Oxidative degradation of haemoglobin by nitrosobenzene in the erythrocyte

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.

Proteomic Analysis of Thiol Modifications and Assessment of Structural Changes in Hemoglobin Induced by the Aniline Metabolites N-Phenylhydroxylamine and Nitrosobenzene

MS-based proteomic analysis was combined with in silico quantum mechanical calculations to improve understanding of protein adduction by N-phenylhydroxylamine (PhNHOH) and nitrosobenzene (NOB), metabolic products of aniline. In vitro adduction of model peptides containing nucleophilic sidechains (Cys, His, and Lys) and selected proteins (bovine and human hemoglobin and β-lactoglobulin-A) were characterized. Peptide studies identified the Cys thiolate as the most reactive nucleophile for these metabolites, a result consistent with in silico calculations of reactivity parameters. For PhNHOH, sulfinamides were identified as the primary adduction products, which were stable following tryptic digestion. Conversely, reactions with NOB yielded an additional oxidized adduct, the sulfonamide. In vitro exposure of human whole blood to PhNHOH and NOB demonstrated that only sulfinamides were formed. In addition to previously reported adduction of β⁹³Cys of human Hb, two novel sites of adduction were found; α¹⁰⁴Cys and β¹¹²Cys. We also report CD and UV-Vis spectroscopy studies of adducted human Hb that revealed loss of α-helical content and deoxygenation. The results provide additional understanding of the covalent interaction of aromatic amine metabolites with protein nucleophiles.

Mass spectrometric identification of hemoglobin modifications induced by nitrosobenzene

Aniline and nitrobenzene (NB) are widely used industrial chemicals. Early effects of aniline toxicity include methemoglobin formation and damage to erythrocytes (Jenkins, F.P., 1972. The no-effect dose of anilne in human subjects and a comparison of aniline toxicity in man and rat. Food Cosmet. Toxicol. 10, 671-679; Bus, J.S., Popp, J.A., 1987. Perspectives on the mechanism of action of the splenic toxicity of aniline and structurally-related. Food Chem. Toxicol. 25, 619-627). In this report, we describe an analytical method, based on LC techniques and mass spectrometry, which could help in monitoring the exposure to aniline and NB. In particular, we describe and characterize the formation of specific adducts during an in vitro reaction of nitrosobenzene (NOB), the main metabolite of aniline and NB, and human hemoglobin.

The effects of xenobiotics on erythrocytes

1. Methemoglobin formation was observed when erythrocytes were incubated with xenobiotics such as hydroxylamines or phenols, other metabolites resulting from the interaction of these compounds with erythrocytes being reactive free radicals derived from the respective xenobiotic, and a ferryl-heme oxo-complex. 2. Steady-state levels of these reaction products depended on the permeability of the erythrocyte membrane for the various methemoglobin (MetHb) generators and the presence of antioxidants that downregulate the radicals formed. 3. Electron spin resonance (ESR) spectra of xenobiotic-derived free radicals could be obtained only from the readily water soluble hydroxylamines, whereas the poorly water soluble phenolic compounds did not allow the use of concentrations required for the generation of detectable amounts of ESR-sensitive metabolites in erythrocytes. 4. Previous investigations with oxyhemoglobin solutions and with the MetHb/H2O2 model systems have shown that, apart from ESR-sensitive radical species, excited reaction intermediates such as compound 1 ferryl hemoglobin can be detected as well by using chemiluminescence measurements. 5. A strong correlation was found between the intensity of the emitted light and the MetHb formation rate, indicating that the production of compound 1 ferryl hemoglobin is closely related to the MetHb formation step. 6. The sensitivity of the photon-counting method allowed measurements of excited species in intact erythrocytes not only with the readily soluble hydroxylamines, but also with the less soluble phenolic compounds. 7. In addition, parameters indicative of xenobiotic-induced oxidative alterations were found: a significant decrease in intraerythrocytic thiol levels was a result of all compounds that initiate MetHb formation, as also described for slowly reacting xenobiotics. 8. With the most reactive compound investigated, unsubstituted hydroxylamine, a significant release of iron from the oxidatively modified hemoglobin was detected, facilitated by binding of this transition metal to hydroxylamine and its final oxidation product, nitric oxide. 9. The use of the ESR spin-labeling technique revealed membrane alterations of erythrocytes exposed to the reducing MetHb generators presented in this study. 10. A direct action of BHA and BHT on the integrity of the erythrocyte membrane was observed, leading to hemolysis independent of the formation of prooxidant species. 11. The presence of strong prooxidants (radicals) was indicated both by fluidity changes in the membrane and by an oxidative decrease in cytosolic thiol levels.

Direct formation of complexes between cytochrome P-450 and nitrosoarenes

The mechanism of the formation of the complexes between various nitrosobenzenes and cytochrome P-450 has been investigated. We have observed the formation of these complexes by a new and, as yet, undescribed route. Nitrosobenzene (NOB) itself reacts with cytochrome P-450 in the iron(III) state, in the absence of any exogenous reducing agent, to produce the iron(II)-NOB complex. Apparently, NOB is a ligand that is capable of causing the spontaneous autoreduction of the iron. The reduction of the iron may occur via ligand-induced oxidation of the axially bound thiolate of cytochrome P-450.

The red cell as a sensitive target for activated toxic arylamines

During biotransformation of arylamines, activated phase I metabolites, like aminophenols and hydroxylamines, occasionally escape the liver and exert allergic, toxic or carcinogenic effects in sensitive target organs. The first organ in contact with these proximate toxic compounds is the blood where oxyhemoglobin activates proximate to ultimate toxic derivatives. Thereby hydroxylamines and oxyhemoglobin are co-oxidized to nitrosoarenes and ferrihemoglobin. Because of an enzymic cycle, severe methemoglobinemia can occur even with small, catalytic amounts of hydroxylamines. Reactive oxygen intermediates, if not eliminated enzymically, may be responsible for hemolysis, Heinz body formation, and green pigments. In addition, nitrosoarenes bind covalently to hemoglobin and membranes and deplete glutathione by formation of glutathione-sulfinamides. Aminophenols, on the other hand, have to be activated first by oxyhemoglobin to phenoxyl radicals and quinonimines, which are reduced back with simultaneous ferrihemoglobin formation. Hence, aminophenols catalytically transfer electrons from iron to oxygen. This catalytic cycle is terminated by side reactions: p-quinonimines form adducts with glutathione and hemoglobin. Thereby the physiological functions of hemoglobin can be greatly altered as shown for 4-dimethylaminophenol. o-Quinonimines either condense to the respective phenoxazones, or if condensation is hindered, they form adducts, mainly with thiols. The different pathways for o-aminophenols are concentration-dependent, with adduct formation being favoured at low concentrations. Thus, methemoglobin formation poorly correlates with the implications of reactive electrophilic intermediates.

Conjugation of ubiquitin to denatured hemoglobin is proportional to the rate of hemoglobin degradation in HeLa cells

Ubiquitin was radioiodinated and introduced into HeLa cells by the erythrocyte-mediated fusion procedure. Fractionation of injected HeLa cells and subsequent NaDodSO4/polyacrylamide gel electrophoresis showed that HeLa nuclei contained two major labeled proteins: ubiquitin and the histone H2A-ubiquitin conjugate, protein A24. HeLa cytosol contained ubiquitin and a series of ubiquitin-protein conjugates of diverse molecular weights. When injected HeLa cells were treated with phenylhydrazine to denature the cotransferred hemoglobin, a series of prominent ubiquitin-globin conjugates appeared. The identity of these conjugates was established by microinjection experiments in which both proteins were labeled. At low doses of phenylhydrazine, the intracellular concentration of globin-ubiquitin conjugates was proportional to the rate of hemoglobin degradation. This result, together with the observation that ubiquitin conjugation to globin is markedly enhanced by phenylhydrazine-induced denaturation of hemoglobin, provides support for the hypothesis that the covalent attachment of ubiquitin to proteins signals proteolysis.

beta-meso-Phenylbiliverdin IX alpha and N-phenylprotoporphyrin IX, products of the reaction of phenylhydrazine with oxyhemoproteins

Oxyhemoglobin and oxymyoglobin were allowed to react aerobically with phenylhydrazine and p-tolylhydrazine. The chloroform extract of each reaction mixture, after treatment with H2SO4/methanol, yielded a blue pigment and a green pigment, which were identified by electronic absorption, mass, and proton NMR spectroscopy as the dimethyl esters of beta-meso-arylbiliverdin IX alpha and N-arylprotoporphyrin IX, respectively. N-Phenylprotoporphyrin IX dimethyl ester formed complexes with Zn2+, Cd2+, and Hg2+ but not with other cations. The proton NMR spectrum of the zinc complex suggested binding of the phenyl group to one of the two pyrrole rings of protoporphyrin IX with a propionic acid substituent. The effectiveness of phenylhydrazine as an inducer of Heinz body formation may be due to destabilization of the hemoglobin molecule by the replacement of heme with phenyl adducts of biliverdin and protoporphyrin.

Intracellular degradation of hemoglobin transferred into fibroblasts by fusion with red blood cells

Hamster fibroblast protein and rabbit hemoglobin were labelled by incubation of fibroblasts (BHK21) or reticulocytes with [3H]leucine. Alternatively, human or rabbit hemoglobin was labelled by carbamoylation of erythrocytes with K14CNO. The labelled hemoglobins were introduced into fibroblasts by virus-mediated fusion between the blood cells and fibroblasts. The hemoglobins became uniformly distributed throughout the cytoplasm. Degradation was assessed from release of acid-soluble radioactivity into the medium. Radioactivity from [14C]-carbamoylhemoglobin was released as carbamoylvaline and homocitrulline, and these compounds were not metabolized or reincorporated by the cells. Intermediate degradation products could not be detected. The degradation of hemoglobin followed first-order kinetics. The half-life of both carbamoylated and native rabbit hemoglobin in hamster fibroblasts was 28 h, and the half-life of carbamoylated human hemoglobin was about 150 h in fibroblasts from hamster (BHK21), mouse (Balb/3T3), and man (MRC 5), corresponding to that of the more stable endogenous proteins. Phenylhydrazine increased the intracellular degradation of carbamoylated human hemoglobin about 13 times, whereas the degradation of endogenous proteins was little affected. Hemoglobin was degraded in homogenates at 31% h-1 at pH 5 and 0.3% h-1 at pH 7.4. Phenylhydrazine increased these rates to 45% h-1 and 9.7% h-1, respectively. Growing hamster fibroblasts, which are brought into quiescence by serum deprivation or by high culture density, increase the degradation of endogenous protein and of hemoglobin in parallel.

Biotransformation of nitrosobenzene in the red cell and the role of glutathione

1. In the red cell nitrosobenzene formed glutathione-sulphinanilide from reduced glutathione, and the corresponding sulphinanilide with the reactive cysteine residues of haemoglobin. 2. Glutathionesulphinanilide was reductively cleaved by an NADPH-linked reductase with formation of free analine half an equivalent of reduced glutathione and half of glutathione sulphinic acid. 3. About three quarters of the aniline produced from nitrosobenzene or phenylhydroxylamine was formed via this pathway within the red cell.