Copper and the oxidation of hemoglobin: a comparison of horse and human hemoglobins

Strain-promoted azide–alkyne cycloaddition for protein–protein coupling in the formation of a bis-hemoglobin as a copper-free oxygen carrier

Conventional chemical approaches to protein–protein coupling present challenges due to the intrinsic competition between the desired interactions of reagents with groups of the protein as well as reactions with water.

Chemical physiology of blood flow regulation by red blood cells: the role of nitric oxide and S-nitrosohemoglobin

Blood flow in the microcirculation is regulated by physiological oxygen (O2) gradients that are coupled to vasoconstriction or vasodilation, the domain of nitric oxide (NO) bioactivity. The mechanism by which the O2 content of blood elicits NO signaling to regulate blood flow, however, is a major unanswered question in vascular biology. While the hemoglobin in red blood cells (RBCs) would appear to be an ideal sensor, conventional wisdom about its chemistry with NO poses a problem for understanding how it could elicit vasodilation. Experiments from several laboratories have, nevertheless, very recently established that RBCs provide a novel NO vasodilator activity in which hemoglobin acts as an O2 sensor and O2-responsive NO signal transducer, thereby regulating both peripheral and pulmonary vascular tone. This article reviews these studies, together with biochemical studies, that illuminate the complexity and adaptive responsiveness of NO reactions with hemoglobin. Evidence for the pivotal role of S-nitroso (SNO) hemoglobin in mediating this response is discussed. Collectively, the reviewed work sets the stage for a new understanding of RBC-derived relaxing activity in auto-regulation of blood flow and O2 delivery and of RBC dysfunction in disorders characterized by tissue O2 deficits, such as sickle cell disease, sepsis, diabetes, and heart failure.

Redox reactions of hemoglobin

Redox reactions of hemoglobin have gained importance because of the general interest of the role of oxidative stress in diseases and the possible role of red blood cells in oxidative stress. Although electron paramagnetic resonance (EPR) is extremely valuable in studying hemoglobin redox reactions it has not been adequately used. We have focused in this review on the important contributions of EPR to our understanding of hemoglobin redox reactions. We have limited our discussion to the redox reactions thought to occur under physiological conditions. This includes autoxidation as well as the reactions of hydrogen peroxide generated by superoxide dismutation. We have also discussed redox reactions associated with nitric oxide produced in the circulation. We have pinpointed the value of using EPR to detect and study the paramagnetic species and free radicals formed during these reactions. We have shown how EPR not only identifies the paramagnetic species formed but can also be used to provide insights into the mechanism involved in the redox reactions.

Protein disulfide isomerase, a multifunctional protein chaperone, shows copper-binding activity

Protein disulfide isomerase (PDI) is a 55 kDa multifunctional protein of the endoplasmic reticulum (ER) involved in protein folding and isomerization. In addition to the chaperone and catalytic functions, PDI is a major calcium-binding protein of the ER. Although the active site of PDI has a similar motif CXXC to the Cu-binding motif in Wilson and Menkes proteins and in other copper chaperones, there has been no report on any metal-binding capability of PDI other than calcium binding. We present evidence that PDI is a copper-binding protein. In the absence of reducing agent freshly reduced PDI can bind a maximum of 4 mol of Cu(II) and convert to Cu(I). These bound Cu(I) are surface exposed as they can be competed readily by BCS reagent, a Cu(I) specific chelator. However, when the binding is performed using the mixture of Cu(II) and 1mM DTT, the total number of Cu(I) bound increases to 10 mol/mol, and it is slower to react with BCS, indicating a more protected environment. In both cases, the copper-bound forms of PDI exist as tetramers while apo-protein is a monomer. These findings suggest that PDI plays a role in intracellular copper disposition.

The oxidative effect of bacterial lipopolysaccharide on native and cross-linked human hemoglobin as a function of the structure of the lipopolysaccharide

The binding of lipopolysaccharide (LPS, also known as bacterial endotoxin) to human hemoglobin is known to result in oxidation of hemoglobin to methemoglobin and hemichrome. We have investigated the effects of the LPSs from smooth and rough Escherichia coli and Salmonella minnesota on the rate of oxidation of native oxyhemoglobin A0 and hemoglobin cross-linked between the alpha-99 lysines. For cross-linked hemoglobin, both smooth LPSs produced a rate of oxidation faster than the corresponding rough LPSs, indicating the importance of the binding of LPS to the hemoglobin. The effect of the LPS appeared to be largely on the initial fast phase of the oxidation reaction, suggesting modification of the heme pocket of the alpha chains. For hemoglobin A0, the rates of oxidation produced by rough and smooth LPSs were very similar, suggesting the possibility that the effect of the LPSs was to cause dissociation of hemoglobin into dimers. The participation of cupric ion in the oxidation process was demonstrated in most cases. In contrast, the rate of oxidation of cross-linked hemoglobin by the LPSs of both the rough and smooth E. coli was not affected by the presence of chelators, suggesting that cupric ion had previously bound to these LPSs. Overall, these data suggest that the physiological effectiveness of hemoglobin solutions now being developed for clinical use may be decreased by the presence of lipopolysaccharide in the circulation of recipients.

Lanthanide ions induce hydrolysis of hemoglobin-bound 2,3-diphosphoglycerate (2,3-DPG), conformational changes of globin and bidirectional changes of 2,3-DPG-hemoglobin's oxygen affinity

The changes in structure and function of 2,3-diphosphoglycerate-hemoglobin (2,3-DPG-Hb) induced by Ln(3+) binding were studied by spectroscopic methods. The binding of lanthanide cations to 2,3-DPG is prior to that to Hb. Ln(3+) binding causes the hydrolysis of either one from the two phosphomonoester bonds in 2,3-DPG non-specifically. The results using the ultrafiltration method indicate that Ln(3+) binding sites for Hb can be classified into three categories: i.e. positive cooperative sites (N(I)), non-cooperative strong sites (N(S)) and non-cooperative weak sites (N(W)) with binding constants in decreasing order: K(I)>K(S)>K(W). The total number of binding sites amounts to about 65 per Hb tetramer. Information on reaction kinetics was obtained from the change of intrinsic fluorescence in Hb monitored by stopped-flow fluorometry. Fluctuation of fluorescence dependent on Ln(3+) concentration and temperature was observed and can be attributed to the successive conformational changes induced by Ln(3+) binding. The results also reveal the bidirectional changes of the oxygen affinity of Hb in the dependence on Ln(3+) concentration. At the range of [Ln(3+)]/[Hb]<2, the marked increase of oxygen affinity (P(50) decrease) with the Ln(3+) concentration can be attributed to the hydrolysis of 2,3-DPG, while the slight rebound of oxygen affinity in higher Ln(3+) concentration can be interpreted by the transition to the T-state of the Hb tetramer induced by Ln(3+) binding. This was indicated by the changes in secondary structure characterized by the decrease of alpha-helix content.

Internal electron transfer between hemes and Cu(II) bound at cysteine beta93 promotes methemoglobin reduction by carbon monoxide

Previous studies showed that CO/H2O oxidation provides electrons to drive the reduction of oxidized hemoglobin (metHb). We report here that Cu(II) addition accelerates the rate of metHb beta chain reduction by CO by a factor of about 1000. A mechanism whereby electron transfer occurs via an internal pathway coupling CO/H2O oxidation to Fe(III) and Cu(II) reduction is suggested by the observation that the copper-induced rate enhancement is inhibited by blocking Cys-beta93 with N-ethylmaleimide. Furthermore, this internal electron-transfer pathway is more readily established at low Cu(II) concentrations in Hb Deer Lodge (beta2His --> Arg) and other species lacking His-beta2 than in Hb A0. This difference is consistent with preferential binding of Cu(II) in Hb A0 to a high affinity site involving His-beta2, which is ineffective in promoting electron exchange between Cu(II) and the beta heme iron. Effective electron transfer is thus affected by Hb type but is not governed by the R left arrow over right arrow T conformational equilibrium. The beta hemes in Cu(II)-metHb are reduced under CO at rates close to those observed for cytochrome c oxidase, where heme and copper are present together in the oxygen-binding site and where internal electron transfer also occurs.

Biphasic nature in the autoxidation reaction of human oxyhemoglobin

In comparison with myoglobin molecule as a reference, we have studied the autoxidation rate of human oxyhemoglobin (HbO2) as a function of its concentration in 0.1 M buffer at 35 degrees C and in the presence of 1 mM EDTA. At pH 6.5, HbA showed a biphasic autoxidation reaction that can be described completely by a first-order rate equation containing two rate constants-kf, for fast autoxidation of the alpha-chain, and ks, for slow autoxidation of the beta-chain, respectively. When tetrameric HbO2 was dissociated into alpha beta-dimers by dilution, the value of kf increased markedly to an extent comparable with the autoxidation rate of horse heart oxymyoglobin (MbO2). The rate constant Ks, on the other hand, was found to remain at an almost constant value over the whole concentration range from 1.0 x 10(-3) M to 3.2 x 10(-6) M in heme. At pH 8.5 and pH 10.0, however, the autoxidation of HbO2 was monophasic, and no enhancement in the rate was observed by diluting hemoglobin solutions. Taking into consideration the effects of 2,3-diphosphoglyceric acid and chloride anion on the autoxidation rate of HbO2, we have characterized the differential susceptibility of the alpha- and beta-chains to the autoxidation reaction in aqueous solution.

Site-directed mutagenesis of histidine residues involved in Cu(II) binding and reduction by sperm whale myoglobin

Sperm whale myoglobin (Mb) reduces Cu(II) through a site-specific mechanism involving complexation by one or more surface histidine residues. Three mutants of Mb, derived from recombinant wild-type Mb, were designed in which surface histidine residues exhibiting strong Cu(II) binding were replaced with amino acids with comparatively poor metal binding characteristics. The kinetics of Cu(II)(Gly)2 reduction by native Mb, recombinant wild-type Mb, and the mutants were compared. Recombinant wild-type Mb reduced Cu(II) at a rate similar to that of native Mb. Two single mutations (His-48----Ala and His-116----Asp) decreased the rate by 31% and 7%, respectively, relative to wild-type Mb and decreased the rate by 38% and 16%, respectively, relative to native Mb. A double mutation (His-113----Ala, His-116----Asp) decreased the rate only slightly more than the single mutation at His-116. Previous NMR studies showed that His-113 exhibits the strongest Cu(II) binding of all surface histidines, but the present experiments suggest that it plays little or no role in the reduction of Cu(II) by Mb. His-48, located 12.7 A from the Fe(II)-heme, participates in one-third of the redox activity of the protein. His-116 appears to play a minor role in the overall redox activity of Mb, but its involvement shows that Mb has the ability to reduce Cu(II) through a histidine residue located more than 20 A from the Fe(II)-heme. These experiments demonstrate that electron transport from the Fe(II)-heme to site-specifically bound Cu(II) can be mediated through multiple pathways in sperm whale Mb.

Glutathione-mediated transfer of copper(I) into American lobster apohemocyanin

Copper in the cytosol of the hepatopancreas of the American lobster, Homarus americanus, occurs as copper-metallothionein [Cu(I)-MT] and as a copper-glutathione complex [Cu(I)-GSH]. The latter can act in vitro as the source of Cu(I) in the reconstitution of lobster apohemocyanin, whereas Cu(I)-MT cannot. Here we report on the mechanism of the GSH-mediated reconstitution. Binding of Cu(I) to apohemocyanin was measured by its effect on the protein's fluorescence, by ultrafiltration experiments and size-exclusion HPLC. Reconstitution of CO and O2 binding was studied using the [Cu(I)...Cu(I)-CO] fluorescence of hemocyanin and its Cu-O2-Cu charge-transfer band as spectral probes. The hemocyanin oligomer has 1 (1.02 +/- 0.09) high-affinity (apparent Kdiss = 1.67 +/- 0.40 microM) external binding site for ionic Cu(I) per subunit. Binding of Cu(I) to this site is fast and reversible and is followed by a slow, irreversible incorporation of copper into the protein matrix. Movement of the first copper through the matrix to the active site is the rate-limiting step in the reconstitution process. Mononuclear copper sites, once formed, are rapidly converted into biologically active, binuclear copper sites. In accordance with this reaction sequence, the restoration of CO/O2 binding by hemocyanin is a first-order reaction with a half-time of 100 +/- 5 min at pH 6.0. Reconstitution is extremely pH-dependent and proceeds best at those pH values where the architecture of the copper pocket of hemocyanin is open as judged from its extremely low affinity for oxygen and its very fast oxygen dissociation rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by thiols of copper(II)-induced oxidation of oxyhemoglobin

The ability of thiols, 2-imidazolethiones and uric acid to protect bovine oxyhemoglobin from copper(II)-induced oxidation to methemoglobin was investigated. The oxidation of oxyhemoglobin by Cu(II) proceeded in two phases: (1) an initial rapid reaction (less than 30 s) followed by (2) a slower reaction that carried it to completion. Thiols, including N-acetyl-L-cysteine, DL-dithiothreitol, reduced glutathione, DL-homocysteine, 2-mercaptoethanol and 2- and 3-mercaptopropionic acid, whose sulfhydryl groups were slowly oxidized by Cu(II) (with the exception of 2-mercaptopropionic acid), protected oxyhemoglobin in both phases of the reaction. Other thiols, including L-cysteine, cysteamine, and D-penicillamine, whose sulfhydryl groups were readily oxidized by Cu(II), protected hemoglobin initially, but within 2-4 min, the rate of methemoglobin formation was the same as Cu(II)-treated oxyhemoglobin. 2-Mercaptoimidazole and 1-methyl-2-mercaptoimidazole, which complex Cu(II) and inhibit Cu(II)-catalyzed oxidation of ascorbic acid, also protected hemoglobin in the initial phase, but not in the second phase. Uric acid, L-ergothioneine, and thiourea did not protect oxyhemoglobin in either the fast or slow phase. Cu(II) may have a coordination site involved in the oxidation of hemoglobin that is not blocked by the 2-imidazolethiones, uric acid, or the oxidized thiols. It is concluded that certain thiols that complex Cu(II) and are not rapidly oxidized will protect oxyhemoglobin from Cu(II)-induced oxidation, but the thiols are no longer effective once they are oxidized.

Interaction of copper-metallothionein from the American lobster, Homarus americanus, with glutathione

Organisms have harnessed the unique chemistry of copper for a variety of purposes. However, that same chemistry makes this essential metal toxic at elevated concentrations. Metallothioneins (MTs), a family of small metal-binding proteins, are thought to play a crucial role in the regulation of this reactive ion. Here we report that copper-metallothioneins from the American lobster, Homarus americanus, interact with the tripeptide glutathione (gamma-Glu-Cys-Gly). Glutathione in the cytosolic fraction prepared from the digestive gland of the American lobster coelutes with copper-metallothionein during size-exclusion chromatography. The latter protein can be separated into three isoforms by anion-exchange chromatography. All three isoforms belong to the class I MTs. CuMT-I and -II are very similar, whereas CuMT-III is distinct from isoforms I and II. The interaction between glutathione and MT isoforms was examined by ultrafiltration experiments and size-exclusion HPLC. CuMT-III forms a stable 1:1 complex with glutathione, with a dissociation constant of 1 microM. CuMT-I/II makes a transient complex with glutathione, which releases copper as a copper-glutathione complex. This complex can function as the source of Cu(I) in the restoration of the oxygen-binding capacity of copper-free hemocyanin. These studies suggest that metallothionein and glutathione are intricately linked in the biochemistry of copper regulation.

Kinetics and mechanisms of reduction of Cu(II) and Fe(III) complexes by soybean leghemoglobin alpha

The reduction of low-molecular-weight Cu(II) and Fe(III) complexes by soybean leghemoglobin alpha was characterized using both kinetic analysis and 1H-NMR experiments. Whereas Fe(III) (CN)6(3-) was reduced through an outer sphere transfer over the exposed heme edge, all other Cu(II) and Fe(III) complexes investigated were reduced via a site-specific binding of the metal to the protein. Reduction of all metal complexes was enhanced by decreasing pH while only Fe(III)NTA reduction kinetics were altered by changes in ionic strength. Rates of reduction for both Cu(II) and Fe(III) were also affected inversely by the effective binding constant of the metal chelate used. NMR data confirmed that both Cu(II)NTA and Fe(III)NTA were bound to specific sites on the protein. Cu(II) bound preferentially to distal His-61 and Fe(III) exerted its greatest effect on two surface lysine residues with epsilon proton resonances at 3.04 and 3.12 ppm. The Fe(III)NTA complex also had a mild but noticeable line broadening effect on the distal His-61 singlet resonance near 5.3 ppm. Like hemoglobin and myoglobin, leghemoglobin might function not only as an oxygen carrier, but also as a biological reductant for low-molecular-weight Cu(II) and Fe(III) complexes.

Cysteine adducts of human haemoglobin measured by isoelectric focusing in polyacrylamide gels with a non-linear pH gradient

The in vitro formation of adducts from human haemoglobin formed by alkylation with methyl-methanesulphonate, dimethyl sulphate and iodoacetamide was determined with isoelectric focusing in ultra-thin polyacrylamide gels with a non-linear pH gradient. The most important adduct seen in the gels was identified as HbA alkylated at the beta-93 cysteine. Influences of the chemical nature of the alkylating agents and of the biological environment are discussed. The method is suggested to be applicable to monitoring the biological effects of low, long-term exposure to mixtures of alkylating agents or of exposure to unknown compounds.

Temperature adaptation of fish hemoglobins reflected in rates of autoxidation

Observation of rapid autoxidation of oxyhemoglobin (HbO2) from a deep-sea fish (Co-ryphaenoides acrolepis) prompted a survey of rates of autoxidation of HbO2 to methemoglobin in vitro from fishes inhabiting vastly different depths (1 to 3800 m) in order to discover any relationship between autoxidation rates and different environmental temperatures and pressures to which hemoglobins may be adapted. The rate of autoxidation was found to be sensitive to temperature in both deep- and shallow-living fishes. Hemoglobin appears to be thermally adapted since autoxidation was 10 times faster at all temperatures tested with HbO2 obtained from a cold-adapted fish (Coryphaenoides armatus variabilis) than from a comparatively warm-adapted one (Paralabrax nebulifer). High hydrostatic pressure does not affect autoxidation rates in either deep- or shallow-dwelling species. Rates of autoxidation may be intimately related to hemoglobin function thus providing a means for studying hemoglobin adaptation in poikilothermic vertebrates, especially among cold-adapted species.

Kinetics and mechanisms of the oxidation of myoglobin by Fe(III) and Cu(II) complexes

Two distinct mechanisms by which sperm whale myoglobin reduces, respectively, complexes of Fe(III) and Cu(II) and, in turn, is oxidized to metmyoglobin have been characterized. For both mechanisms, deoxymyoglobin is the active reductant. An outer sphere electron transfer, probably at the edge of the heme, is involved for Fe(III)NTA (NTA is nitrilotriacetic acid). This pathway does not involve ionic binding of the Fe(III) complex to the protein. The most reactive species of Fe(III)NTA is uncharged. No inhibition is observed with Ni(II) or Zn(II). An outer sphere site specific electron transfer is operative for reduction of Cu(II) complexes. The site has been characterized using NMR spectroscopy and involves one or more histidines. There is an initial binding of the Cu(II) chelate. The ternary complex of chelator-Cu(II)-deoxymyoglobin is a mandatory intermediate. Ni(II) and Zn(II) compete with Cu(II) for the binding site. A scheme for the participation of either or both of these mechanisms in reduction reactions of heme proteins is proposed. Both the overall redox potential, delta E0, and the stability constant for the ternary complex, K, govern the pathway and the reaction rate.

Nitrosyliron(III) hemoglobin: autoreduction and spectroscopy

Nitrosyl complexes of the iron(III) forms of myoglobin, human hemoglobin, Glycera dibranchiata hemoglobins (Hbm and Hbh), and model iron(II) and iron(III) synthetic porphyrins including octaethylporphyrin (OEP) have been prepared. The iron(III) heme proteins are electron spin (paramagnetic) resonance (ESR) silent, while hexacoordinate solution structures are indicated for [Fe(OEP)(NO)2]ClO4 and for Hbm(II)NO, which has an ESR spectrum similar to that of Mb(II)NO and the hexacoordinate iron(II) model complex Fe(OEP)NO(BzIm). The splitting of the alpha- and beta-bands in the optical spectrum of Mb(III)NO and Hbh(III)NO contrasts markedly with the sharp, single bands observed in that of Hbm-(III)NO. The nondegeneracy of the dxz and dyz orbitals in Mb(III)NO and Hbh(III)NO is attributed to the influence of the distal histidine. Circular dichroism spectra were obtained for Hbm(III)NO, Hbm(II)NO, Hbh(III)NO, Hbh(II)NO, Mb(II)NO, and Mb(III)NO. The vicinal chiral center contribution that governs the heme protein CD leads to low Kuhn anisotropies, which have been used to assign certain electronic transitions. The Hb(III)NO spectrum is not stable but transforms into that of Hb(II)NO. This autoredox process follows kinetics that are first order in FeIIINO. The relative rates of autoreduction (25 degrees C, 1 atm NO) are Mb(III)NO less than Hbm(III)NO less than Hb alpha(III)NO less than HbA(III)NO. At high NO partial pressure or after "recycling" of HbA, the rates of reduction decrease. The first step in the reaction of NO with the ferric heme is the reversible formation of the formally iron(III) adduct. This reacts with another molecule of NO, generating the final heme(II)-NO via nitrosylation of NO itself or of an endogenous nucleophile. Kinetic and spectroscopic evidence shows involvement of trans-heme-(NO)2 in the reaction. The activation parameters delta H and delta S were determined. The overall reaction is photoenhanced.

Interaction between bound cupric ion and spin-labeled cysteine beta-93 in human and horse hemoglobins

The location of the various copper binding sites for horse and human hemoglobin was probed using spin labels attached to the beta-93 cysteine residue. Dipole-dipole interactions between the spin label and bound copper produce a decrease in the amplitude of the spin label spectrum which was used to estimate the Cu(II) spin label distance. By comparing the results with horse and human hemoglobin at 298 and 77 K four different Cu(II) binding sites were identified. The low affinity horse hemoglobin site with the sulfhydryl blocked (site 1) was found to be located 10-13 A from the sulfhydryl spin label on the surface of the molecule. Only with a free sulfhydryl is the site (site 2) in the pocket between the F and H helices closer to the SH-group and the iron populated. It is site 2 which is responsible for the oxidation. In frozen solutions a Cu-nitroxide distance of about 17 A was determined with human hemoglobin. This distance is consistent with the previously postulated location of the "high affinity" human hemoglobin site near the amino terminus of the beta-chain. At 298 K a much shorter Cu-nitroxide distance of about 7 A was calculated for human hemoglobin. This shorter distance at higher temperature also correlated with a slightly smaller value of g11 and A11 for the Cu(II) ESR spectrum. It is postulated that in solution cross-linking between nitrogenous ligands in the region of the amino terminus of one beta-chain and the carboxyl terminus of the other beta-chain can explain this shorter distance. This cross-link could involve histidine beta-143, which is one of the ligands thought to be also involved in site 1. Binding to the "high-affinity" site in solution thus stabilizes the "low-affinity" site 2 relative to site 1 explaining the reported interaction between the "high-affinity" and "low-affinity" sites.

Temperature dependent spectral changes of iron and nickel hemoglobins and their derivatives

Temperature dependent absolute and difference spectra for deoxy and oxy human hemoglobin, alpha and beta subunits, NiHbA, carboxypeptidase A treated deoxy HbA and NiHbA have been investigated. It is shown for the first time that the alpha subunits are mainly responsible for the temperature dependent spectral changes in the absorption spectra of Hb in the range from 0 degrees C to 40 degrees C. It has also been found that in the R state the spectral alterations caused by temperature variation are about 85% of those found for the T state of Hb. The value of following the temperature dependence of the porphyrin bands of hemoproteins, as a sensitive probe for subtle changes in the region of the heme, is demonstrated.

Interaction between low-affinity cupric ion and human methemoglobin

Human hemoglobin has been shown to contain a high- as well as a low-affinity binding site for cupric ion on each of its constitutent beta chains. The copper that is bound to the low-affinity site has been implicated in the selective oxidation of the beta hemes. In the present work a low-affinity binding site for cupric ion has been located within 10 A of the heme iron in human hemoglobin. It is suggested that the proximal histidine is involved in the binding of copper at this site and that it participates in the oxidation of heme iron and reduction of cupric ion.

Effects of 2-formylpyridine monothiosemicarbazonato copper II on red cell components

1-Formylpyridine monothiosemicarbazonato copper II (CuL+) is readily taken up by red cells and is initially bound to glutathione and hemoglobin. Glutathione was depleted within 5 hr of incubation, presumably by oxidation mediated by CuL+ and O2 with concomitant generation of toxic oxygen species. Cupric ion was slowly transferred from CuL+ to hemoglobin within about 7 hr and hemoglobin was oxidized until the major form prevailing after 10 hr was alpha 2 beta 2+. Little increase in hemolysis due to addition of CuL+ dissolved in the radical scavenger dimethyl sulfoxide was observed with prolonged incubation. Strong inhibition of red cell hexokinase by CuL+ was observed when the enzymes in red cell lysates and hemoglobin-free red cell lysates were examined. CuL+ was also an effective inhibitor of yeast hexokinase. However, the inhibitory effect of CuL+ within the red cells was less pronounced. It is suggested that even though intracellular accumulation of CuL+ creates an oxidizing environment and is potentially capable of inhibiting thiol enzymes such as hexokinase, protective effects are exerted in the red cell by the presence of hemoglobin, of radical scavengers, and of high levels of enzymes that detoxify toxic oxygen species.

Oxygen requirement for cupric ion induced hemolysis

The Cu(II) induced lysis of rabbit erythrocytes occurs in two phases. First there is a lag period of 60 min where few cells lyse, followed by a period of rapid lysis where most of the remaining cells undergo hemolysis. Lysis is effectively inhibited if the incubation is conducted in nitrogen or if the erythrocytes are pre-saturated with carbon monoxide. These results suggest that oxygenated hemoglobin is necessary for lysis. It has been reported that copper binds to oxyhemoglobin and releases superoxide ion. This additional oxidative stress can initiate lipid peroxidation which leads to cell hemolysis.

The oxidation of cat, human, and the cat-human hybrid hemoglobins alpha 2 human beta 2 cat and alpha 2 cat beta 2 human by copper(II)

The heme iron of the beta chains of mammalian hemoglobins are rapidly and selectively oxidized in the presence of excess Cu(II) ions in a reaction that requires the presence of a free -SH groups on the beta globin chain. The presence of freely reactive -SH groups on the alpha chains of cat and sheep hemoglobins does not alter the course of this reaction; only the beta hemes are oxidized rapidly by Cu(II) in these hemoglobins. Two equivalents of copper are required for the rapid oxidation of the two beta chain hemes per mole of cat hemoglobin, in contrast with the four equivalents that are required for reaction with human hemoglobin. The human-cat hybrid hemoglobins, alpha 2 Human beta 2 Cat and alpha 2 Cat beta 2 Human, required two and four equivalents of copper/mol, respectively, for the reaction. Thus, the kinetics and stoichimetry of the reaction are determined by the nature of the beta subunit. Analysis of the esr spectra of the products of the reaction of Cu(II) with these hemoglobins indicate that human hemoglobin and the hybrid alpha 2 Cat beta 2 Human contain tight binding sites for two equivalents of Cu(II) that are not involved in the oxidation reaction and are not present in cat hemoglobin or alpha 2 Human beta 2 Cat. Cat beta globin like others (sheep, bovine) that lack the tight binding site, has no histidine residue at 2 beta. It has phenylalanine in this position. These results support the suggestion of Rifkind et al. (Biochemistry 15,5337[1976]) that the tight binding site is near the amino terminal region of the beta chain and is associated with histidine 2 beta.