Electron spin resonance studies on a flavoprotein in neutrophil plasma membranes. Redox potentials of the flavin and its participation in NADPH oxidase

Sources of Vascular Nitric Oxide and Reactive Oxygen Species and Their Regulation

Nitric oxide (NO) is a small free radical with critical signaling roles in physiology and pathophysiology. The generation of sufficient NO levels to regulate the resistance of the blood vessels and hence the maintenance of adequate blood flow is critical to the healthy performance of the vasculature. A novel paradigm indicates that classical NO synthesis by dedicated NO synthases is supplemented by nitrite reduction pathways under hypoxia. At the same time, reactive oxygen species (ROS), which include superoxide and hydrogen peroxide, are produced in the vascular system for signaling purposes, as effectors of the immune response, or as byproducts of cellular metabolism. NO and ROS can be generated by distinct enzymes or by the same enzyme through alternate reduction and oxidation processes. The latter oxidoreductase systems include NO synthases, molybdopterin enzymes, and hemoglobins, which can form superoxide by reduction of molecular oxygen or NO by reduction of inorganic nitrite. Enzymatic uncoupling, changes in oxygen tension, and the concentration of coenzymes and reductants can modulate the NO/ROS production from these oxidoreductases and determine the redox balance in health and disease. The dysregulation of the mechanisms involved in the generation of NO and ROS is an important cause of cardiovascular disease and target for therapy. In this review we will present the biology of NO and ROS in the cardiovascular system, with special emphasis on their routes of formation and regulation, as well as the therapeutic challenges and opportunities for the management of NO and ROS in cardiovascular disease.

Evaluation of radical scavenging properties of shikonin

With the aim of developing effective anti-inflammatory drugs, we have been investigating the biochemical effects of shikonin of “Shikon” roots, which is a naphthoquinone with anti-inflammatory and antioxidative properties. Shikonin scavenged reactive oxygen species like hydroxyl radical, superoxide anion (O₂^•−) and singlet oxygen in previous studies, but its reactivity with reactive oxygen species is not completely understood, and comparison with standard antioxidants is lacking. This study aimed elucidation of the reactivity of shikonin with nitric oxide radical and reactive oxygen species such as alkyl-oxy radical and O₂^•−. By using electron paramagnetic resonance spectrometry, shikonin was found unable of reacting with nitric oxide radical in a competition assay with oxyhemoglobin. However, shikonin scavenged alkyl-oxy radical from 2,2'-azobis(2-aminopropane) dihydrochloride with oxygen radical absorbance capacity, ORAC of 0.25 relative to Trolox, and showed a strong O₂^•−-scavenging ability (42-fold of Trolox; estimated reaction rate constant: 1.7 × 10⁵ M⁻¹s⁻¹) in electron paramagnetic resonance assays with CYPMPO as spin trap. Concerning another source of O₂^•−, the phagocyte NADPH oxidase (Nox2), shikonin inhibited the Nox2 activity by impairing catalysis when added before enzyme activation (IC₅₀: 1.1 µM; NADPH oxidation assay). However, shikonin did not affect the preactivated Nox2 activity, although having potential to scavenge produced O₂^•−. In conclusion, shikonin scavenged O₂^•− and alkyl-oxy radical, but not nitric oxide radical.

Oxidative stress: an essential factor in the pathogenesis of gastrointestinal mucosal diseases

Reactive oxygen species (ROS) are generated as by-products of normal cellular metabolic activities. Superoxide dismutase, glutathione peroxidase, and catalase are the enzymes involved in protecting cells from the damaging effects of ROS. ROS are produced in response to ultraviolet radiation, cigarette smoking, alcohol, nonsteroidal anti-inflammatory drugs, ischemia-reperfusion injury, chronic infections, and inflammatory disorders. Disruption of normal cellular homeostasis by redox signaling may result in cardiovascular, neurodegenerative diseases and cancer. ROS are produced within the gastrointestinal (GI) tract, but their roles in pathophysiology and disease pathogenesis have not been well studied. Despite the protective barrier provided by the mucosa, ingested materials and microbial pathogens can induce oxidative injury and GI inflammatory responses involving the epithelium and immune/inflammatory cells. The pathogenesis of various GI diseases including peptic ulcers, gastrointestinal cancers, and inflammatory bowel disease is in part due to oxidative stress. Unraveling the signaling events initiated at the cellular level by oxidative free radicals as well as the physiological responses to such stress is important to better understand disease pathogenesis and to develop new therapies to manage a variety of conditions for which current therapies are not always sufficient.

Nitroarachidonic acid prevents NADPH oxidase assembly and superoxide radical production in activated macrophages

Nitration of arachidonic acid (AA) to nitroarachidonic acid (AANO2) leads to anti-inflammatory intracellular activities during macrophage activation. However, less is known about the capacity of AANO2 to regulate the production of reactive oxygen species under proinflammatory conditions. One of the immediate responses upon macrophage activation involves the production of superoxide radical (O2(•-)) due to the NADPH-dependent univalent reduction of oxygen to O2(•-) by the phagocytic NADPH oxidase isoform (NOX2), the activity of NOX2 being the main source of O2(•-) in monocytes/macrophages. Because the NOX2 and AA pathways are connected, we propose that AANO2 can modulate macrophage activation by inhibiting O2(•-) formation by NOX2. When macrophages were activated in the presence of AANO2, a significant inhibition of NOX2 activity was observed as evaluated by cytochrome c reduction, luminol chemiluminescence, Amplex red fluorescence, and flow cytometry; this process also occurs under physiological mimic conditions within the phagosomes. AANO2 decreased O2(•-) production in a dose- (IC50=4.1±1.8 μM AANO2) and time-dependent manner. The observed inhibition was not due to a decreased phosphorylation of the cytosolic subunits (e.g., p40(phox) and p47(phox)), as analyzed by immunoprecipitation and Western blot. However, a reduction in the migration to the membrane of p47(phox) was obtained, suggesting that the protective actions involve the prevention of the correct assembly of the active enzyme in the membrane. Finally, the observed in vitro effects were confirmed in an in vivo inflammatory model, in which subcutaneous injection of AANO2 was able to decrease NOX2 activity in macrophages from thioglycolate-treated mice.

F420H2-dependent degradation of aflatoxin and other furanocoumarins is widespread throughout the actinomycetales

Two classes of F₄₂₀-dependent reductases (FDR-A and FDR-B) that can reduce aflatoxins and thereby degrade them have previously been isolated from Mycobacterium smegmatis. One class, the FDR-A enzymes, has up to 100 times more activity than the other. F₄₂₀ is a cofactor with a low reduction potential that is largely confined to the Actinomycetales and some Archaea and Proteobacteria. We have heterologously expressed ten FDR-A enzymes from diverse Actinomycetales, finding that nine can also use F₄₂₀H₂ to reduce aflatoxin. Thus FDR-As may be responsible for the previously observed degradation of aflatoxin in other Actinomycetales. The one FDR-A enzyme that we found not to reduce aflatoxin belonged to a distinct clade (herein denoted FDR-AA), and our subsequent expression and analysis of seven other FDR-AAs from M. smegmatis found that none could reduce aflatoxin. Certain FDR-A and FDR-B enzymes that could reduce aflatoxin also showed activity with coumarin and three furanocoumarins (angelicin, 8-methoxysporalen and imperatorin), but none of the FDR-AAs tested showed any of these activities. The shared feature of the compounds that were substrates was an α,β-unsaturated lactone moiety. This moiety occurs in a wide variety of otherwise recalcitrant xenobiotics and antibiotics, so the FDR-As and FDR-Bs may have evolved to harness the reducing power of F₄₂₀ to metabolise such compounds. Mass spectrometry on the products of the FDR-catalyzed reduction of coumarin and the other furanocoumarins shows their spontaneous hydrolysis to multiple products.

Role of putative second transmembrane region of Nox2 protein in the structural stability and electron transfer of the phagocytic NADPH oxidase

Flavocytochrome b(558) (cytb) of phagocytes is a heterodimeric integral membrane protein composed of two subunits, p22(phox) and gp91(phox). The latter subunit, also known as Nox2, has a cytosolic C-terminal "dehydrogenase domain" containing FAD/NADPH-binding sites. The N-terminal half of Nox2 contains six predicted transmembrane α-helices coordinating two hemes. We studied the role of the second transmembrane α-helix, which contains a "hot spot" for mutations found in rare X(+) and X(-) chronic granulomatous disease. By site-directed mutagenesis and transfection in X-CGD PLB-985 cells, we examined the functional and structural impact of seven missense mutations affecting five residues. P56L and C59F mutations drastically influence the level of Nox2 expression indicating that these residues are important for the structural stability of Nox2. A53D, R54G, R54M, and R54S mutations do not affect spectral properties of oxidized/reduced cytb, oxidase complex assembly, FAD binding, nor iodonitrotetrazolium (INT) reductase (diaphorase) activity but inhibit superoxide production. This suggests that Ala-53 and Arg-54 are essential in control of electron transfer from FAD. Surprisingly, the A57E mutation partially inhibits FAD binding, diaphorase activity, and oxidase assembly and affects the affinity of immunopurified A57E cytochrome b(558) for p67(phox). By competition experiments, we demonstrated that the second transmembrane helix impacts on the function of the first intracytosolic B-loop in the control of diaphorase activity of Nox2. Finally, by comparing INT reductase activity of immunopurified mutated and wild type cytb under aerobiosis versus anaerobiosis, we showed that INT reduction reflects the electron transfer from NADPH to FAD only in the absence of superoxide production.

Regulation of NADPH oxidase activity in phagocytes: relationship between FAD/NADPH binding and oxidase complex assembly

The X(+)-linked chronic granulomatous disease (X(+)-CGD) variants are natural mutants characterized by defective NADPH oxidase activity but with normal Nox2 expression. According to the three-dimensional model of the cytosolic Nox2 domain, most of the X(+)-CGD mutations are located in/or close to the FAD/NADPH binding regions. A structure/function study of this domain was conducted in X(+)-CGD PLB-985 cells exactly mimicking 10 human variants: T341K, C369R, G408E, G408R, P415H, P415L, Δ507QKT509-HIWAinsert, C537R, L546P, and E568K. Diaphorase activity is defective in all these mutants. NADPH oxidase assembly is normal for P415H/P415L and T341K mutants where mutation occurs in the consensus sequences of NADPH- and FAD-binding sites, respectively. This is in accordance with their buried position in the three-dimensional model of the cytosolic Nox2 domain. FAD incorporation is abolished only in the T341K mutant explaining its absence of diaphorase activity. This demonstrates that NADPH oxidase assembly can occur without FAD incorporation. In addition, a defect of NADPH binding is a plausible explanation for the diaphorase activity inhibition in the P415H, P415L, and C537R mutants. In contrast, Cys-369, Gly-408, Leu-546, and Glu-568 are essential for NADPH oxidase complex assembly. However, according to their position in the three-dimensional model of the cytosolic domain of Nox2, only Cys-369 could be in direct contact with cytosolic factors during oxidase assembly. In addition, the defect in oxidase assembly observed in the C369R, G408E, G408R, and E568K mutants correlates with the lack of FAD incorporation. Thus, the NADPH oxidase assembly process and FAD incorporation are closely related events essential for the diaphorase activity of Nox2.

Modulation of the cytochrome P450 reductase redox potential by the phospholipid bilayer

Cytochrome P450 reductase (CPR) is a tethered membrane protein which transfers electrons from NADPH to microsomal P450s. We show that the lipid bilayer has a role in defining the redox potential of the CPR flavin domains. In order to quantitate the electrochemical behavior of this central redox protein, full-length CPR was incorporated into soluble nanometer scale discoidal membrane bilayers (nanodiscs), and potentials were measured using spectropotentiometry. The redox potentials of both FMN and FAD were found to shift to more positive values when in a membrane bilayer as compared to a solubilized version of the reductase. The potentials of the semiquinone/hydroquinone couple of both FMN and FAD are altered to a larger extent than the oxidized/semiquinone couple which is understood by a simple electrostatic model. When anionic lipids were used to change the membrane composition of the CPR-nanodisc, the redox potential of both flavins became more negative, favoring electron transfer from CPR to cytochrome P450.

The respiratory burst oxidase

Sbarra and Karnovsky were the first to present evidence suggesting the presence in phagocytes of a special enzyme designed to generate reactive oxidants for purposes of host defense. In the years since their report appeared, a great deal has been learned about this enzyme, now known as the respiratory burst oxidase. It has been found to be a plasma membrane-bound heme- and flavin-containing enzyme, dormant in resting cells, that catalyzes the one-electron reduction of oxygen to O2- at the expense of NADPH: O2 + NADPH----O2- + NADP+ + H+ Its behavior in whole cells and its response to various activating stimuli have been described in detail, although important insights continue to emerge, as for example a very interesting new series of observations on differences in oxidase activation patterns between suspended and adherent cells. The enzyme has been shown by biochemical and genetic studies to consist of at least six components. In the resting cell, three of these components are in the cytosol and three in the plasma membrane, but when the cell passes from its resting to its activated state the cytosolic components are all transferred to the plasma membrane, presumably assembling the oxidase. Of the components initially bound to the membrane, two constitute cytochrome b558, a heme protein characteristic of the respiratory burst oxidase, and the third may represent an oxidase flavoprotein. With regard to the cytosolic components, one is a phosphoprotein and another is the NADPH-binding component, possibly a second oxidase flavoprotein. The nature of the third (p67phox) is a puzzle. Four of the six oxidase components have now been cloned and sequenced. These findings only scratch the surface, however, and many questions remain. How many oxidase components, for example, remain to be discovered, and how do they fit together to form the active enzyme? How is the route of activation of the oxidase integrated into the general signal transduction systems of the cell? How did the oxidase come to be? Could there be a widespread system that generates small amounts of O2- as an intercellular signaling molecule, as recent work is beginning to suggest, and did the ever-destructive respiratory burst oxidase arise from that innocuous system as the creation of some evolutionary Frankenstein--an oxidase from hell? Finally, will it be possible to develop drugs that specifically block the respiratory burst oxidase, and will such drugs prove to be clinically useful as anti-inflammatory agents?(ABSTRACT TRUNCATED AT 400 WORDS)

The NADPH oxidase of professional phagocytes--prototype of the NOX electron transport chain systems

The NADPH oxidase is an electron transport chain in "professional" phagocytic cells that transfers electrons from NADPH in the cytoplasm, across the wall of the phagocytic vacuole, to form superoxide. The electron transporting flavocytochrome b is activated by the integrated function of four cytoplasmic proteins. The antimicrobial function of this system involves pumping K+ into the vacuole through BKCa channels, the effect of which is to elevate the vacuolar pH and activate neutral proteases. A number of homologous systems have been discovered in plants and lower animals as well as in man. Their function remains to be established.

Binding of FAD to cytochrome b558 is facilitated during activation of the phagocyte NADPH oxidase, leading to superoxide production

The superoxide-producing phagocyte NADPH oxidase can be reconstituted in a cell-free system. The activity of NADPH oxidase is dependent on FAD, but the physiological status of FAD in the oxidase is not fully elucidated. To clarify the role of FAD in NADPH oxidase, FAD-free full-length recombinant p47(phox), p67(phox), p40(phox), and Rac were prepared, and the activity was reconstituted with these proteins and purified cytochrome b(558) (cyt b(558)) with different amounts of FAD. A remarkably high activity, over 100 micromol/s/micromol heme, was obtained in the oxidase with purified cyt b(558), ternary complex (p47-p67-p40(phox)), and Rac. From titration with FAD of the activity of NADPH oxidase reconstituted with purified FAD-devoid cyt b, the dissociation constant K(d) of FAD in cyt b(558) of reconstituted oxidase was estimated as nearly 1 nm. We also examined addition of FAD on the assembly process in reconstituted oxidase. The activity was remarkably enhanced when FAD was present during assembly process, and the efficacy of incorporating FAD into the vacant FAD site in purified cyt b(558) increased, compared when FAD was added after assembly processes. The absorption spectra of reconstituted oxidase under anaerobiosis showed that incorporation of FAD into cyt b(558) recovered electron flow from NADPH to heme. From both K(d) values of FAD and the amount of incorporated FAD in cyt b(558) of reconstituted oxidase, in combination with spectra, we propose the model in which the K(d) values of FAD in cyt b(558) is changeable after activation and FAD binding works as a switch to regulate electron transfer in NADPH oxidase.

Development of novel fluorescence probes that can reliably detect reactive oxygen species and distinguish specific species

We designed and synthesized 2-[6-(4'-hydroxy)phenoxy-3H-xanthen-3-on-9-yl]benzoic acid (HPF) and 2- [6-(4'-amino)phenoxy-3H-xanthen-3-on-9-yl]benzoic acid (APF) as novel fluorescence probes to detect selectively highly reactive oxygen species (hROS) such as hydroxyl radical (*OH) and reactive intermediates of peroxidase. Although HPF and APF themselves scarcely fluoresced, APF selectively and dose-dependently afforded a strongly fluorescent compound, fluorescein, upon reaction with hROS and hypochlorite ((-)OCl), but not other reactive oxygen species (ROS). HPF similarly afforded fluorescein upon reaction with hROS only. Therefore, not only can hROS be differentiated from hydrogen peroxide (H(2)O(2)), nitric oxide (NO), and superoxide (O2*-) by using HPF or APF alone, but (-)OCl can also be specifically detected by using HPF and APF together. Furthermore, we applied HPF and APF to living cells and found that HPF and APF were resistant to light-induced autoxidation, unlike 2',7'-dichlorodihydrofluorescein, and for the first time we could visualize (-)OCl generated in stimulated neutrophils. HPF and APF should be useful as tools to study the roles of hROS and (-)OCl in many biological and chemical applications.

The heme component of the neutrophil NADPH oxidase complex is a target for aryliodonium compounds

The redox core of the neutrophil NADPH oxidase complex is a membrane-bound flavocytochrome b in which FAD and heme b are the two prosthetic redox groups. Both FAD and heme b are able to react with diphenylene iodonium (DPI) and iodonium biphenyl (IBP), two inhibitors of NADPH oxidase activity. In this study, we show that the iodonium modification of heme b contributes predominantly to the inhibition of NADPH oxidase. This conclusion is based on the finding that both iodonium compounds decreased the absorbance of the Soret peak of flavocytochrome b in neutrophil membranes incubated with NADPH, and that this decrease was strictly correlated with the loss of oxidase activity. Furthermore, the heme component of purified flavocytochrome b reduced to no more than 95% by a limited amount of sodium dithionite could be oxidized by DPI or IBP. Butylisocyanide which binds to heme iron precludes heme b oxidation. In activated neutrophil membranes, competitive inhibition of O2 uptake by DPI or IBP occurred transiently and was followed by a noncompetitive inhibition. These results, together with those of EPR spectroscopy experiments, lead us to postulate that DPI or IBP first captures an electron from the reduced heme iron of flavocytochrome b to generate a free radical. Then, the binding of this radical to the proximate environment of the heme iron, most probably on the porphyrin ring, results in inhibition of oxidase activity. In the presence of an excess of sodium dithionite, DPI and IBP produced a biphasic decrease of the Soret band of flavocytochrome b, with a break in the dose effect curve occurring at 50% of the absorbance loss. This was consistent with the presence of two hemes in flavocytochrome b that differ by their sensitivity to DPI or IBP.

Physiological production of singlet molecular oxygen in the myeloperoxidase-H2O2-chloride system

The putative role of singlet oxygen (1O2) in the respiratory burst of neutrophils has remained elusive due to the lack of reliable means to study its quantitative production. To measure 1O2 directly from biological or chemical reactions in the near infrared region, we have developed a highly sensitive detection system which employs two InGaAs/InP pin photodiodes incorporated with a dual charge integrating amplifier circuit. Using this detection system, we detected light emission derived from a myeloperoxidase (MPO)-mediated reaction in physiological conditions: pH 7.4, 1-30 nM MPO, 10-100 microM H2O2 and 100-130 mM CI in place of Br without the use of deuterium oxide. The MNPO-H2O2-CI(-) system exhibited a single emission peak at 1.27 microm with a spectral distribution identical to that of delta singlet oxygen. Our results suggest physiological production of 1O2 in the MPO-H2O2-CI(-) system at an intravacuolar neutral pH. The MPO-mediated generation of 1O2, which may have an important role in host defense mechanisms, is discussed in connection with previous results.

Spectroscopic identification of the heme axial ligation of cytochrome b558 in the NADPH oxidase of porcine neutrophils

The combination of electron paramagnetic resonance (EPR), near-infrared magnetic circular dichroism (NIR-MCD) and resonance Raman (RR) spectroscopies at cryogenic temperatures has been used to identify the axial heme ligation of the low spin cytochrome b558 component of NADPH oxidase from porcine blood neutrophils. The EPR and NIR-MCD results indicate the presence of two distinct forms in frozen solution; one with a low field g-value at 3.23 and porphyrin(pi)-to-Fe(III) charge transfer maximum at 1660 nm and the other a low field g-value at 3.00 and porphyrin(pi)-to-Fe(III) charge transfer maximum at 1510 nm. On the basis of these properties and the RR studies, both are attributed to forms of cytochrome b558 with bis-histidine axial ligation. The origin of the observed heterogeneity, the location and identity of the specific histidines involved in ligating the heme, and the role of the heme prosthetic group in O2- production are discussed in light of these results.

Reconstitution of flavin-depleted neutrophil flavocytochrome b558 with 8-mercapto-FAD and characterization of the flavin-reconstituted enzyme

Cytochrome b558 isolated from human neutrophils was inactive and contained no detectable FAD. However, high NADPH oxidase activity was seen upon reconstitution of the cytochrome with either native FAD or 8-mercapto-FAD in the presence of phospholipids (phosphatidylcholine/phosphatidylethanolamine/phosphatidylinositol/ sphingomyelin/cholesterol, 4:2:1:3:3 (w/w)). Their cell-free superoxide-generating activities were 40.5 and 35.5 mol/s/mol of heme, respectively, which corresponded to 70 and 61% of the original activity of the plasma membranes. Both flavins co-eluted with heme and protein on gel exclusion chromatography. The respective specific flavin content was 6.45 and 7.93 nmol/mg of protein and corresponded to a flavin:heme molar ratio of 0.41 and 0.51 consistent with a 2:1 ratio of heme to flavin. Mixing of 8-mercapto-FAD with flavin-depleted cytochrome b558 caused a red-shift of the flavin absorption maximum from 520 nm to around 560 nm, as has been seen when a variety of other apoflavoprotein dehydrogenases bind this analog. The 8-mercapto-FAD reconstituted into the cytochrome reacted readily with either iodoacetamide (k = 38.8 M-1.min-1) or iodoacetic acid (k = 12.1 M-1.min-1) to give a fluorescence spectrum characteristic of a 8-mercaptoflavin derivative, 8-SCH2CONH2 FAD or 8-SCH2COOH FAD. These results indicate that position 8 of FAD bound to the protein is freely accessible to solvent. These studies support the idea that cytochrome b558 is a flavocytochrome.

Antiinflammatory effects of NADPH oxidase inhibitors

Proinflammatory cytokines prime the membrane-bound NADPH oxidase of neutrophils and monocytes of mice suffering from experimental arthritis so as to attain an activated state, which, upon a second stimulus, releases 6-fold increased levels of reactive oxygen species (ROS) than do unprimed phagocytes. Enhanced NADPH oxidase activity deregulates ROS-dependent signal transduction pathways of inflammation, which play a crucial role in the pathogenesis of arthritis. The antiarthritic reactivity of two inhibitors of NADPH oxidase, diphenylene iodoniumchloride (DPI) and staurosporine, was tested in male DBA/1 x B10A(4R) hybrid mice suffering from potassium peroxochromate arthritis. Daily doses of 2.8 mumol/kg of DPI or 30 nmol/kg of staurosporine sufficed to inhibit the arthritis by 50%. A complete inhibition was obtained with 10 mumol/kg of DPI, and 100 nmol/kg of staurosporine suppressed the arthritis by 85%. The onset, progression, and remission of arthritis correlated to both the activity of phagocytic NADPH oxidase (r = 0.750) and to overt disease symptoms as judged by the arthritis index. Our data support the hypothesis that oxidative stress plays a pivotal role in the pathology of arthritis, which can be therapeutically targeted by NADPH oxidase inhibitors.

Kinetic characterization of the redox components in solubilized membranes from porcine neutrophils: reduction with dithionite and photoexcited NAD(P)H

Cytochrome b558 in solubilized membranes prepared from porcine neutrophils was reduced by dithionite with a second-order rate constant of 2.5 x 10(6) M-1 s-1 at pH 7.4 and 20 degrees C accompanied by spectral changes with peaks at 428 nm and 560 nm and isosbestic points at 420 and 441 nm. When an anaerobic mixture of solubilized membranes and NAD(P)H was exposed to a white light, cytochrome b558 was reduced biphasically but with almost the same spectral profiles as in the dithionite reduction. Thus, participation of redox component(s) of unknown nature in the photochemical reduction was suggested. The NAD(P). radical generated by photoexcitation of NAD(P)H with a 355 nm laser pulse under anaerobic conditions also reduced cytochrome b558 with a high rate constant of 4.3 x 10(8) M-1 s-1 at pH 7.4 and 20 degrees C. The reduction of cytochrome b558 accompanied a simultaneous reduction of a component having an absorption band around 420 nm, suggesting participation of an iron-sulfur (Fe-S) cluster. The cytochrome b558 reduction was followed by its reoxidation by another component with an apparent second-order rate constant of 6.5 x 10(5) M-1 s-1. During the reoxidation, the Fe-S-like component remained in the reduced state, and thus its role other than as electron mediator in neutrophils NADPH oxidase is suggested. Not only the rate constant but also the extent of cytochrome b558 reoxidation decreased as the same reaction mixture was exposed to the laser pulse repeatedly. This result clearly indicates that an electron accumulates in this electron-accepting component designated tentatively as the omega component.

Modulation of the heme environment of neutrophil cytochrome b558 to a "cytochrome P450-like" structure by pyridine

The effect of pyridine on the heme environment of cytochrome b558 was studied using ESR and optical absorption spectroscopy in relation to the O2(-)-generating activity in the NADPH oxidase system of stimulated pig neutrophils. As the concentration of pyridine increased, the absorption maxima of the alpha- and gamma-bands of cytochrome b558 shifted which correlated with a concomitant decrease in O2(-)-generating activity. In addition, the g = 3.2 signal of cytochrome b558 decreased with the concomitant appearance of a new ESR spectrum that strikingly resembled that of cytochrome P450. The results suggest that pyridine induces a structural modification in the heme environment of cytochrome b558 by shifting the 5th heme ligand (histidine) to a nearby thiolate group without direct binding of pyridine to the heme. The existence of a reactive thiolate near the heme iron was confirmed by pretreatment of blocked cytochrome b558 with p-chloromercuribenzoate, which completely inhibited the formation of the cytochrome P450-like ESR spectrum. The results provide further evidence that a low-spin heme iron of cytochrome b558 with a g-value of 3.2 is essential to the O2(-)-forming reaction of the NADPH oxidase system. From sequence alignments of cytochrome P450 with those of large and small subunits of cytochrome b558, the heme in cytochrome b558 appears to be specifically associated with the large subunit.

Small G proteins and the neutrophil NADPH oxidase

The NADPH oxidase of phagocytic cells is a multimeric enzyme complex activated during phagocytosis. It catalyzes the production of the superoxide anion, precursor of many toxic oxygen metabolites involved in the defense against microorganisms. The enzyme becomes active after assembly on a membrane bound flavocytochrome b of cytosolic factors p47 phox, p67 phox and p40 phox and of low molecular mass GTP binding proteins. This paper reviews recent results concerning the role of two small G proteins, Rac and Rap 1A in oxidase activation. Native prenylated small G proteins are either in the form of a complex in which the GDP bound G protein is associated with a guanine nucleotide dissociation inhibitor, GDI, or in an active GTP bound form able to trigger the activity of its effector. Rac and Rho share a common GDI. As chemotaxis, under Rho control, and oxidase activation, under Rac control, show mutually exclusive signalling pathways, we propose a model where the GDI would switch from one pathway to the other by sequestering either Rac or Rho.

Specific interaction of pancreatic elastase and leucocytes to produce oxygen radicals and its implication in pancreatitis

Many previous reports using experimental animal models of pancreatitis have suggested that oxygen free radicals play an important part in initiation and development of pancreatitis. Infiltration of inflammatory cells--that is, neutrophils, lymphocytes, and monocytes--has been seen in damaged pancreatic glands of animal models and patients with pancreatitis. As neutrophils are known to be the highest producer of oxygen free radicals among these inflammatory cells, it seems plausible that oxygen free radicals produced by neutrophils have some pathoaetiological meaning in pancreatitis. This study measured the superoxide production by neutrophils obtained from patients with acute and chronic pancreatitis and then examined the effects of pancreatic enzymes on superoxide production. Patients showed significantly higher superoxide production by 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) stimulated neutrophils than healthy controls. Among the three pancreatic enzymes, amylase, trypsin, and elastase, elastase was the only one that increased the superoxide production by PMA stimulated neutrophils, by an increment of 1.5-fold. It also increased the activity of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase prepared from PMA stimulated neutrophils by a factor of 2.1. High affinity and low affinity binding sites for elastase on neutrophils were identified. These results suggest that elastase plays a part in the development of pancreatitis by enhancing superoxide production of neutrophils.

Mechanisms for the activation/electron transfer of neutrophil NADPH-oxidase complex and molecular pathology of chronic granulomatous disease

Professional phagocytes, neutrophils, possess a unique membrane-associated NADPH-oxidase system, dormant in resting cells, which becomes activated upon exposure to the appropriate stimuli and catalyzes the one-electron reduction of molecular oxygen to superoxide, O2-. Oxidase activation involves the assembly, in the plasma membrane, of membrane-bound and cytosolic constituents of the oxidase system, which are disassembled in the resting state. The oxidase system consists of two plasma membrane-bound components; low-potential cytochrome b558, which is composed of two subunits of 22-kDa, and 91-kDa, and a possible flavoprotein related to the electron transport between NADPH and cytochrome b558. Recent reports have indicated that FAD-binding sites of the oxidase are contained in cytochrome b558. At least two cytosolic components, 67-kDa protein and a phosphorylated 47-kDa protein, are known to translocate to the plasma membrane, ensuring assembly of an active O2(-)-generating NADPH-oxidase system. It is the purpose of this review to focus on recent data concerning electron transfer mechanisms of the activated neutrophil NADPH-oxidase complex and molecular pathology of chronic granulomatous disease.

Superoxide production by cytochrome b559. Mechanism of cytosol-independent activation

Purified cytochrome b559 relipidated with either a mixture of phosphatidylcholine and phosphatidic acid or with phosphatidylcholine only exhibits high and low superoxide (O2-) producing ability, respectively, in the absence of cytosolic activators [Koshkin, V. and Pick, E. (1993) FEBS Lett. 327, 57-62]. This system was used as a model for the study of the mechanism of NADPH oxidase activation. It is shown that, depending on the composition of the phospholipid environment, cytochrome b599 binds FAD with high or low affinity, this being accompanied by changes in flavin absorbance and fluorescence. High affinity binding of FAD to cytochrome b559 relipidated with phosphatidylcholine combined with phosphatidic acid is associated with an enhanced NADPH-driven O2- producing capacity. A kinetic study of O2- production by cytochrome b559 reflavinated under stoichiometric FAD binding conditions revealed an FAD/heme ratio of 1:2. A further kinetic study of O2- production by high- and low-activity relipidated and reflavinated cytochrome b559, at varying substrate concentrations, and the determination of steady-state difference spectra of such preparations, reduced by NADPH, indicated that O2- production is activated by facilitation of electron transfer from NADPH to FAD rather than by an enhancement of NADPH binding.

Activation factors of neutrophil NADPH oxidase complex

Professional phagocytes, neutrophils, possess a unique membrane-associated NADPH oxidase system, dormant in resting cells, which becomes activated upon exposure to the appropriate stimuli and catalyzes the one-electron reduction of molecular oxygen to superoxide, O2-. Oxidase activation involves the assembly, in the plasma membrane, of membrane-bound and cytosolic constituents of the oxidase system, which are disassembled in the resting state. The oxidase system consists of two plasma membrane-bound components; low-potential cytochrome b558, which is composed of two subunits of 22 kDa and 91 kDa, and a flavoprotein related to the electron transport between NADPH and heme-binding domains of the oxidase. Recent reports have indicated that FAD-binding sites of the oxidase are contained in cytochrome b558 (flavocytochrome b558). At least two cytosolic components, 67 kDa protein and a phosphorylated 47 kDa protein, are known to translocate to the plasma membrane, ensuring assembly of an active O2(-)-generating NADPH oxidase system. More recently, the membrane (Raps) and cytosolic (Racs) GTP-binding proteins have been established as essential to oxidase assembly. It is the purpose of this review to focus on recent data concerning the regulatory mechanisms which lead to organization and activation of the neutrophil NADPH oxidase system.

Nucleoside-triphosphate binding of the two cytosolic components of the respiratory burst oxidase system: evidence for its inhibition by the 2',3'-dialdehyde derivative of NADPH and desensitization in their translocated states

Affinity labeling of the two cytosolic components of the respiratory burst oxidase system, p49-phox and p63-phox, from resting porcine neutrophils was carried out with [32P]NADPH dialdehyde (oNADPH), [32P]oGTP and [32P]oATP. p49-phox and p63-phox showed 10-times higher affinities for both oGTP and oATP than for oNADPH, suggesting that they are nucleoside triphosphate (NTP)-binding proteins, rather than the NADPH-binding site of the oxidase. In addition, oNADPH markedly inhibited the affinity labeling of p49-phox with [32P]oGTP and [32P]oATP, well reflecting its inhibitory effect on the oxidase activity in the cell-free system, which was previously reported to propose the NADPH-binding site in a cytosolic component. Stimulation of porcine neutrophils with either myristic acid or phorbol myristate acetate resulted in great enhancement of the oxidase activity, and in considerable translocation of p49-phox and p63-phox. Nevertheless, the affinity labeling of the stimulated cell membranes in both cases revealed no labeled bands corresponding to molecular masses of 49 kDa and 63 kDa. p49-phox derived from the stimulated membranes had lost its [32P]oGTP binding ability in contrast with that from resting cytosol, suggesting that the NTP-binding sites of the two cytosolic components may be desensitized on NTP binding in their translocated states.

A structural model for the nucleotide binding domains of the flavocytochrome b-245 beta-chain

NADPH is a system in phagocytic cells that generates O2- and hydrogen peroxide in the endocytic vacuole, both of which are important for killing of the engulfed microbe. Dysfunction of this oxidase results in the syndrome of chronic granulomatous disease, characterized by a profound predisposition to bacterial and fungal infections. A flavocytochrome b is the site of most of the mutations causing this syndrome. The FAD and NADPH binding sites have been located on the beta subunit of this molecule, the C-terminal half of which showed weak sequence similarity to other reductases, including the ferredoxin-NADP reductase (FNR) of known structure. This enabled us to build a model of the nucleotide binding domains of the flavocytochrome using this structure as a template. The model was built initially using a novel automatic modeling method based on distance-matrix projection and then refined using energy minimization with appropriate side-chain torsional constraints. The resulting model rationalized much of the observed sequence conservation and identified a large insertion as a potential regulatory domain. It confirms the inclusion of the neutrophil flavocytochrome b-245 (Cb-245) as a member of the FNR family of reductases and strongly supports its function as the proximal electron transporting component of the NADPH oxidase.

Generation of superoxide by purified and relipidated cytochrome b559 in the absence of cytosolic activators

Purified cytochrome b559 from guinea pig macrophages was relipidated with several phospholipid mixtures. Relipidated cytochrome b559 was found capable of NADPH-dependent superoxide (O2-) production in the absence of the cytosolic components of the NADPH oxidase complex. The rate of O2- generation by cytochrome b559 varied with the type of phospholipid utilized for relipidation, was absolutely dependent on exogenous FAD, and was enhanced by a critical concentration of anionic amphiphile. It is demonstrated that exogenous FAD acts by binding to cytochrome b559. These results provide firm experimental evidence for the proposal that cytochrome b559 comprises the complete electron transporting apparatus of the O2- forming NADPH oxidase and that the cytosolic components function merely as activators.

Electron paramagnetic resonance studies on cytochrome b-558 and peroxidases of pig blood granulocytes

Low-temperature electron paramagnetic resonance (EPR) spectrometry on granulocytes prepared from pig blood was carried out with concentrated cellular and subcellular fractions to characterize EPR signals of cytochrome b-558 (cyt b-558). A thick cell suspension (approximately 2 x 10(9) cells/ml), containing mostly neutrophils, showed typical high-spin EPR signals due to myeloperoxidase (MPO) and a low spin signal at a g value of around 3.2. A similar thick granulocyte suspension containing eosinophils showed not only these signals but also low spin heme signals at g values of 2.86, 2.13, and 1.66, which have been reported to be of cyt b-558 (Ueno et al. 1991, FEBS Lett. 281, 130-132). MPO and eosinophil peroxidase (EPO) were released from the membrane fractions with 50 mM phosphate buffer (pH 7.0) containing 1 M NaCl, and then were highly concentrated, in which no cyt b-558 was detected by absorption spectra. The signal at a g value of 2.86 was found only in the EPO fraction, suggesting that this signal is derived from a low-spin form of an EPO-complex, but neither from MPO nor cyt b-558. The O2(-)-forming NADPH oxidase associated in the membranes was solubilized with heptyl-thio-glucoside at 0 degree C and concentrated up to 45 microM cyt b-558 with no modification of the heme moiety confirmed by its O2(-)-generating activity and lack of carbon monoxide-binding capacity. Cyt b-558 showed an anisotropic signal at a g value of 3.2 +/- 0.05, which was cyanide-insensitive and reducible with reductants. The signal intensity was concentration dependent, suggesting that the g = 3.2 signal is characteristic of the low-spin heme iron in cyt b-558.

Reconstitution and characterization of the human neutrophil respiratory burst oxidase using recombinant p47-phox, p67-phox and plasma membrane

Human neutrophil respiratory burst oxidase (NADPH-oxidase) activity can be reconstituted in a cell-free system consisting of plasma membrane, cytosol and an anionic amphiphile [e.g., sodium dodecyl sulfate (SDS) or arachidonate]. Herein, we report reconstitution of oxidase activity using isolated neutrophil plasma membrane together with purified recombinant p47-phox and p67-phox which had been produced using a baculovirus expression system. Activity required an anionic amphiphile (SDS or arachidonate) and was potentiated by diacylglycerol and GTP gamma S. Serial washes of the plasma membrane failed to affect its ability to reconstitute activity, indicating that a dissociable membrane component was not present. The Km for NADPH, 43 microM, was the same as that determined using cytosol in place of recombinant factors. The EC50 values for p47-phox and p67-phox under optimal activation conditions were 220 nM and 80 nM, respectively, indicating a relatively high affinity of these components in an activation complex. Since neither cytosolic component contains a nucleotide binding consensus sequence, these data indicate that the NADPH binding component of the oxidase resides in the plasma membrane.

Determination of flavin contents in neutrophils by a sensitive chemiluminescence assay: evidence for no translocation of flavoproteins from the cytosol to the membrane upon cell stimulation

A sensitive and specific chemiluminescence (CL) method with bacterial luciferase was adapted for accurate measurement of the flavins FAD and FMN in the membrane and cytosolic fractions of neutrophils prepared from pig and human blood. The FAD and FMN contents (FAD/FMN = 100:2) in the membranes were essentially the same in resting (R) and myristate-stimulated (S) cells, although O2(-)-generation was markedly enhanced exclusively in S membranes. The O2(-)-forming activity of S samples remained unchanged or even increased after washing the membranes with buffer, although one-third of the FAD was lost during washing (a decrease from 140 to 95 pmol/10(8) cell-equivalent (CE) during washing). The cytosol is known to contain at least three components that are essential for O2- production (p47-phox, p67-phox, and a G-protein), and that are translocated to membranes upon activation, but its flavin content was one tenth of that of the membranes. The cytosol was treated with fatty acids in the absence of membranes to induce substantial precipitation of p47-phox, p67-phox and a protein of 32 kDa. No difference relative to a solvent-control was noted in the low flavin content of the precipitate indicating that these cytosolic components are not flavoproteins. These results do not support the possibility of translocation of a cytosolic flavoprotein to the membrane upon activation of the respiratory burst.

The involvement of oxygen radicals in microbicidal mechanisms of leukocytes and macrophages

Phagocytic leukocytes generate large amounts of reactive oxygen compounds during and after phagocytosis of micro-organisms. These compounds are essential for the killing of a wide variety of microbes. The enzyme responsible for this process is NADPH:O2 oxidoreductase (NADPH oxidase), which utilizes the reduction equivalents of NADPH to reduce atmospheric oxygen to superoxide (O2-.). Subsequently, superoxide is converted by the leukocytes to other reactive compounds, such as hydrogen peroxide (H2O2), hypochlorous acid (HOCl) and N-chloramines (RNCl). Each of these compounds has potent microbicidal properties. Under resting, non-phagocytizing conditions, phagocytes do not produce reactive oxygen compounds. However, within 15-30 sec after binding of micro-organisms to cell surface receptors, superoxide generation starts. This phenomenon is called the respiratory burst. This phenomenon is called the respiratory burst. The activation of the NADPH oxidase is caused by the assembly of components of this enzyme into an active complex. Under resting conditions, at least three components reside in the cytoplasm and at least two are located in the plasma membrane. Activation of the NADPH oxidase results in translocation of cytosolic components to the plasma membrane and formation of an active enzymatic complex in the plasma membrane.

Electron transfer reactions in the NADPH oxidase system of neutrophils--involvement of an NADPH-cytochrome c reductase in the oxidase system

Membrane-bound NADPH oxidase of pig blood neutrophils was solubilized with heptylthioglucoside in a high yield. The solubilized preparation from myristate-stimulated cells (sample S) showed high O2- generating activity, and the preparation from resting cells (sample R) had no activity, but the two samples had equal amounts of flavins and cytochrome b-558 (cyt b-558). The electron transfer reactions to exogenous cytochrome c (cyt c) or cyt b-558 in samples S and R were examined. Under anaerobic conditions, NADPH-dependent cyt c reductase activity appeared higher in sample S than in sample R, and the addition of FMN and FAD greatly enhanced the reductase activity of sample S, but not that of sample R. No marked difference between the reductase activities of samples S and R was seen with NADH. Photoreduction of the NADPH oxidase system was examined in the absence of NADPH under anaerobic conditions by monitoring the reduction rates of exogenous cyt c using a flashlight with cut-off filters between 400 and 500 nm. Cyt c reduction was much higher in sample S than in sample R on photoexcitation at about 450 nm. Photoreduction was carried out with a band-pass filter for selective irradiation at 450 nm. Marked reduction of exogenous cyt c was observed only in sample S: the small reduction of cyt c by sample R was independent of the light wavelength and was equal to the blank level. In contrast, no difference in the reduction of cyt b-558 by the two samples was found by either NADPH or photoreduction. Under aerobic conditions, no direct reduction of either cyt c or cyt b-558 was observed. These results suggest that an NADPH-cyt c reductase (a membrane-bound flavoprotein) is involved in the NADPH oxidase system of stimulated neutrophils.

The superoxide-generating oxidase of phagocytic cells. Physiological, molecular and pathological aspects

Professional phagocytes (neutrophils, eosinophils, monocytes and macrophages) possess an enzymatic complex, the NADPH oxidase, which is able to catalyze the one-electron reduction of molecular oxygen to superoxide, O2-. The NADPH oxidase is dormant in non-activated phagocytes. It is suddenly activated upon exposure of phagocytes to the appropriate stimuli and thereby contributes to the microbicidal activity of these cells. Oxidase activation in phagocytes involves the assembly, in the plasma membrane, of membrane-bound and cytosolic components of the oxidase complex, which were diassembled in the resting state. One of the membrane-bound components in resting phagocytes has been identified as a low-potential b-type cytochrome, a heterodimer composed of two subunits of 22-kDa and 91-kDa. The link between NADPH and cytochrome b is probably a flavoprotein whose subcellular localization in resting phagocytes remains to be determined. Genetic defects in the cytochrome b subunits and in the cytosolic factors have been shown to be the molecular basis of chronic granulomatous disease, a group of inherited disorders in the host defense, characterized by severe, recurrent bacterial and fungal infections in which phagocytic cells fail to generate O2- upon stimulation. The present review is focused on recent data concerning the signaling pathway which leads to oxidase activation, including specific receptors, the production of second messengers, the organization of the oxidase complex and the molecular defects responsible for granulomatous disease.

Two cytosolic components of the neutrophil NADPH oxidase, P47-phox and P67-phox, are not flavoproteins

Two cytosolic proteins, p47-phox and p67-phox, have been shown to be essential components of the NADPH-dependent oxidase of human neutrophils, although the specific role of each of these proteins in the multicomponent electron transport complex is undetermined. The superoxide-generating activity of this oxidase can be reproduced in a cell-free system, combining cytosol and membranes from unstimulated neutrophils in the presence of fatty acid and NADPH. In the present studies, cytosol was treated with myristic acid, arachidonic acid, or sodium dodecyl sulfate in the absence of membranes and the resultant precipitate collected by centrifugation and analyzed. Both p47-phox and p67-phox precipitated in the presence of fatty acid. However, neither FAD nor FMN was localized in the precipitates, even though substantial amounts of p47-phox and p67-phox precipitated. These results suggest that neither p47-phox nor p67-phox is a flavoprotein and that neither, therefore, is the oxidase component which accepts electrons from NADPH.

NADPH: nitroblue tetrazolium reductase found in plasma membrane of human neutrophil

After phorbol 12-myristate 13-acetate (PMA) stimulation the increase of NADPH:nitroblue tetrazolium reductase activity in the plasma membrane almost corresponded with the stimulated activity of respiratory burst oxidase. Solubilization of plasma membranes from PMA-activated neutrophils with n-octyl glucoside resulted in high recoveries of the two enzymatic activities. When solubilized plasma membrane was subjected to non-denaturing polyacrylamide gel electrophoresis in the presence of 35 mM n-octyl glucoside, we could see three major bands stained with NADPH-dependent nitroblue reductase activity giving molecular masses of approx. 95, 45 and 40 kDa, respectively. Activity was specific for NADPH but not for NADH. These bands also stained weakly in the plasma membranes obtained from resting cells. The activities for NADPH oxidase and nitroblue tetrazolium reductase were found to elute as a very similar protein peak on an anion-exchange HPLC, at about 0.32 M KCl. This elution peak also contains 45 and 40 kDa proteins showing NADPH:nitroblue tetrazolium reductase activity.

Leukocytic oxygen activation and microbicidal oxidative toxins

Following a brief introduction of cellular response to stimulation comprising leukocyte activation, three major areas are discussed: (1) the neutrophil oxidase; (2) myeloperoxidase (MPO)-dependent oxidative microbicidal reactions; and (3) MPO-independent oxidative reactions. Topics included in section (A) are current views on the activation mechanism, redox composition, structural and topographic organization of the oxidase, and its respiratory products. In section (B), emphasis is placed on recent research on cidal mechanisms of HOCl, including the oxidative biochemistry of active chlorine compounds, identification of sites of lesions in bacteria, and attendant metabolic consequences. In section (C), we review the (bio)chemistry of H2O2 and .OH microbicidal reactions, with particular attention being given to addressing the controversial issue of probe methods to identify .OH radical and critical assessment of the recent proposal that MPO-independent killing arises from site-specific metal-catalyzed Fenton-type chemistry.

Subcellular localization and dynamics of components of the respiratory burst oxidase

Membrane and cytosolic factors cooperate to generate NADPH-oxidase. The study of the syndrome of NADPH-oxidase deficiencies, chronic granulomatous disease, has enabled the identification of two membrane factors: a flavin adenine dinucleotide flavoprotein and a b cytochrome. The nature of the cytosolic components is still unknown, but a 47-kD protein, whose phosphorylation occurs in parallel with the generation of a respiratory burst in intact cells, seems to be one of the cytosolic factors. The subcellular localization of the membrane-bound NADPH-oxidase components has been studied in neutrophils: In unstimulated cells, only a minute fraction of the NADPH-oxidase components is localized in the plasma membrane, whereas approximately 80% is localized in the membrane of the specific granules and the majority of the rest is in a newly described membrane-bound compartment, the secretory granules, identified by latent alkaline phosphatase. During stimulation, these NADPH-oxidase components are translocated to the plasma membrane as a result of fusion of granule membrane with plasma membrane. Only the NADPH-oxidase components present in the plasma membrane are incorporated in the respiratory burst oxidase generated in intact cells.

Isolation of the respiratory burst oxidase: the role of a flavoprotein component

The article reviews the enzymatic and electron transfer properties of a low-potential FAD-dependent flavoprotein that is a component of the NADPH-dependent O2-.-generating respiratory burst oxidase of phagocytes. Current methods available for isolation of the respiratory burst oxidase and the flavoprotein component of the complex are also reviewed. These studies and data obtained from affinity-labeling of respiratory burst oxidase components, suggest that the flavoprotein has a molecular weight of 65-67 kD. The prevailing evidence suggests that the flavoprotein functions as a dehydrogenase/electron transferase and can directly catalyse NADPH-dependent O2-.formation when isolated. However, in neutrophil plasma membranes, the prevailing evidence suggests that the flavoprotein functions primarily to transfer electrons from NADPH to cytochrome b-245 and that this latter redox component is the catalytic side of O2-.formation. A working model for the arrangement of the flavoprotein and cytochrome b-245 components of the respiratory burst oxidase in neutrophil membranes is proposed.