Covalent alteration of the prosthetic heme of human hemoglobin by BrCCl3. Cross-linking of heme to cysteine residue 93

A novel single sensor hemoglobin domain from the thermophilic cyanobacteria Thermosynechococcus elongatus BP-1 exhibits higher pH but lower thermal stability compared to globins from mesophilic organisms

Thermosynechococcus elongatus-BP1 belongs to the class of photoautotrophic cyanobacterial organisms. The presence of chlorophyll a, carotenoids, and phycocyanobilin are the characteristics that categorize T. elongatus as a photosynthetic organism. Here, we report the structural and spectroscopic characteristics of novel hemoglobin (Hb) Synel Hb from T.elongatus, synonymous with Thermosynechococcus vestitus BP-1. The X-ray crystal structure (2.15 Å) of Synel Hb suggests the presence of a globin domain with a pre-A helix similar to the sensor domain (S) family of Hbs. The rich hydrophobic core accommodates heme in a penta-coordinated state and readily binds an extraneous ligand(imidazole). The absorption and circular dichroic spectral analysis of Synel Hb reiteratedthat the heme is in Fe^III+ state with a predominantly α-helical structure similar to myoglobin. Synel Hb displays higher resistance to structural perturbations induced via external stresses like pH and guanidium hydrochloride, which is comparable to Synechocystis Hb. However, Synel Hb exhibited lower thermal stability compared to mesophilic hemoglobins. Overall, the data is suggestive of the structural sturdiness of Synel Hb, which probably corroborates its origin in extreme thermophilic conditions. The stable globin provides scope for further investigation and may lead to new insights with scope for engineering stability in hemoglobin-based oxygen carriers.

Reductive Activation of Carbon Tetrachloride by Human Haemoglobin

The reductive metabolism of carbon tetrachloride (CC14) by human haemoglobin (Hb) was observed in vitro by absolute absorption spectra recorded under anaerobic conditions. The following results were obtained: 1) a decrease of the 430nm peak typical of free reduced Hb (Hb2+); 2) the formation of a shoulder of absorbance, attributable to the production of a complex between Hb2+and a metabolite of CC14carbon monoxide (Hb-CO); and 3) the oxidation of some Hb2+to methaemoglobin (Hb3+). The concentration of these three forms — Hb2+, Hb-CO and Hb3+— during anaerobic incubation of Hb with CC14was calculated algebraically from the absolute spectra. CO production was then calculated from the concentration of Hb-CO, using a suitable calibration curve. Interestingly, under identical experimental conditions, a substrate-dependent loss of Hb-derived haem, but not of Hb itself nor of haem-derived porphyrin fluorescence, was measured. Preliminary HPLC studies to clarify the discrepancy and, in particular, the role and fate of the haem group, showed two substrate-dependent modified haem products. The results indicate that human Hb is able to catalyse the reductive activation of CCl4, and suggest that, during the process, its prosthetic group haem may be modified by CC14metabolites to products which maintain a tetrapyrrolic structure but are unable to react with pyridine.

Hemoglobin and red blood cells catalyze atom transfer radical polymerization

Hemoglobin (Hb) is a promiscuous protein that not only transports oxygen, but also catalyzes several biotransformations. A novel in vitro catalytic activity of Hb is described. Bovine Hb and human erythrocytes were found to display ATRPase activity, i.e., they catalyzed the polymerization of vinyl monomers under conditions typical for atom transfer radical polymerization (ATRP). N-isopropylacrylamide (NIPAAm), poly(ethylene glycol) methyl ether acrylate (PEGA), and poly(ethylene glycol) methyl ether methacrylate (PEGMA) were polymerized using organobromine initiators and the reducing agent ascorbic acid in acidic aqueous solution. In order to avoid chain transfer from polymer radicals to Hb's cysteine residues, the accessible cysteines were blocked by a reaction with a maleimide. The formation of polymers with bromine chain ends, relatively low polydispersity indices (PDI), first order kinetics and an increase in the molecular weight of poly(PEGA) and poly(PEGMA) upon conversion indicate that control of the polymerization by Hb occurred via reversible atom transfer between the protein and the growing polymer chain. For poly(PEGA) and poly(PEGMA), the reactions proceeded with a good to moderate degree of control. Sodium dodecyl sulfate (SDS) gel electrophoresis, circular dichroism spectroscopy, and time-resolved ultraviolet-visible (UV-vis) spectroscopy revealed that the protein was stable during polymerization, and only underwent minor conformational changes. As Hb and erythrocytes are readily available, environmentally friendly, and nontoxic, their ATRPase activity is a useful tool for synthetic polymer chemistry. Moreover, this novel activity enhances the understanding of Hb's redox chemistry in the presence of organobromine compounds.

Reduction of hypervalent states of myoglobin and hemoglobin to their ferrous forms by thymoquinone: the role of GSH, NADH and NADPH

The reactivity of thymoquinone towards different redox states of hemoglobin and myoglobin in the presence of GSH, NADH, and NADPH was evaluated by optical spectral analysis. Thymoquinone reduces the ferryl forms (HbIV/MbIV) of both met-hemoglobin (HbIII) and met-myoglobin (MbIII) to oxy-hemoglobin (HbIIO2) and oxy-myoglobin (MbIIO2) under physiological conditions. The reaction is mediated by the intermediate quinone forms of TQ, that is, glutathionyl-dihydrothymoquinone (DHTQ-GS) and dihydrothymoquinone (DHTQ), formed from direct interaction of TQ with GSH or NADH (NADPH). In vitro incubation of oxidized human erythrocytes with TQ, DHTQ, and the GSH/TQ mixture reduces the intracellular met-Hb at different rates. In the present study, we report that TQ and its reduced derivatives can also prevent lipid peroxidation induced by the MbFeIII/H2O2 system. In this system, lipid peroxidation is induced by MbIV or a putative MbIV/.MbVI composite; it is plausible that the antioxidant function of TQ derivatives is related to their ability to reduce these oxidizing species. This is of particular biological significance, as natural quinones may participate in reducing processes that lead to recovery of hemoglobin and myoglobin during oxidative stress.

The influence of ferrylhemoglobin and methemoglobin on the human erythrocyte membrane

The aim of the study was to examine and compare the effects of methemoglobin (metHb) and ferrylhemoglobin (ferrylHb) on the erythrocyte membrane. Kinetic studies of the decay of ferrylhemoglobin (*HbFe(IV)=O denotes ferryl derivative of hemoglobin present 5 min after initiation of the reaction of metHb with H(2)O(2); ferrylHb) showed that autoredecay of this derivative is slower than its decay in the presence of whole erythrocytes and erythrocyte membranes. It provides evidence for interactions between ferrylHb and the erythrocyte membrane. Both hemoglobin derivatives induced small changes in the structure and function of the erythrocyte membrane which were more pronounced for ferrylHb. The amount of ferrylHb bound to erythrocyte membranes increased with incubation time and, after 2 h, was twice that of membrane-bound metHb. The incubation of erythrocytes with metHb or ferrylHb did not influence osmotic fragility and did not initiate peroxidation of membrane lipids in whole erythrocytes as well as in isolated erythrocyte membranes. Membrane acetylcholinesterase activity increased by about 10% after treatment of whole erythrocytes with both metHb and ferrylHb. ESR spectra of membrane-bound maleimide spin label demonstrated minor changes in the conformation of label-binding proteins in ferrylHb-treated erythrocyte membranes. The fluidity of the membrane surface layer decreased slightly after incubation of erythrocytes and isolated erythrocyte membranes with ferrylHb and metHb. In whole erythrocytes, these changes were not stable and disappeared during longer incubation.

Identification and characterization of heme-interacting proteins in the malaria parasite, Plasmodium falciparum

The degradation of hemoglobin by the malaria parasite, Plasmodium falciparum, produces free ferriprotoporphyrin IX (FP) as a toxic by-product. In the presence of FP-binding drugs such as chloroquine, FP detoxification is inhibited, and the build-up of free FP is thought to be a key mechanism in parasite killing. In an effort to identify parasite proteins that might interact preferentially with FP, we have used a mass spectrometry approach. Proteins that bind to FP immobilized on agarose include P. falciparum glyceraldehyde-3-phosphate dehydrogenase (PfGAPDH), P. falciparum glutathione reductase (PfGR), and P. falciparum protein disulfide isomerase. To examine the potential consequences of FP binding, we have examined the ability of FP to inhibit the activities of GAPDH and GR from P. falciparum and other sources. FP inhibits the enzymic activity of PfGAPDH with a Ki value of 0.2 microm, whereas red blood cell GAPDH is much less sensitive. By contrast, PfGR is more resistant to FP inhibition (Ki > 25 microm) than its human counterpart. We also examined the ability of FP to inhibit the activities of the additional antioxidant enzymes, P. falciparum thioredoxin reductase, which exhibits a Ki value of 1 microm, and P. falciparum glutaredoxin, which shows more moderate sensitivity to FP. The exquisite sensitivity of PfGAPDH to FP may indicate that the glycolytic pathway of the parasite is particularly susceptible to modulation by FP stress. Inhibition of this pathway may drive flux through the pentose phosphate pathway ensuring sufficient production of reducing equivalents to counteract the oxidative stress induced by FP build-up.

Immunochemical detection of hemoglobin-derived radicals formed by reaction with hydrogen peroxide: involvement of a protein-tyrosyl radical

To investigate the involvement of a hemoglobin radical in the human oxyhemoglobin (oxyHb) or metHb/H2O2 system, we have used a new approach called "immuno-spin trapping," which combines the specificity and sensitivity of both spin trapping and antigen:antibody interactions. Previously, a novel rabbit polyclonal anti-DMPO nitrone adduct antiserum, which specifically recognizes protein radical-derived nitrone adducts, was developed and validated in our laboratory. In the present study, the formation of nitrone adducts on hemoglobin was shown to depend on the oxidation state of the iron heme, the concentrations of H2O2 and DMPO, and time as determined by enzyme-linked immunosorbent assay (ELISA) and by Western blotting. The presence of reduced glutathione or L-ascorbate significantly decreased the level of nitrone adducts on metHb in a dose-dependent manner. To confirm the ELISA results, Western blotting analysis showed that only the complete system (oxy- or metHb/DMPO/H2O2) generates epitopes recognized by the antiserum. The specific modification of tyrosine residues on metHb by iodination nearly abolished antibody binding, while the thiylation of cysteine residues caused a small but reproducible decrease in the amount of nitrone adducts. These findings strongly suggest that tyrosine residues are the site of formation of the immunochemically detectable hemoglobin radical-derived nitrone adducts. In addition, we were able to demonstrate the presence of hemoglobin radical-derived nitrone adducts inside red blood cells exposed to H2O2 and DMPO. In conclusion, our new approach showed several advantages over EPR spin trapping with the anti-DMPO nitrone adduct antiserum by demonstrating the formation of tyrosyl radical-derived nitrone adduct(s) in human oxyHb/metHb at much lower concentrations than was possible with EPR and detecting radicals inside RBC exposed to H2O2.

Alteration of the heme prosthetic group of neuronal nitric-oxide synthase during inactivation by N(G)-amino-L-arginine in vitro and in vivo

It is established that N(G)-amino-L-arginine (NAA) is a metabolism-based inactivator of all three major nitric-oxide synthase (NOS) isoforms. The mechanism by which this inactivation occurs, however, is not well understood. In the current study, we discovered that inactivation of the neuronal isoform of NOS (nNOS) by NAA in vitro results in covalent alteration of the heme prosthetic group, in part, to products that contain an intact porphyrin ring and are either dissociable from or irreversibly bound to the protein. The alteration of the heme is concomitant with the loss of nNOS activity. Studies with nNOS containing a 14C-labeled prosthetic heme moiety indicate that the major dissociable product and the irreversibly bound heme adduct account for 21 and 28%, respectively, of the heme that is altered. Mass spectral analysis of the major dissociable product gave a molecular ion of m/z 775.3 that is consistent with the mass of an adduct of heme and NAA minus a hydrazine group. Peptide mapping of the irreversibly bound heme adduct indicates that the heme is bound to a residue in the oxygenase domain of nNOS. We show for the first time that metabolism-based inactivation of nNOS occurs in vivo as highly similar heme products are formed. Because inactivation and alteration may trigger ubiquitination and proteasomal degradation of nNOS, NAA may be a useful biochemical tool for the study of these basic regulatory processes.

The origin of red cell fluorescence caused by hydrogen peroxide treatment

Fluorescence in red cells following hydrogen peroxide treatment has been attributed to lipid peroxidation of the membrane. The putative relationship between lipid peroxidation and fluorescence was questioned by the finding that BHT and alpha-tocopherol, which are thought to inhibit lipid peroxidation, do not inhibit the fluorescence detected by flow cytometry. Furthermore, lipid peroxidation induced in red cells by the Fe(III)-ADP-ascorbate system did not produce fluorescence. These results require an alternative explanation for the hydrogen peroxide-induced fluorescence. A role for reduced hemoglobin is indicated by the inhibition of fluorescence by pretreatment of cells with CO that binds strongly to ferrohemoglobin and nitrite that oxidizes ferrohemoglobin. Our earlier studies have shown the formation of fluorescent heme degradation products during the reaction of purified hemoglobin with hydrogen peroxide, which was also inhibited by CO and nitrite pretreatment. The fluorescence produced in red cells after the addition of hydrogen peroxide can, therefore, be attributed to fluorescent heme degradation products.

Laboratory of Chemical Pharmacology, National Heart, Lung, and Blood Institute, NIH: a short history

The Laboratory of Chemical Pharmacology (LCP) began in 1950 as the Section of Pharmacology within the National Heart Institute, the National Institutes of Health. Its first chief was Bernard B. Brodie, considered by many to be one of the fathers of modern pharmacology. Since its inception, LCP has made many significant contributions to the fields of pharmacology and toxicology. LCP was among the first to study (a) the effects of drugs on the turnover of serotonin and norepinephrine in brain and other tissues, (b) the absorption of drugs from the gastrointestinal tract and their passage across the blood-brain barrier, (c) the oxidation and reduction of drugs and other foreign compounds by liver microsomal enzymes (later known as the cytochrome P450 enzymes) and inhibitors and inducers of these enzymes, (d) the formation of toxic chemically reactive metabolites of drugs and other foreign compounds, and (e) mechanisms of immunological responses. Approximately 300 scientists worked in LCP during its existence, and they and their collaborators published more than 1,300 papers. This is a short history of the people who worked in it and of their contributions to biomedical sciences.

Aminoguanidine-mediated inactivation and alteration of neuronal nitric-oxide synthase

It is established that aminoguanidine (AG) is a metabolism-based inactivator of the three major isoforms of nitric-oxide synthase. AG is thought to be of potential use in diseases, such as diabetes, where pathological overproduction of NO is implicated. We show here that during the inactivation of neuronal nitric-oxide synthase (nNOS) by AG that the prosthetic heme is altered, in part, to dissociable and protein-bound adducts. The protein-bound heme adduct is the result of cross-linking of the heme to residues in the oxygenase domain of nNOS. The dissociable heme product is unstable and reverts back to heme upon isolation. The alteration of the heme is concomitant with the loss in the ability to form the ferrous-CO complex of nNOS and accounts for at least two-thirds of the activity loss. Studies with [(14)C]AG indicate that alteration of the protein, in part on the reductase domain of nNOS, also occurs but at low levels. Thus, heme alteration appears to be the major cause of nNOS inactivation. The elucidation of the mechanism of inactivation of nNOS will likely lead to a better understanding of the in vivo effects of NOS inhibitors such as AG.

Chemiluminescence assay for oxidatively modified myoglobin

Treatment of myoglobin with H2O2 results in covalent alteration of the heme prosthetic group, in part, to protein-bound adducts. These protein-bound heme adducts are known to be redox active and are suspected to participate in oxidative tissue injury. In the course of our studies on the toxicological role of these heme adducts, we sought to develop a sensitive assay for their detection and quantitation. We have discovered that protein-bound heme adducts, due to their inherent peroxidase activity, can be detected with the use of enhanced chemiluminescence detection reagents, following SDS-PAGE and electroblotting. The assay is specific for protein-bound heme adducts as we have identified conditions where noncovalently bound hemes are completely dissociated from the protein during electrophoresis. Signal intensity was quantified by laser densitometry and found to be linear over a concentration range of 0.44-22 pmol of protein-bound heme adduct, which represented a 20-fold greater sensitivity than the currently available HPLC method. Moreover, we have identified tris(2-carboxyethyl)phosphine as a thiol reducing agent that does not interfere with the detection of the heme-mediated peroxidase activity. The current method may be utilized to identify heme-binding regions of proteins in addition to the detection of oxidatively modified myoglobin.

Identification of the heme-modified peptides from cumene hydroperoxide-inactivated cytochrome P450 3A4

Cumene hydroperoxide-mediated (CuOOH-mediated) inactivation of cytochromes P450 (CYPs) results in destruction of their prosthetic heme to reactive fragments that irreversibly bind to the protein. We have attempted to characterize this process structurally, using purified, 14C-heme labeled, recombinant human liver P450 3A4 as the target of CuOOH-mediated inactivation, and a battery of protein characterization approaches [chemical (CNBr) and proteolytic (lysylendopeptidase-C) digestion, HPLC-peptide mapping, microEdman sequencing, and mass spectrometric analyses]. The heme-peptide adducts isolated after CNBr/lysylendopeptidase-C digestion of the CuOOH-inactivated P450 3A4 pertain to two distinct P450 3A4 active site domains. One of the peptides isolated corresponds to the proximal helix L/Cys-region peptide 429-450 domain and the others to the K-region (peptide 359-386 domain). Although the precise residue(s) targeted remain to be identified, we have narrowed down the region of attack to within a 17 amino acid peptide (429-445) stretch of the 55-amino acid proximal helix L/Cys domain. Furthermore, although the exact structures of the heme-modifying fragments and the nature of the adduction remain to be established conclusively, the incremental masses of approximately 302 and 314 Da detected by electrospray mass spectrometric analyses of the heme-modified peptides are consistent with a dipyrrolic heme fragment comprised of either pyrrole ring A-D or B-C, a known soluble product of peroxidative heme degradation, as a modifying species.

Formation of fluorescent heme degradation products during the oxidation of hemoglobin by hydrogen peroxide

Hemoglobin and methemoglobin oxidized by hydrogen peroxide generate ferrylhemoglobin and oxoferrylhemoglobin, respectively. Two fluorescent compounds were found to be produced during the reaction of oxyhemoglobin, but not methemoglobin, with H2O2. These two compounds had excitation wavelengths of 321 nm and 460 nm, respectively, with emission wavelengths of 465 nm and 525 nm, respectively. The formation of the same fluorescent products during the reaction of H2O2 with ferroprotoporphyrin-IX and ferriprotoporphyrin-IX demonstrate that these compounds originate from the heme moiety. The release of heme iron during the formation of these fluorescent compounds indicates that they are associated with heme degradation. The time course for the formation of fluorescent products show that the extent of heme degradation is dependent on H2O2 concentration. The results of this investigation indicate that the heme moiety of Fe(II) hemoglobin undergoes degradation in presence of H2O2. The ability to detect this process by fluorescence provides a sensitive marker in order to asses hemoglobin and RBC oxidative stress under pathological conditions.

Heme oxygenase active-site residues identified by heme-protein cross-linking during reduction of CBrCl3

The reduction of CBrCl3 by the heme-heme oxygenase complex forms dissociable and covalently bound heme products. No such products are formed with mesoheme in which the heme vinyl substituents are replaced by ethyl groups. The dissociable heme products are chromatographically similar but not identical to those obtained in the analogous reaction with myoglobin. Tryptic digestion of the heme-protein adduct and Edman sequencing and mass spectrometric analysis of the heme-linked peptide identify His-25, the proximal iron ligand, as the alkylated residue. Reaction of CBrCl3 with the heme complexes of the T135V mutant and a Delta221 C-terminal truncated protein yields heme-linked peptides in addition to that from the wild-type reaction. The sequence of the principal labeled peptide from the T135V reaction, 205TAFLLNIQLFEELQELLTHDTK226 , and the lability of the adduct suggest the heme is attached to one of the carboxylic acid residues. A carboxylic acid residue is probably also labeled in the modified peptide 49LVMASLYHIYVALEEEIER67 from the Delta221 truncated protein. Thus, addition of the reductively generated trichloromethyl radical to a heme vinyl group produces a species that alkylates active-site residues. The changes in the alkylated residue caused by the Thr-135 mutation or truncation of the protein places residues in the sequences 49-67 and 205-226 within the active site. Furthermore, this is the first demonstration that heme oxygenase, like cytochrome P450, may catalyze the reductive metabolism of halocarbons and thus contribute to the toxicity of these agents.

Covalent crosslinking of the heme prosthetic group to myoglobin by H2O2: toxicological implications

It is known that treatment of myoglobin with H2O2 leads to covalent alteration of the heme prosthetic group with concomitant formation of a protein bound heme adduct and transforms myoglobin from an oxygen storage protein to an oxidase. In the current study it was shown, with the use of 14C-labeled heme reconstituted into apomyoglobin, that up to 88% of the oxidatively altered heme can be accounted for by the protein bound product. Furthermore, a partially purified preparation of the protein bound heme adduct was introduced into human fibroblasts using the method of osmotic lysis of pinosomes and found to cause cell death (40%) within 1 h, as evidenced by trypan blue exclusion. Native myoglobin introduced into cells in the same manner or extracellular treatment by the protein bound heme adduct had no effect on cell viability. The extent of cell death could be decreased (50%) by N-acetyl-L-cysteine, indicating a potential role for reactive oxygen intermediates in this process. These results show that the covalently altered myoglobin can elicit cellular damage and suggests that similar processes may occur in vivo in pathologic conditions such as that involving cardiac ischemia and reperfusion injury, where covalently altered myoglobin may form.

Suicidal inactivation of haemoproteins by reductive metabolites of halomethanes: a structure-activity relationship study

Human haemoglobin (Hb), methaemalbumin (MHA) or rat liver microsomal cytochrome P-450 (P-450) were incubated anaerobically at microM concentrations with 1 mM carbon tetrachloride (CCl4), trichlorobromomethane (CCl3Br), chloroform (CHCl3) or methylene chloride (CH2Cl2) in presence of 1 mM sodium dithionite as the reducing agent. At the end of a 5-min incubation, haem was measured by various methods, i.e. binding spectrum with CO, pyridine-haemochromogen haem assay and porphyrin fluorescence, and compared for the four analogues. Statistically significant losses were observed, with all three haemo-protein systems, for CCi3Br, CCl4 and CHCl3, but not CH2Cl2. For Hb, the loss was greater with CCl3Br (haem assay, 63%; porphyrin fluorescence, 48%; CO binding, 24%) than with CCl4 (haem assay, 31%) or CHCl3 (haem assay, 13%). On the other hand, with MHA, CCl4 gave a dramatic loss (haem assay, 88%; porphyrin fluorescence, 83%; CO binding, 67%), which was greater than that observed with CCl3Br (haem assay, 49%; porphyrin fluorescence, 38%; CO binding, 25%). No loss was found with CHCl3. Finally, with microsomes, the inactivation was larger with CCl4 (CO binding, 58%; haem assay, 50%; porphyrin fluorescence, 33%) than with CCl3Br (CO binding, 33%; haem assay, 10%) or CHCl3 (haem assay, 9%; CO binding, 6%). In a separate set of similar experiments, an ion-pairing reverse phase HPLC method showed the formation of substrate-dependent hae-derived products during incubation of CCl3Br with Hb or microsomes, and of CCl4 with Hb. A correlation between potential for free radical formation (CCl3Br > CCl4 > CHCl3 > CH2Cl2) and extent of haem inactivation was observed with all methods for Hb, but not for microsomal P-450 or MHA. The results indicate that these halomethanes may be activated differently by different haemoproteins and suggest that their potential ability to undergo reductive metabolism may not be the only critical factor involved in P-450 haem inactivation by these chemicals.

Reductive activation of halothane by human haemoglobin results in the modification of the prosthetic haem

The metabolic activation of halothane by human haemoglobin (Hb) under reducing conditions in vitro is reported. Absolute spectra of sodium dithionite-reduced Hb, recorded during its anaerobic incubation in the presence of the substrate, showed decreasing concentrations of reduced Hb (Hb2+) with time. The loss of Hb2+ was accompanied, although only to some extent, by a concurrent oxidation to methaemoglobin (Hb3+), suggesting that electron transfer from Hb to the substrate had occurred. Reductive halothane metabolism was observed under these conditions as indicated by a dose-dependent inorganic fluoride (F-) production, which was, however, lower than that observed with heated Hb or a water soluble haem preparation (methaemalbumin). A rapid, partial loss of Hb was found upon addition of the substrate to the incubation mixture, as indicated by a decrease of the typical peak at 418 nm in the absolute spectra recorded in the presence of carbon monoxide (CO). This effect was associated with a loss of the Hb prosthetic group, haem, as shown by a decrease of the pyridine-haemochromogen reaction. Both effects were time and dose dependent. The inhibition of the Hb inactivation reaction by adding exogenous CO or the spin trapping agent N-t-butyl-alpha-phenylnitrone (PBN) to the incubation mixture beforehand indicated that (a) a reduced and free haem iron is required by Hb to activate halothane, and (b) the formation of free radical reactive metabolites of halothane is likely to be responsible for Hb inactivation. The mechanism of the reaction may involve the attack of these metabolites on the haem group of Hb, as indicated by the detection, with a reverse-phase ion-pairing HPLC system, of two Hb-derived products showing a typical haem-like absorption spectrum. The present results resemble those obtained recently with carbon tetrachloride (Ferrara et al., Alternatives to Laboratory Animals 21: 57-64, 1993) and suggest a common mechanism of activation of the two polyhalogenated alkanes by Hb.

Differential susceptibilities of the prosthetic heme of hemoglobin-based red cell substitutes. Implications in the design of safer agents

One approach to the development of an effective red cell substitute has been chemical modification of human hemoglobin to optimize oxygen transport and plasma half-life. Human hemoglobin A0 and two of these modified hemoglobins, one prepared from the cross-linking of the alpha-chains at lysine residue 99 by bis(3,5-dibromosalicyl)fumarate (Hb-DBBF) and the other by acylation of lysine residue 82 of the beta-chain by mono-(3,5-dibromosalicyl)fumarate (Hb-FMDA), were tested by HPLC for their susceptibility to oxidative damage caused by H2O2. Such oxidative insult may occur during ischemia and reperfusion of tissues after transfusion of red cell substitutes to patients with hypovolemic shock and trauma. Hb-DBBF was extremely susceptible to damage of its heme and protein moieties with stoichiometric amounts of H2O2, whereas Hb-FMDA was highly resistant, even at 10-fold molar excess and at an acidic pH of 4.7. Hemoglobin A0 was of intermediate susceptibility, exhibiting alteration of heme and protein moieties at acidic but not neutral pH. Since the degradation of heme can release the potentially toxic agent iron, Hb-FMDA may be a more promising candidate than Hb-DBBF for development as a red cell substitute. A similar approach may be used to assess the susceptibility of other hemoglobin-based red cell substitutes to oxidative damage in order to determine the molecular basis of heme and protein alteration.

Thiols, gold-thiols, zinc-thiols and the redox state of hemoglobin

The beta subunit of human hemoglobin can be oxidized site-specifically through beta-Cys-93 by Cu(II)(His)2. A series of thiol ligands, gold thiols and zinc(II) inhibit this oxidation. The thiol inhibitors formed a transient ternary intermediate involving Cu(I) with consequent inhibition of electron transfer from the Fe(II)-heme. The intermediate led to the formation of a disulfide at the beta-Cys-93 site. The most effective thiols achieved maximum inhibition at one equivalent per beta heme. Gold thiols and zinc complexes inhibited heme oxidation by competing with the Cu(II)(His)2 for the beta-Cys-93 site.