenicol: magic bullet or double-edge sword?

The genetic and genotoxic potentials of chloramphenicol are reviewed and analyzed. Although this widely used antimicrobial agent appears to cause chromosomal effects in somatic cells, in view of the consistent absence of other genetic effects, these cytogenetic abnormalities are ascribed to non-genotoxic causes. It is pointed out that despite its widespread use in human medicine, chloramphenicol has not been systematically tested for genotoxicity.

Formation of 4-ethoxy-4'-nitrosodiphenylamine in the reaction of the phenacetin metabolite 4-nitrosophenetol with glutathione

Phenacetin, a constituent of several analgesic and antipyretic formulations has been made responsible for a variety of toxic and carcinogenic actions. 4-Nitrosophenetol, the N-oxydation product of intermediate 4-phenetidine, forms methemoglobin and binds covalently to sulfhydryl groups of proteins and glutathione. In the reaction of 4-nitrosophenetol with glutathione and other thiols an intermediate so-called "semimercaptal" is formed from which N-(thiol-S-yl)-4-phenetidine S-oxide, N-(thiol-S-yl)-4-phenetidine and 4-phenetidine derive. Besides thiol adducts, a yellow compound is formed which was isolated as a pure crystalline product (elemental analysis) and identified by FAB-MS, EI-MS, 13C-, 1H-NMR, and UV-VIS spectroscopy as 4-ethoxy-4'-nitrosodiphenylamine. This nitrosoarene is formed by an unknown mechanism from 4-nitrosophenetol and 4-phenetidine under liberation of ethanol. In human erythrocytes this compound is easily reduced to 4-amino-4'-ethoxydiphenylamine (FAB-MS, EI-MS, 13C-NMR). During the reaction of 4-nitrosophenetol with red cells only traces of 4-ethoxy-4'-nitrosodiphenylamine were formed, whereas up to 10% appeared as the reduction product 4-amino-4'-ethoxydiphenylamine. This latter compound is unstable in red cells and is metabolized further to unidentified products.