Human neutrophil cytochrome b contains multiple hemes. Evidence for heme associated with both subunits

NADPH Oxidase 3: Beyond the Inner Ear

Reactive oxygen species (ROS) were formerly known as mere byproducts of metabolism with damaging effects on cellular structures. The discovery and description of NADPH oxidases (Nox) as a whole enzyme family that only produce this harmful group of molecules was surprising. After intensive research, seven Nox isoforms were discovered, described and extensively studied. Among them, the NADPH oxidase 3 is the perhaps most underrated Nox isoform, since it was firstly discovered in the inner ear. This stigma of Nox3 as "being only expressed in the inner ear" was also used by me several times. Therefore, the question arose whether this sentence is still valid or even usable. To this end, this review solely focuses on Nox3 and summarizes its discovery, the structural components, the activating and regulating factors, the expression in cells, tissues and organs, as well as the beneficial and detrimental effects of Nox3-mediated ROS production on body functions. Furthermore, the involvement of Nox3-derived ROS in diseases progression and, accordingly, as a potential target for disease treatment, will be discussed.

NADPH Oxidases (NOX): An Overview from Discovery, Molecular Mechanisms to Physiology and Pathology

The reactive oxygen species (ROS)-producing enzyme NADPH oxidase (NOX) was first identified in the membrane of phagocytic cells. For many years, its only known role was in immune defense, where its ROS production leads to the destruction of pathogens by the immune cells. NOX from phagocytes catalyzes, via one-electron trans-membrane transfer to molecular oxygen, the production of the superoxide anion. Over the years, six human homologs of the catalytic subunit of the phagocyte NADPH oxidase were found: NOX1, NOX3, NOX4, NOX5, DUOX1, and DUOX2. Together with the NOX2/gp91phox component present in the phagocyte NADPH oxidase assembly itself, the homologs are now referred to as the NOX family of NADPH oxidases. NOX are complex multidomain proteins with varying requirements for assembly with combinations of other proteins for activity. The recent structural insights acquired on both prokaryotic and eukaryotic NOX open new perspectives for the understanding of the molecular mechanisms inherent to NOX regulation and ROS production (superoxide or hydrogen peroxide). This new structural information will certainly inform new investigations of human disease. As specialized ROS producers, NOX enzymes participate in numerous crucial physiological processes, including host defense, the post-translational processing of proteins, cellular signaling, regulation of gene expression, and cell differentiation. These diversities of physiological context will be discussed in this review. We also discuss NOX misregulation, which can contribute to a wide range of severe pathologies, such as atherosclerosis, hypertension, diabetic nephropathy, lung fibrosis, cancer, or neurodegenerative diseases, giving this family of membrane proteins a strong therapeutic interest.

The NADPH Oxidase Family and Its Inhibitors

Significance: The oxidative stress, resulting from an imbalance in the production and scavenging of reactive oxygen species (ROS), is known to be involved in the development and progression of several pathologies. The excess of ROS production is often due to an overactivation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOX) and for this reason these enzymes became promising therapeutic targets. However, even if NOX are now well characterized, the development of new therapies is limited by the lack of highly isoform-specific inhibitors. Recent Advances: In the past decade, several groups and laboratories have screened thousands of molecules to identify new specific inhibitors with low off-target effects. These works have led to the characterization of several new potent NOX inhibitors; however, their specificity varies a lot depending on the molecules. Critical Issues: Here, we are reviewing more than 25 known NOX inhibitors, focusing mainly on the newly identified ones such as APX-115, NOS31, Phox-I1 and 2, GLX7013114, and GSK2795039. To have a better overall view of these molecules, the inhibitors were classified according to their specificity, from pan-NOX inhibitors to highly isoform-specific ones. We are also presenting the use of these compounds both in vitro and in vivo. Future Directions: Several of these new molecules are potent and very specific inhibitors that could be good candidates for the development of new drugs. Even if the results are very promising, most of these compounds were only validated in vitro or in mice models and further investigations will be required before using them as potential therapies.

NADPH oxidase depletion in neutrophils from patients with cirrhosis and restoration via toll-like receptor 7/8 activation

OBJECTIVE

Cirrhosis downregulates phagocyte oxidant production via their antibacterial superoxide-generating system, NADPH oxidase (NOX2) and increases patients' susceptibility to infection and mortality rate. To explore novel biochemical parameters that explain susceptibility to infections, we investigated the expression of NOX2 and partners in neutrophils of patients with severe alcoholic cirrhosis and have provided a novel approach to restore superoxide production capacity in patients' neutrophils and blood.

DESIGN

Neutrophils were isolated from patients with decompensated alcoholic cirrhosis. NOX2 activity was assessed after stimulation of purified neutrophils or whole blood with the bacterial-derived peptide fMet-Leu-Phe. The expression of NOX2 and partners was studied by western blot analysis, flow cytometry and reverse transcription-PCR.

RESULTS

The impaired superoxide production by patients' neutrophils was associated with a severe deficient expression of the NADPH oxidase catalytic core flavocytochrome-b558 (gp91 ^phox /NOX2 and p22 ^phox ), its cytosolic partner p47 ^phox but not p67 ^phox . NOX2 expression decreased rapidly by protein degradation involving elastase released during degranulation of healthy neutrophils stimulated with fMet-Leu-Phe, or highly present in patients' plasma. Interestingly, the deficient superoxide production was reversed by treatment of patients' neutrophils and whole blood with toll-like receptor 7/8 (TLR7/8) agonists. This treatment stimulated a rapid NOX2 transcription and translation through a process involving mammalian target of rapamycin (mTOR) whose expression was also deficient in patients' neutrophils. NOX2 expression was also increased by the TLR4 agonist lipopolysaccharide but with only a modest improvement of reactive oxygen species production.

CONCLUSION

Impairment of neutrophil oxidants production in alcoholic cirrhosis is associated with NOX2 degradation and deficient mTOR-dependent translational machinery. The NOX2 depletion can be reversed via TRL7/8 activation and might be used to restore antimicrobial responses of immunocompromised patients.

CYBA encoding p22(phox), the cytochrome b558 alpha polypeptide: gene structure, expression, role and physiopathology

P22(phox) is a ubiquitous protein encoded by the CYBA gene located on the long arm of chromosome 16 at position 24, containing six exons and spanning 8.5 kb. P22(phox) is a critical component of the superoxide-generating nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs). It is associated with NOX2 to form cytochrome b558 expressed mainly in phagocytes and responsible for the killing of microorganisms when bacterial and fungal infections occur. CYBA mutations lead to one of the autosomal recessive forms of chronic granulomatous disease (AR22(0)CGD) clinically characterized by recurrent and severe infections in early childhood. However, p22(phox) is also the partner of NOX1, NOX3 and NOX4, but not NOX5, which are analogs of NOX2, the first identified member of the NOX family. P22(phox)-NOX complexes have emerged as one of the most relevant sources of reactive oxygen species (ROS) in tissues and cells, and are associated with several diseases such as cardiovascular and cerebrovascular diseases. The p22(phox)-deficient mouse strain nmf333 has made it possible to highlight the role of p22(phox) in the control of inner ear balance in association with NOX3. However, the relevance of p22(phox) for NOX3 function remains uncertain because AR22(0)CGD patients do not suffer from vestibular dysfunction. Finally, a large number of genetic variations of CYBA have been reported, among them the C242T polymorphism, which has been extensively studied in association with coronary artery and heart diseases, but conflicting results continue to be reported.

Nox NADPH oxidases and the endoplasmic reticulum

SIGNIFICANCE

Understanding isoform- and context-specific subcellular Nox reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase compartmentalization allows relevant functional inferences. This review addresses the interplay between Nox NADPH oxidases and the endoplasmic reticulum (ER), an increasingly evident player in redox pathophysiology given its role in redox protein folding and stress responses.

RECENT ADVANCES

Catalytic/regulatory transmembrane subunits are synthesized in the ER and their processing includes folding, N-glycosylation, heme insertion, p22phox heterodimerization, as shown for phagocyte Nox2. Dual oxidase (Duox) maturation also involves the regulation by ER-resident Duoxa2. The ER is the activation site for some isoforms, typically Nox4, but potentially other isoforms. Such location influences redox/Nox-mediated calcium signaling regulation via ER targets, such as sarcoendoplasmic reticulum calcium ATPase (SERCA). Growing evidence suggests that Noxes are integral signaling elements of the unfolded protein response during ER stress, with Nox4 playing a dual prosurvival/proapoptotic role in this setting, whereas Nox2 enhances proapoptotic signaling. ER chaperones such as protein disulfide isomerase (PDI) closely interact with Noxes. PDI supports growth factor-dependent Nox1 activation and mRNA expression, as well as migration in smooth muscle cells, and PDI overexpression induces acute spontaneous Nox activation.

CRITICAL ISSUES

Mechanisms of PDI effects include possible support of complex formation and RhoGTPase activation. In phagocytes, PDI supports phagocytosis, Nox activation, and redox-dependent interactions with p47phox. Together, the results implicate PDI as possible Nox organizer.

FUTURE DIRECTIONS

We propose that convergence between Noxes and ER may have evolutive roots given ER-related functional contexts, which paved Nox evolution, namely calcium signaling and pathogen killing. Overall, the interplay between Noxes and the ER may provide relevant insights in Nox-related (patho)physiology.

Peroxiredoxin-6 and NADPH oxidase activity

Peroxiredoxins (Prdxs) are a family of proteins which catalyze the reduction of H2O2 through the interaction of active site cysteine residues. Conserved within all plant and animal kingdoms, the function of these proteins is related to protection from oxidation or participation of signaling through degradation of H2O2. Peroxiredoxin 6 (Prdx6), a protein belonging to the class of 1-cys Prdxs, was identified in polymorphonuclear leukocytes or neutrophils, defined by amino acid sequence and activity, and found associated with a component of the NADPH oxidase (Nox2), p67(phox). Prdx6 plays an important role in neutrophil function and supports the optimal activity of Nox2. In this chapter, methods are described for determining the Prdx activity of Prdx6. In addition, the approach for assessing the effect of Prdx6 on Nox2 in the SDS-activated, cell-free system of NADPH oxidase activity is presented. Finally, the techniques for suppressing Prdx6 expression in phox-competent K562 cells and cultured myeloid cells with siRNA and shRNA methods are described. With these approaches, the role of Prdx6 in Nox2 activity can be explored with intact cells. The biochemical mechanisms of the Prdx6 effect on the NADPH oxidase can be investigated with the experimental strategies described.

Invariant local conformation in p22phox p.Y72H polymorphisms suggested by mass spectral analysis of crosslinked human neutrophil flavocytochrome b

The NADPH oxidase of phagocytic leukocytes generates superoxide that plays a critical role in innate immunity and inflammatory responses. The integral membrane protein flavocytochrome b (Cyt b, a.k.a. cytochrome b(558/559)) is the catalytic core of the complex and serves as a prototype for homologs important in regulating signaling networks in a wide variety of animal and plant cells. Our analysis identifies a naturally-occurring Tyr72/His72 polymorphism (p.Y72H) in the p22(phox) subunit of Cyt b at the protein level that has been recognized at the nucleotide level (c.214T > C, formerly C242T) and implicated in cardiovascular disease. In the present study, Cyt b was isolated from human neutrophils and reacted with chemical crosslinkers for subsequent structure analysis by MALDI mass spectrometry. Following mild chemical modification of Cyt b with two pairs of isotopically-differentiated lysine crosslinkers: BS(2)G-d(0)/d(4) and BS(3)-d(0)/d(4), the reaction mixtures were digested with trypsin and purified on C(18)ZipTips to generate samples for mass analysis. MALDI analysis of tryptic digests from each of the above reactions revealed a series of masses that could be assigned to p22(phox) residues 68-85, assuming an intra-molecular crosslink between Lys71 and Lys78. In addition to the 30 ppm mass accuracy obtained with internal mass calibration, increased confidence in the assignment of the crosslinks was provided by the presence of the diagnostic mass patterns resulting from the isotopically-differentiated crosslinking reagent pairs and the Tyr72/His72 p22(phox) polymorphisms in the crosslinked peptides. This work identifies a novel, low-resolution distance constraint in p22(phox) and suggests that the medically-relevant p.Y72H polymorphism has an invariant structural motif in this region. Because position 72 in p22(phox) lies outside regions identified as interactive with other oxidase components, the structural invariance also provides additional support for maturational differences as the source of the wide variation in observed reactive oxygen species production by cells expressing p.Y72H.

Cell death and stress signaling in glycogen storage disease type I

Cell death has been traditionally classified in apoptosis and necrosis. Apoptosis, known as programmed cell death, is an active form of cell death mechanism that is tightly regulated by multiple cellular signaling pathways and requires ATP for its appropriate process. Apoptotic death plays essential roles for successful development and maintenance of normal cellular homeostasis in mammalian. In contrast to apoptosis, necrosis is classically considered as a passive cell death process that occurs rather by accident in disastrous conditions, is not required for energy and eventually induces inflammation. Regardless of different characteristics between apoptosis and necrosis, it has been well defined that both are responsible for a wide range of human diseases. Glycogen storage disease type I (GSD-I) is a kind of human genetic disorders and is caused by the deficiency of a microsomal protein, glucose-6-phosphatase-α (G6Pase-α) or glucose-6-phosphate transporter (G6PT) responsible for glucose homeostasis, leading to GSD-Ia or GSD-Ib, respectively. This review summarizes cell deaths in GSD-I and mostly focuses on current knowledge of the neutrophil apoptosis in GSD-Ib based upon ER stress and redox signaling.

Does botrytis cinerea Ignore H(2)O(2)-induced oxidative stress during infection? Characterization of botrytis activator protein 1

Botrytis cinerea is a phytopathogen infecting a broad range of plants including strawberries and grapevine. During infection, the necrotrophic fungus is exposed to reactive oxygen species (ROS) released by the oxidative burst, an early plant defense reaction. B. cinerea even produces ROS itself in planta. This raises questions about how the pathogen senses and responds to the host defense reaction and which role the pathogen's oxidative stress response systems play. Functional analysis of the AP-1 transcription factor Bap1 confirmed its role as a pivotal regulator of ROS detoxification in vitro. Macroarray analysis revealed 99 H(2)O(2)-induced Bap1 target genes, of which several genes encoded ROS-degrading enzymes as well as other central components of the cellular redox status. However, Bap1 is not essential for pathogenesis. In planta analyses revealed that the Bap1 target genes were not expressed 2 days postinoculation although H(2)O(2) was detectable, proving that the normal virulence of the Deltabap1 mutant is not due to alternative regulation of the major oxidative stress response system in planta. The fungus obviously does not suffer H(2)O(2)-induced oxidative stress in planta, questioning classical ideas about the role of the oxidative burst in the infection process.

Microevolution of cytochrome bd oxidase in Staphylococci and its implication in resistance to respiratory toxins released by Pseudomonas

Pseudomonas aeruginosa and Staphylococcus aureus are opportunistic pathogens and frequently coinfect the lungs of cystic fibrosis patients. P. aeruginosa secretes an arsenal of small respiratory inhibitors, like pyocyanin, hydrogen cyanide, or quinoline N-oxides, that may act against the commensal flora as well as host cells. Here, we show that with respect to their susceptibility to these respiratory inhibitors, staphylococcal species can be divided into two groups: the sensitive group, comprised of pathogenic species such as S. aureus and S. epidermidis, and the resistant group, represented by nonpathogenic species such as S. carnosus, S. piscifermentans, and S. gallinarum. The resistance in the latter group of species was due to cydAB genes that encode a pyocyanin- and cyanide-insensitive cytochrome bd quinol oxidase. By exchanging cydB in S. aureus with the S. carnosus-specific cydB, we could demonstrate that CydB determines resistance. The resistant or sensitive phenotype was based on structural alterations in CydB, which is part of CydAB, the cytochrome bd quinol oxidase. CydB represents a prime example of both microevolution and the asymmetric pattern of evolutionary change.

Deletion mutagenesis of p22phox subunit of flavocytochrome b558: identification of regions critical for gp91phox maturation and NADPH oxidase activity

The heterodimeric flavocytochrome b558, comprised of the two integral membrane proteins p22phox and gp91phox, mediates the transfer of electrons from NADPH to molecular oxygen in the phagocyte NADPH oxidase to generate the superoxide precursor of microbicidal oxidants. This study uses deletion mutagenesis to identify regions of p22phox required for maturation of gp91phox and for NADPH oxidase activity. N-terminal, C-terminal, or internal deletions of human p22phox were generated and expressed in Chinese hamster ovary cells with transgenes for gp91phox and two other NADPH oxidase subunits, p47phox, and p67phox. The results demonstrate that p22phox-dependent maturation of gp91phox carbohydrate, cell surface expression of gp91phox, and the enzymatic function of flavocytochrome b558 are closely correlated. Whereas the 5 N-terminal and 25 C-terminal amino acids are dispensable for these functions, the N-terminal 11 amino acids of p22phox are required, as is a hydrophilic region between amino acids 65 and 90. Upon deletion of 54 residues at the C terminus of p22phox (amino acids 142-195), maturation and cell surface expression of gp91phox was still preserved, although NADPH oxidase activity was absent, as expected, due to removal of a proline-rich domain between amino acids 151-160 that is required for recruitment of p47phox. Antibody binding studies indicate that the extreme N terminus of p22phox is inaccessible in the absence of cell permeabilization, supporting a model in which both the N- and C-terminal domains of p22phox extend into the cytoplasm, anchored by two membrane-embedded regions.

Structural organization of the neutrophil NADPH oxidase: phosphorylation and translocation during priming and activation

The reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is part of the microbicidal arsenal used by human polymorphonuclear neutrophils (PMNs) to eradicate invading pathogens. The production of a superoxide anion (O2-) into the phagolysosome is the precursor for the generation of more potent products, such as hydrogen peroxide and hypochlorite. However, this production of O2- is dependent on translocation of the oxidase subunits, including gp91phox, p22phox, p47phox, p67phox, p40phox, and Rac2 from the cytosol or specific granules to the plasma membrane. In response to an external stimuli, PMNs change from a resting, nonadhesive state to a primed, adherent phenotype, which allows for margination from the vasculature into the tissue and chemotaxis to the site of infection upon activation. Depending on the stimuli, primed PMNs display altered structural organization of the NADPH oxidase, in that there is phosphorylation of the oxidase subunits and/or translocation from the cytosol to the plasma or granular membrane, but there is not the complete assembly required for O2- generation. Activation of PMNs is the complete assembly of the membrane-linked and cytosolic NADPH oxidase components on a PMN membrane, the plasma or granular membrane. This review will discuss the individual components associated with the NADPH oxidase complex and the function of each of these units in each physiologic stage of the PMN: rested, primed, and activated.

Oxidative stress and cardiovascular risk: the role of vascular NAD(P)H oxidase and its genetic variants

Several risk factors for coronary artery disease (CAD) induce atherosclerosis through endothelial activation and dysfunction, and ample evidence now suggests that the balance between production and removal of reactive oxygen species (ROS) - a condition termed oxidative stress - is implicated in such processes. A main source of ROS in vascular cells is the reduced nicotinamide adenine dinucleotide/nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase system. This is a membrane-associated enzyme, composed of five subunits, catalyzing the one-electron reduction of oxygen, using NADH or NADPH as the electron donor. One of the system subunits, termed p22-phox, has a polymorphic site on exon 4, associated with variable enzyme activity. This polymorphism is generated by a point mutation (C(242)T) producing a substitution of histidine with tyrosine at position 72, which affects one of the heme binding sites essential for the NAD(P)H enzyme activity. The consequent decrease of superoxide production thus characterizes a phenotype candidate for conferring to the carrier a reduced susceptibility to CAD. At present, however, the body of evidence from current literature is not yet sufficient to confirm or exclude the hypothesis that the C(242)T polymorphism protects from CAD. The functional effects of this polymorphism and the potential and its pathophysiological consequences also need further investigation.

Monoclonal antibody CL5 recognizes the amino terminal domain of human phagocyte flavocytochrome b558 large subunit, gp91phox

Human phagocyte flavocytochrome b558 (Cytb) is a heterodimeric integral membrane protein that serves as the electron transferase of the beta-nicotinamide adenine dinucleotidephosphate, reduced (NADPH)-oxidase, an enzyme complex important in the host defense function of phagocytic cells. In this study, we report the characterization of monoclonal antibody (mAb) CL5 that is specific for the large subunit, gp91phox, of the oxidase protein. This antibody recognizes gp91phox by immunoblot analysis of membrane extracts and samples of the immunopurified gp91phox/p22phox heterodimer, prepared on anti-p22phox affinity matrices. Phage display analysis confirmed this specificity, indicating that the CL5 epitope contains the region 135-DPYSVALSELGDR of gp91phox. The antibody was used to probe for the presence of gp91phox in membrane preparations from neutrophils of patients with nine genetically distinct forms of X-linked chronic granulomatous disease (CGD). The causative mutations included missense errors as well as nonsense errors that result in premature termination of gp91phox synthesis. Analysis of the CGD samples by immunoblotting indicated that CL5 recognizes only the full-length wild-type and two missense mutations, consistent with the absence of stable short gp91phox peptide expression in CGD neutrophils. Interestingly, CL5 was also shown to be cross-reactive with cytosolic and membrane-bound gelsolin, identified by purification, mass spectrometry and immunoblot analysis. CL5 probably cross-reacts with the sequence 771-DPLDRAMAEL in the C-terminus of gelsolin. We conclude that mAb CL5 is a useful probe for detection of full length and possibly truncated N-terminal fragments of gp91phox from membranes of Cytb-producing cells.

Site-specific inhibitors of NADPH oxidase activity and structural probes of flavocytochrome b: characterization of six monoclonal antibodies to the p22phox subunit

The integral membrane protein flavocytochrome b (Cyt b) is the catalytic core of the human phagocyte NADPH oxidase, an enzyme complex that initiates a cascade of reactive oxygen species important in the elimination of infectious agents. This study reports the generation and characterization of six mAbs (NS1, NS2, NS5, CS6, CS8, and CS9) that recognize the p22(phox) subunit of the Cyt b heterodimer. Each of the mAbs specifically detected p22(phox) by Western blot analysis but did not react with intact neutrophils in FACS studies. Phage display mapping identified core epitope regions recognized by mAbs NS2, NS5, CS6, CS8, and CS9. Fluorescence resonance energy transfer experiments indicated that mAbs CS6 and CS8 efficiently compete with Cascade Blue-labeled mAb 44.1 (a previously characterized, p22(phox)-specific mAb) for binding to Cyt b, supporting phage display results suggesting that all three Abs recognize a common region of p22(phox). Energy transfer experiments also suggested the spatial proximity of the mAb CS9 and mAb NS1 binding sites to the mAb 44.1 epitope, while indicating a more distant proximity between the mAb NS5 and mAb 44.1 epitopes. Cell-free oxidase assays demonstrated the ability of mAb CS9 to markedly inhibit superoxide production in a concentration-dependent manner, with more moderate levels of inhibition observed for mAbs NS1, NS5, CS6, and CS8. A combination of computational predictions, available experimental data, and results obtained with the mAbs reported in this study was used to generate a novel topology model of p22(phox).

Functional association of nox1 with p22phox in vascular smooth muscle cells

The vascular NAD(P)H oxidases constitute important sources of ROS in the vessel wall and have been implicated in vascular disease. Vascular smooth muscle cells (VSMCs) from conduit arteries express two gp91phox homologs, Nox1 and Nox4, of which Nox1 is agonist-sensitive. Because p22phox has been shown to be functionally important in vascular cells stimulated with vasoactive hormones, the relationship of Nox1 and p22phox was investigated in VSMCs from rat and human aortas. Coimmunoprecipitation studies demonstrated that p22phox and hemagglutinin-tagged Nox1 associate in unstimulated VSMCs. These findings were confirmed by confocal microscopy, showing colocalization of the two proteins in their native states in the plasma membrane and submembrane areas of the cell. NADPH-driven superoxide production, as measured by electron spin resonance using 1-hydroxy-3-carboxypyrrolidine as a spin probe, is dependent on the coexpression of both subunits, suggesting the importance of the association for the functional integrity of the enzyme. These results indicate that in contrast to the neutrophil enzyme, VSMCs can use Nox1 rather than gp91phox as a catalytic center in the p22phox-based oxidase and that these two proteins are preassembled at or near the plasma membrane and submembrane vesicular structures in unstimulated cells.

Structure and regulation of the neutrophil respiratory burst oxidase: comparison with nonphagocyte oxidases

Neutrophils play an essential role in the body's innate defense against pathogens and are one of the primary mediators of the inflammatory response. To defend the host, neutrophils use a wide range of microbicidal products, such as oxidants, microbicidal peptides, and lytic enzymes. The generation of microbicidal oxidants by neutrophils results from the activation of a multiprotein enzyme complex known as the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, which is responsible for transferring electrons from NADPH to O2, resulting in the formation of superoxide anion. During oxidase activation, cytosolic oxidase proteins translocate to the phagosome or plasma membrane, where they assemble around a central membrane-bound component known as flavocytochrome b. This process is highly regulated, involving phosphorylation, translocation, and multiple conformational changes. Originally, it was thought that the NADPH oxidase was restricted to phagocytes and used solely in host defense. However, recent studies indicate that similar NADPH oxidase systems are present in a wide variety of nonphagocytic cells. Although the nature of these nonphagocyte NADPH oxidases is still being defined, it is clear that they are functionally distinct from the phagocyte oxidases. It should be noted, however, that structural features of many nonphagocyte oxidase proteins do seem to be similar to those of their phagocyte counterparts. In this review, key structural and functional features of the neutrophil NADPH oxidase and its protein components are described, including a consideration of transcriptional and post-translational regulatory features. Furthermore, relevant details about structural and functional features of various nonphagocyte oxidase proteins will be included for comparison.

Single-step immunoaffinity purification and characterization of dodecylmaltoside-solubilized human neutrophil flavocytochrome b

Flavocytochrome b (Cyt b) is a heterodimeric, integral membrane protein that serves as the central component of an electron transferase system employed by phagocytes for elimination of bacterial and fungal pathogens. This report describes a rapid and efficient single-step purification of Cyt b from human neutrophil plasma membranes by solubilization in the nonionic detergent dodecylmaltoside (DDM) and immunoaffinity chromatography. A similar procedure for isolation of Cyt b directly from intact neutrophils by a combination of heparin and immunoaffinity chromatography is also presented. The stability of Cyt b was enhanced in DDM relative to previously employed solubilizing agents as determined by both monitoring the heme spectrum in crude membrane extracts and assaying resistance to proteolytic degradation following purification. Gel filtration chromatography and dynamic light scattering indicated that DDM maintains a predominantly monodisperse population of Cyt b following immunoaffinity purification. The high degree of purity obtained with this isolation procedure allowed for direct determination of a 2:1 heme to protein stoichiometry, confirming previous structural models. Analysis of the isolated heterodimer by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry allowed for accurate mass determination of p22(phox) as indicated by the gene sequence. Affinity-purified Cyt b was functionally reconstituted into artificial bilayers and demonstrated that catalytic activity of the protein was efficiently retained throughout the purification procedure.

Inhibition of NADPH oxidase activation in endothelial cells by ortho-methoxy-substituted catechols

NADPH oxidase is a major enzymatic source of oxygen free radicals in stimulated endothelial cells (ECs). The ortho-methoxy-substituted catechol, apocynin (4-hydroxy-3-methoxyacetophenone), isolated from the traditional medicinal plant Picrorhiza kurroa, inhibits the release of superoxide anion (O2*-) by this enzyme. The compound acts by blocking the assembly of a functional NADPH oxidase complex. The underlying chemistry of this inhibitory activity, and its physiological significance to EC proliferation, have been investigated. A critical event is the reaction of ortho-methoxy-substituted catechols with reactive oxygen species (ROS) and peroxidase. Analysis of this reaction reveals that apocynin is converted to a symmetrical dimer through the formation of a 5,5' carbon-carbon bond. Both reduced glutathione and L-cysteine inhibit this dimerization process. Catechols without the ortho-methoxy-substituted group do not undergo this chemical reaction. Superoxide production by an endothelial cell-free system incubated with apocynin was nearly completely inhibited after a lagtime for inhibition of ca. 2 min. Conversely, O2*- production was nearly completely inhibited, without a lagtime, by incubation with the dimeric form of apocynin. The apocynin dimer undergoes a two-electron transfer reaction with standard redox potentials of -0.75 and -1.34 V as determined by cyclic voltammetry. Inhibition of endothelial NADPH oxidase by apocynin caused a dose-dependent inhibition of cell proliferation. These findings identify a metabolite of an ortho-methoxy-substituted catechol, which may be the active compound formed within stimulated ECs that prevents NADPH oxidase complex assembly and activation.

Structural changes are induced in human neutrophil cytochrome b by NADPH oxidase activators, LDS, SDS, and arachidonate: intermolecular resonance energy transfer between trisulfopyrenyl-wheat germ agglutinin and cytochrome b(558)

Anionic amphiphiles such as sodium- and lithium dodecyl sulfate (SDS, LDS), or arachidonate (AA) initiate NADPH oxidase and proton channel activation in cell-free systems and intact neutrophils. To investigate whether these amphiphiles exert allosteric effects on cytochrome b, trisulfopyrenyl-labeled wheat germ agglutinin (Cascade Blue-wheat germ agglutinin, CCB-WGA) was used as an extrinsic fluorescence donor for resonance energy transfer (RET) to the intrinsic heme acceptors of detergent-solubilized cytochrome b. In solution, cytochrome b complexed with the CCB-WGA causing a rapid, saturable, carbohydrate-dependent quenching of up to approximately 55% of the steady-state fluorescence. Subsequent additions of SDS, LDS, or AA to typical cell-free oxidase assay concentrations completely relaxed the fluorescence quenching. The relaxation effects were specific, and not caused by dissociation of the CCB-WGA-cytochrome b complex or alterations in the spectral properties of the chromophores. In contrast, addition of the oxidase antagonist, arachidonate methyl ester, caused an opposite effect and was able to partially reverse the activator-induced relaxation. We conclude that the activators induce a cytochrome b conformation wherein the proximity or orientation between the hemes and the extrinsic CCB fluorescence donors has undergone a significant change. These events may be linked to NADPH oxidase assembly and activation or proton channel induction.

A 29-kDa protein associated with p67phox expresses both peroxiredoxin and phospholipase A2 activity and enhances superoxide anion production by a cell-free system of NADPH oxidase activity

Production of toxic oxygen metabolites provides a mechanism for microbicidal activity of the neutrophil. The NADPH oxidase enzyme system initiates the production of oxygen metabolites by reducing oxygen to form superoxide anion (O(2)()). With stimulation of the respiratory burst, cytosolic oxidase components, p47(phox), p67(phox), and Rac, translocate to the phagolysomal and plasma membranes where they form a complex with cytochrome b(558) and express enzyme activity. A 29-kDa neutrophil protein (p29) was identified by co-immunoprecipitation with p67(phox). N-terminal sequence analysis of p29 revealed homology to an open reading frame gene described in a myeloid leukemia cell line. A cDNA for p29 identical to the open reading frame protein was amplified from RNA of neutrophils. Significant interaction between p29 and p67(phox) was demonstrated using a yeast two-hybrid system. A recombinant (rh) p29 was expressed in Sf9 cells resulting in a protein with an apparent molecular weight of 34,000. The rh-p29 showed immunoreactivity with the original rabbit antiserum that detected p47(phox) and p67(phox). In addition, rh-p29 exhibited PLA(2) activity, which was Ca(2+) independent, optimal at low pH, and preferential for phosphatidylcholine substrates. The recombinant protein protected glutathione synthetase and directly inactivated H(2)O(2). By activity and sequence homology, rh-p29 can be classified as a peroxiredoxin. Finally, O(2)() production by plasma membrane and recombinant cytosolic oxidase components in the SDS-activated, cell-free NADPH oxidase system were enhanced by rh-p29. This effect was not inhibited by PLA(2) inhibitors. Thus, p29 is a novel protein that associates with p67 and has peroxiredoxin activity. This protein has a potential role in protecting the NADPH oxidase by inactivating H(2)O(2) or altering signaling pathways affected by H(2)O(2).

Mutagenesis of p22(phox) histidine 94. A histidine in this position is not required for flavocytochrome b558 function

The NADPH oxidase is a multicomponent enzyme that transfers electrons from NADPH to O2 to generate superoxide (O2*-), the precursor of microbicidal oxygen species that play an important role in host defense. Flavocytochrome b558, a heterodimeric oxidoreductase comprised of gp91(phox) and p22(phox) subunits, contains two nonidentical, bis-histidine-ligated heme groups imbedded within the membrane. Four histidine residues that appear to serve as noncovalent axial heme ligands reside within the hydrophobic N terminus of gp91(phox), but the role of p22(phox) in heme binding is unclear. We compared biochemical and functional features of wild type flavocytochrome b558 with those in cells co-expressing gp91(phox) with p22(phox) harboring amino acid substitutions at histidine 94, the only invariant histidine residue within the p22(phox) subunit. Substitution with leucine, tyrosine, or methionine did not affect heterodimer formation or flavocytochrome b558 function. The heme spectrum in purified preparations of flavocytochrome b558 containing the p22(phox) derivative was unaffected. In contrast, substitution of histidine 94 with arginine appeared to disrupt the intrinsic stability of p22(phox) and, secondarily, the stability of mature gp91(phox) and abrogated O2*- production. These findings demonstrate that His94 p22(phox) is not required for heme binding or function of flavocytochrome b558 in the NADPH oxidase.

The S100A8/A9 protein as a partner for the cytosolic factors of NADPH oxidase activation in neutrophils

In a previous study, the S100A8/A9 protein, a Ca2+- and arachidonic acid-binding protein, abundant in neutrophil cytosol, was found to potentiate the activation of the redox component of the O2- generating oxidase in neutrophils, namely the membrane-bound flavocytochrome b, by the cytosolic phox proteins p67phox, p47phox and Rac (Doussière J., Bouzidi F. and Vignais P.V. (2001) Biochem. Biophys. Res. Commun.285, 1317-1320). This led us to check by immunoprecipitation and protein fractionation whether the cytosolic phox proteins could bind to S100A8/A9. Following incubation of a cytosolic extract from nonactivated bovine neutrophil with protein A-Sepharose bound to anti-p67phox antibodies, the recovered immunoprecipitate contained the S100 protein, p47phox and p67phox. Cytosolic protein fractionation comprised two successive chromatographic steps on hydroxyapatite and DEAE cellulose, followed by isoelectric focusing. The S100A8/A9 heterodimeric protein comigrated with the cytosolic phox proteins, and more particularly with p67phox and Rac2, whereas the isolated S100A8 protein displayed a tendancy to bind to p47phox. Using a semirecombinant cell-free system of oxidase activation consisting of recombinant p67phox, p47phox and Rac2, neutrophil membranes and arachidonic acid, we found that the S100A8/A9-dependent increase in the elicited oxidase activity corresponded to an increase in the turnover of the membrane-bound flavocytochrome b, but not to a change of affinity for NADPH or O2. In the absence of S100A8/A9, oxidase activation departed from michaelian kinetics above a critical threshold concentration of cytosolic phox proteins. Addition of S100A8/A9 to the cell-free system rendered the kinetics fully michaelian. The propensity of S100A8/A9 to bind the cytosolic phox proteins, and the effects of S100A8/A9 on the kinetics of oxidase activation, suggest that S100A8/A9 might be a scaffold protein for the cytosolic phox proteins or might help to deliver arachidonic acid to the oxidase, thus favoring the productive interaction of the cytosolic phox proteins with the membrane-bound flavocytochrome b.

Intestinal dipeptidyl peptidase IV is efficiently sorted to the apical membrane through the concerted action of N- and O-glycans as well as association with lipid microdomains

The apical sorting of human intestinal dipeptidyl peptidase IV (DPPIV) occurs through complex N-linked and O-linked carbohydrates. Inhibition of O-linked glycosylation by benzyl-N-acetyl-alpha-d-galactosaminide affects significantly the sorting behavior of DPPIV in intestinal Caco-2 and HT-29 cells. However, random delivery to the apical and basolateral membranes and hence a more drastic effect on the sorting of DPPIV in both cell types is only observed when, in addition to O-glycans, the processing of N-glycans is affected by swainsonine, an inhibitor of mannosidase II. Together the data indicate that both types of glycosylation are critical components of the apical sorting signal of DPPIV. The sorting mechanism of DPPIV implicates its association with detergent-insoluble membrane microdomains containing cholesterol and sphingolipids, whereas an efficient association largely depends on the presence of a fully complex N- and O-linked glycosylated DPPIV. Interestingly, cholesterol is a more critical component in this context than sphingolipids, because cholesterol depletion by beta-cyclodextrin affects the detergent solubility and the sorting behavior of DPPIV more strongly than fumonisin, an inhibitor of sphingolipid synthesis.

Identification of a spectrally stable proteolytic fragment of human neutrophil flavocytochrome b composed of the NH2-terminal regions of gp91(phox) and p22(phox)

A heme-bearing polypeptide core of human neutrophil flavocytochrome b(558) was isolated by applying high performance, size exclusion, liquid chromatography to partially purified Triton X-100-solubilized flavocytochrome b that had been exposed to endoproteinase Glu-C for 1 h. The fragment was composed of two polypeptides of 60-66 and 17 kDa by SDS-polyacrylamide gel electrophoresis and retained a native heme absorbance spectrum that was stable for several days when stored at 4 degrees C in detergent-containing buffer. These properties suggested that the majority of the flavocytochrome b heme environment remained intact. Continued digestion up to 4.5 h yielded several heme-associated fragments that were variable in composition between experiments. Digestion beyond 4.5 h resulted in a gradual loss of recoverable heme. N-Linked deglycosylation and reduction and alkylation of the 1-h digestion fragment did not affect the electrophoretic mobility of the 17-kDa fragment but reduced the 60-66-kDa fragment to 39 kDa. Sequence and immunoblot analyses identified the fragments as the NH(2)-terminal 320-363 amino acid residues of gp91(phox) and the NH(2)-terminal 169-171 amino acid residues of p22(phox). These findings provide direct evidence that the primarily hydrophobic NH(2)-terminal regions of flavocytochrome b are responsible for heme ligation.

Heme-ligating histidines in flavocytochrome b(558): identification of specific histidines in gp91(phox)

The phagocyte NADPH-dependent oxidase generates superoxide (O(2)) by reducing molecular oxygen through flavocytochrome b(558) (flavocytochrome b), a heterodimeric oxidoreductase composed of gp91(phox) and p22(phox) subunits. Although each flavocytochrome b molecule contains two heme groups, their precise distribution within the heterodimer is unknown. Among functionally and/or structurally related oxidoreductases, histidines at codons 101, 111, 115, 119, 209, 210, and 222 of gp91(phox) are conserved and potential candidates to ligate heme. We compared biochemical and functional features of normal flavocytochrome b with those in cells expressing gp91(phox) harboring amino acid substitutions at each of these histidines. Surface expression of flavocytochrome b and heterodimer formation were relatively unaffected in cells expressing gp91(phox) H111L, H119L, or H210L. These mutations also had no effect on the flavocytochrome b heme spectrum, although NADPH oxidase activity was decreased in cells expressing gp91(phox) H119L or H210L. In contrast, gp65 was not processed to gp91(phox), heterodimers did not form, and flavocytochrome b was not expressed on the surface of cells expressing gp91(phox) H101L, H115L, H115D, H209C, H209Y, H222L, H222C, or H222R. Similarly, this subset of mutants lacked detectable O(2)-generating activity, and flavocytochrome b purified from these cells contained little or no heme. These findings demonstrate that His(101), His(115), His(209), and His(222) of gp91(phox) are critical for heme binding and biosynthetic maturation of flavocytochrome b.

Endothelial cell oxidant production: effect of NADPH oxidase inhibitors

The effects of known leukocyte NADPH oxidase inhibitors on general cellular oxidant production in cultured human endothelial cells (EC) has been investigated. EC were stimulated with 10 nM phorbol 12-myristate 13-acetate and cellular oxidant production measured in the presence and absence of inhibitors that act on various substituents of the oxidase complex and its activation pathways. The effects of the cytosolic oxidase subunit translocation inhibitors, catechols (3,4-dihydroxybenzaldehyde, caffeic acid, and protocatechuic acid), ortho-methoxy-substituted catechols (apocynin, vanillin, and 4-nitroguaiacol), and quinone, 1,4-naphthoquinone; flavoprotein inhibitors, diphenylene iodonium and quinacrine; haem ligands, imidazole and pyridine; directly acting thiol reagents, disulfiram and penicillamine; NADPH analogue, Cibacron Blue; redox active inhibitors, quercetin and esculetin; intracellular calcium antagonist, TMB-8; and calmodulin antagonists, W-7 and trifluoperazine, were determined. All compounds reduced oxidant production in stimulated EC. These findings add to previous observations suggesting the presence of a functionally active NADPH oxidase in EC. Identifying the major cellular reactive oxygen species source in perturbed EC will provide new insights into our understanding of endothelial dysfunction, which has been hypothesized to be a major contributing factor in the pathogenesis of atherosclerosis.

Phosphorylation of p22phox is mediated by phospholipase D-dependent and -independent mechanisms. Correlation of NADPH oxidase activity and p22phox phosphorylation

Human neutrophils participate in the host innate immune response, partly mediated by the multicomponent superoxide-generating enzyme NADPH oxidase. A correlation between phosphorylation of cytosolic NADPH oxidase components and enzyme activation has been identified but is not well understood. We previously showed that p22(phox), the small subunit of the membrane-bound oxidase component flavocytochrome b(558), is an in vitro substrate for both a phosphatidic acid-activated kinase and conventional protein kinase C isoforms (Regier, D. S., Waite, K. A., Wallin, R., and McPhail, L. C. (1999) J. Biol. Chem. 274, 36601-36608). Here we show that several neutrophil agonists (phorbol myristate acetate, opsonized zymosan, and N-formyl-methionyl-leucyl-phenylalanine) induce p22(phox) phosphorylation in intact neutrophils. To determine if phospholipase D (PLD) is needed for p22(phox) phosphorylation, cells were pretreated with ethanol, which reduces phosphatidic acid production by PLD in stimulated cells. Phorbol myristate acetate-induced phosphorylation of p22(phox) and NADPH oxidase activity were not reduced by ethanol. In contrast, ethanol reduced both activities when cells were stimulated by N-formyl-methionyl-leucyl-phenylalanine or opsonized zymosan. Varying the time of stimulation with opsonized zymosan showed that the phosphorylation of p22(phox) coincides with NADPH oxidase activation. GF109203X, an inhibitor of protein kinase C and the phosphatidic acid-activated protein kinase, decreased both p22(phox) phosphorylation and NADPH oxidase activity in parallel in opsonized zymosan-stimulated cells. Stimulus-induced phosphorylation of p22(phox) was on Thr residue(s), in agreement with in vitro results. Overall, these data show that NADPH oxidase activity and p22(phox) phosphorylation are correlated and suggest two mechanisms (PLD-dependent and -independent) by which p22(phox) phosphorylation occurs.

Processing and maturation of flavocytochrome b558 include incorporation of heme as a prerequisite for heterodimer assembly

The phagocyte NADPH-dependent oxidase generates superoxide by reducing molecular oxygen through a transmembrane heterodimer known as flavocytochrome b(558) (flavocytochrome b). We investigated the biosynthesis of flavocytochrome b subunits gp91(phox) and p22(phox) to elucidate features of flavocytochrome b processing in myeloid cells. Although the gp91(phox) precursor, gp65, was processed to gp91(phox) within 4-8 h of chase, unassembled gp65 and p22(phox) monomers were degraded by the cytosolic proteasome. gp65 associated with p22(phox) post-translationally, within 1-4 h of chase, but prior to its modification in the Golgi complex. Moreover, p22(phox) coprecipitated with unglycosylated gp91(phox) primary translation product made in the presence of tunicamycin, suggesting that heterodimer formation does not require glycosylation. Blocking heme synthesis with succinyl acetone completely inhibited heterodimer formation, although biogenesis of gp65 and p22(phox) was unaffected. In succinyl acetone-treated cells, p22(phox) and gp65 were degraded completely by 8 h of chase, a process mediated by the cytosolic proteasome. Taken together, these data suggest that the formation of the gp65-p22(phox) heterodimer is relatively inefficient and that acquisition of heme by gp65 precedes and is required for its association with p22(phox), a process that requires neither the addition of N-linked oligosaccharides nor modification in the Golgi complex.

A phosphatidic acid-activated protein kinase and conventional protein kinase C isoforms phosphorylate p22(phox), an NADPH oxidase component

Using a phosphorylation-dependent cell-free system to study NADPH oxidase activation (McPhail, L. C., Qualliotine-Mann, D., and Waite, K. A. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 7931-7935), we previously showed that p47(phox), a cytosolic NADPH oxidase component, is phosphorylated. Now, we show that p22(phox), a subunit of the NADPH oxidase component flavocytochrome b(558), also is phosphorylated. Phosphorylation is selectively activated by phosphatidic acid (PA) versus other lipids and occurs on a threonine residue in p22(phox). We identified two protein kinase families capable of phosphorylating p22(phox): 1) a potentially novel, partially purified PA-activated protein kinase(s) known to phosphorylate p47(phox) and postulated to mediate the phosphorylation-dependent activation of NADPH oxidase by PA and 2) conventional, but not novel or atypical, isoforms of protein kinase C (PKC). In contrast, all classes of PKC isoforms could phosphorylate p47(phox). In a gel retardation assay both the phosphatidic acid-dependent kinase and conventional PKC isoforms phosphorylated all molecules of p22(phox). These findings suggest that phosphorylation of p22(phox) by conventional PKC and/or a novel PA-activated protein kinase regulates the activation/assembly of NADPH oxidase.

Cytochrome b558 (p22phox) in the guinea-pig adrenal medulla

Paraganglionic cells are sensitive to hypoxia, and the involvement of a plasmalemmal cytochrome b558-like protein in oxygen sensing by these cells has been suggested, but neither the identity of the immunoreactive protein detected by immunohistochemistry nor its anticipated subcellular (i.e., plasmalemmal) localization were directly proven. Thus, we extended these studies to the largest paraganglion, i.e., the adrenal medulla, in the guinea-pig, which, due to its size and accessibility, allowed us to address both of these issues utilizing antisera raised against synthetic peptides of the small (22 kD) subunit of cytochrome b558, p22phox. Cytochrome b558 was originally identified in granulocytes and macrophages, and antisera against this phagocyte p22phox were utilized. Immunoreactivity to p22phox was observed in all adrenal medullary endocrine cells, and the identity of the immunoreactive protein to the small cytochrome b558-subunit was confirmed by Western blotting. Immuno-electron microscopy of ultrathin cryosections and of resin-embedded tissue demonstrated its subcellular localization in the dense core vesicles of endocrine A-cells but not in the plasma membrane. In conclusion, the present study documents the presence of the small subunit of cytochrome b558 in guinea-pig adrenal medullary cells, but its subcellular vesicular localization does not support the initial interpretation of cytochrome b558 serving as a plasmalemmal oxygen sensor.

Simultaneous presence of p47(phox) and flavocytochrome b-245 are required for the activation of NADPH oxidase by anionic amphiphiles. Evidence for an intermediate state of oxidase activation

We have examined the kinetics of NADPH oxidase activation induced by arachidonic acid or SDS in a cell-free system using mixtures of recombinant Phox proteins and purified flavocytochrome b-245. Activation of oxidase activity required the simultaneous presence of p47(phox), flavocytochrome b-245, and the anionic amphiphile. The activation of electron transfer reactions was much more rapid when iodonitrotetrazolium violet was used as electron acceptor than when oxygen alone was the acceptor. We propose that this difference represents an intermediate activation state of NADPH oxidase in which electron flow can proceed from NADPH to enzyme flavin (and hence to iodonitrotetrazolium violet) but not from flavin to heme (or not between the hemes). A model for NADPH oxidase activation is presented that is consistent with these observations.

Mutation at histidine 338 of gp91(phox) depletes FAD and affects expression of cytochrome b558 of the human NADPH oxidase

Defective NADPH oxidase components prevent superoxide (O-2) generation, causing chronic granulomatous disease (CGD). X-linked CGD patients have mutations in the gene encoding the gp91(phox) subunit of cytochrome b558 and usually lack gp91(phox) protein completely (X91(0)). gp91(phox) is considered to be a flavocytochrome that contains binding sites for NADPH, FAD, as well as heme. We here report a rare X-linked CGD patient whose neutrophils entirely failed to produce O-2, but presented a diminished expression of gp91(phox) containing about one-third of the heme present in normal individuals by Soret absorption. Translocation of cytosolic factors p67(phox) and p47(phox) was normal. However, the FAD content in his neutrophil membranes was as low as that of X91(0) patients, suggesting complete depletion of FAD in his gp91(phox). This was in agreement with the finding that a single base substitution (C1024 to T) changed His-338 to Tyr in gp91(phox) in a predicted FAD-binding domain of the flavocytochrome model. The loss of FAD could not be corrected even after addition of reagent FAD or a FAD-rich dehydrogenase fraction isolated from normal neutrophils to the patient's membranes, in a reconstitution in vitro with normal cytosol. These results indicate that His-338 is a very critical residue for FAD incorporation into the NADPH oxidase system. This is the first such mutation found in CGD.

Extracellular Ca2+ regulates the respiratory burst of human neutrophils

The role of extracellular calcium in the activation of respiratory burst in human neutrophils was studied by using the receptor agonist, N-formyl-methionyl-leucyl-phenylalanine (fMLP), and the activator of protein kinase C phorbol myristate acetate (PMA). The level of intracellular free calcium was measured by using both cell suspensions and single cells in the presence and absence of extracellular calcium. The Ca2+-ATPase inhibitor, thapsigargin, was used to activate higher Ca2+ influx, while a novel calcium channel blocker, panax notoginseng saponins (PNGS) was used to block the Ca2+ entry from extracellular space during the responding period of cells. It was found that about two-thirds of the activation of respiratory burst initiated by the receptor agonist were attributed to the Ca2+ influx under normal physiological conditions. The higher Ca2+ influx resulted in tremendous enhancement of the intensity of respiratory burst initiated by fMLP and marked acceleration of the onset of the respiratory burst stimulated by PMA. It is evident that both intra- and extracellular Ca2+ are required for full activation of the respiratory burst of human neutrophils, and the Ca2+ influx from extracellular space plays an important role either in generation of reactive oxygen metabolites or in activation of protein kinase C.

Gp91(phox) is the heme binding subunit of the superoxide-generating NADPH oxidase

The phagocyte NADPH oxidase flavocytochrome b558 is a membrane-bound heterodimer comprised of a glycosylated subunit, gp91(phox), and a nonglycosylated subunit, p22(phox). It contains two nonidentical heme groups that mediate the final steps of electron transfer to molecular oxygen (O2), resulting in the generation of superoxide ion (O2-). However, the location of the hemes within the flavocytochrome heterodimer remains controversial. In this study, we have used transgenic COS7 cell lines expressing gp91(phox), p22(phox), or both polypeptides to examine the relative role of each flavocytochrome b558 subunit in heme binding and O2- formation. A similar membrane localization was observed when gp91(phox) and p22(phox) were either expressed individually or coexpressed, as analyzed by confocal microscopy and immunoblotting of subcellular fractions. Spectral analysis of membranes prepared from COS7 cell lines expressing either gp91(phox) or both gp91(phox) and p22(phox) showed a b-type cytochrome with spectral characteristics identical to those of human neutrophil flavocytochrome b558. In contrast, no heme spectrum was detected in wild-type COS7 membranes or those containing only p22(phox). Furthermore, redox titration studies suggested that two heme groups were contained in gp91(phox) expressed in COS7 membranes, with midpoint potentials of -264 and -233 mV that were very similar to those obtained for neutrophil flavocytochrome b558. These results provide strong support for the hypothesis that gp91(phox) is the sole heme binding subunit of flavocytochrome b558. However, coexpression of gp91(phox) and p22(phox) in COS7 membranes was required to support O2- production in combination with neutrophil cytosol, indicating that the functional assembly of the active NADPH oxidase complex requires both subunits of flavocytochrome b558.

Regulation of the neutrophil respiratory burst oxidase. Identification of an activation domain in p67(phox)

Superoxide generation by the neutrophil respiratory burst oxidase (NADPH oxidase) can be reconstituted in a cell-free system using flavocytochrome b558 and the cytosolic proteins p47(phox), p67(phox), and Rac. p47(phox) functions as an adaptor protein; it increases the affinity of p67(phox) and Rac in the NADPH oxidase complex, but is not essential when high concentrations of these proteins are used (Freeman, J. L., and Lambeth, J. D. (1996) J. Biol. Chem. 271, 22578-22582), implying that p67(phox) and/or Rac directly regulates enzyme activity. Herein, we describe an activation domain in p67(phox) that is essential for NADPH oxidase activity. A series of C-terminal truncation mutants of p67(phox) showed that residues 211 to the C terminus (residue 526) are not needed for cell-free activity. However, shorter truncations were inactive, pointing to an activation domain within the region spanning residues 199-210. p67(phox) mutated at single amino acid residues within this region showed diminished activity, and p67(phox) V204A was completely inactive. The effects of mutations on activity were independent of p47(phox), and mutations did not affect the binding of p67(phox) to Rac. In the presence of wild-type p67(phox), the V204A mutant was a potent inhibitor of superoxide generation, and inhibition was partially reversed by high concentrations of p67(phox), but not by p47(phox) or Rac. The V204A mutant competed with native p67(phox) for translocation to neutrophil plasma membrane, indicating that p67(phox) V204A assembles to form an inactive complex. The data imply a direct activation of flavocytochrome b558 by an activation domain in p67(phox).

The molecular basis of chronic granulomatous disease

CGD is a rare inherited immunodeficiency syndrome, caused by the phagocytes' inability to produce (sufficient) reactive oxygen metabolites. This dysfunction is due to a defect in the NADPH oxidase, the enzyme responsible for the production of superoxide. It is composed of several subunits, two of which, gp91phox and p22phox, form the membrane-bound cytochrome b558, while its three cytosolic components, p47phox, p67phox and p40phox, have to translocate to the membrane upon activation. This is a tightly and intricately controlled process that involves, among others, several low-molecular weight GTP-binding proteins. Gp91phox is encoded on the X-chromosome and p22phox, p47phox and p67phox on different autosomal chromosomes, and a defect in one of these components leads to CGD. This explains the variable mode of inheritance seen in this syndrome. Clinically CGD manifests itself typically already at a very young age with recurrent and serious infections, most often caused by catalase-positive pathogens. Modern treatment options, including prophylaxis with trimethoprim-sulfamethoxazole and rIFN-gamma as well as early and aggressive anti-infection therapy, have improved the prognosis of this disease dramatically. CGD, as a very well-characterized inherited affection of the hematopoietic stem cells, is predestined to be among the first diseases to profit from the advances in cutting-edge therapeutics, such as gene therapy and in utero stem cell transplantation.

Probing the role of the carboxyl terminus of the gp91phox subunit of neutrophil flavocytochrome b558 using site-directed mutagenesis

Site-directed mutagenesis was used to generate a series of substitutions and deletions in the carboxyl-terminal 11 residues of gp91phox, the 91-kDa subunit of the phagocyte NADPH oxidase flavocytochrome b558. This region encompasses 559RGVHFIF565, implicated as a contact point for the cytosolic oxidase subunit p47phox during oxidase activation, and a carboxyl-terminal phenylalanine (Phe570), which corresponds in position to a highly conserved aromatic residue that interacts with the flavin group in the ferredoxin-NADP+ reductase flavoenzyme family, of which gp91phox is a member. Mutant proteins were expressed in human myeloid leukemia cells which lack expression of endogenous gp91phox due to targeted disruption of the X-linked gp91phox gene. Although specific residues within 559RGVHFIF565 had previously been identified by alanine scanning as essential for peptide inhibition of oxidase activity in a cell-free assay, comparable substitutions in the gp91phox polypeptide had either no or only a modest effect on oxidase activity in whole cells. Replacement of nonpolar with polar or charged residues had greater effects on oxidase activity, but were also associated with decreased gp91phox expression, suggesting that overall protein structure was perturbed. No stable gp91phox protein was detected upon deletion of the terminal 11 amino acids. Alanine substitution or deletion of the carboxyl-terminal Phe570 in gp91phox resulted in a 2-fold reduction in superoxide production. This contrasts with a approximately 300-800-fold reduction reported for comparable mutations in pea ferredoxin-NADP+ reductase, which suggests that structural or functional differences exist between the carboxyl terminus of gp91phox and other ferredoxin-NADP+ reductases.

Biosynthesis of the phagocyte NADPH oxidase cytochrome b558. Role of heme incorporation and heterodimer formation in maturation and stability of gp91phox and p22phox subunits

The NADPH oxidase cytochrome b558 is a membrane heterodimer comprised of a glycosylated 91-kDa subunit, gp91(phox), and a nonglycosylated 22-kDa subunit, p22(phox). The role of heme in cytochrome b558 biosynthesis was studied using succinyl acetone, an inhibitor of heme synthesis, in PLB-985 myeloid cells undergoing granulocytic differentiation. Succinyl acetone markedly reduced expression of p22(phox) and the mature 91-kDa form of gp91(phox) but not its 65-kDa high mannose precursor, in association with a profound reduction in NADPH oxidase activity. Expression of non-heme-containing cytosolic oxidase components was unaffected. The reduction in cytochrome b558 expression and NADPH oxidase activity was prevented by adding exogenous heme and was reversible upon removal of succinyl acetone. Transgenic expression of gp91(phox) in monkey COS-7 and murine 3T3 cells, both of which lacked endogenous p22(phox) mRNA, demonstrated that p22(phox) was not required for maturation of gp91(phox) carbohydrate to complex oligosaccharides. However, coexpression of transgenic p22(phox) increased the abundance of the mature gp91(phox) glycoprotein. These results suggest that heme incorporation plays an important role in cytochrome b558 assembly and provide further support for the concept that stability of p22(phox) and the mature gp91(phox) subunit is increased by heterodimer formation.

Purified leukocyte cytochrome b558 incorporated into liposomes catalyzes a cytosolic factor dependent diaphorase activity

The leukocyte iodonitrotetrazolium violet (INT) reductase activity of disrupted bovine polymorphonuclear neutrophils is closely associated with the activation of the O2(-)-generating NADPH oxidase in a cell-free system. It is dependent upon NADPH, cytosolic factors, and amphiphiles (such as arachidonate), the same factors required for O2- generation. Both O2- generation and INT reductase activity are inhibited by phenylarsine oxide, an inhibitor of the activation of the NADPH oxidase [Li, J., & Guillory, R. J. (1997) J. Biochem. Mol. Biol. Biophys. (in press)]. In this report, the INT diaphorase activity of disrupted bovine polymorphonuclear neutrophils is shown to be resolved by DEAE-Sepharose chromatography into two fractions: an NADPH-cytochrome c reductase-containing fraction and a cytochrome b558-associated fraction. The diaphorase activity in the NADPH-cytochrome c reductase-containing portion is not dependent upon the presence of an amphiphile or phospholipid and is not associated with O2- generation. Upon incorporation into liposomes, the cytochrome b558-containing fraction demonstrates high O2- and INT reductase activities in the presence of cytosolic factors. Both O2- generation and INT reductase activities are SDS and FAD dependent and further stimulated by GTPgammaS. Phenylarsine oxide inhibits both O2- generation and INT reductase activities when added prior to activation by SDS. With the cytochrome b-containing liposomes, the Km values (O2- formation) for NADPH and NADH are 27.2 microM and 810 microM, and for INT reductase the Km values are 27.5 microM and 1017 microM, respectively. Under anaerobic conditions and thus in the absence of O2- formation, the NADPH-dependent INT reductase activity does not change, indicating that the dye reduction is not due to its direct reduction by O2 anion but is an intrinsic property of the superoxide-generating NADPH oxidase. Cytochrome b558 is the essential component of the NADPH oxidase and contains all the redox centers necessary for electron flow between NADPH and oxygen. The correlation of the activation and inhibition patterns for O2- generation and INT reduction by cytochrome b558 incorporated into artificial liposomes strongly indicates that the two activities are associated with the same membrane protein, cytochrome b558.

Membrane association of Rac is required for high activity of the respiratory burst oxidase

NADPH-dependent superoxide generation can be reconstituted in a cell-free system using recombinant cytosolic factors (p47-phox, p67-phox, and Rac) plus flavocytochrome b558. Rac1 and Rac2 are closely related small GTPases, differing primarily in the C-terminal 10 residues where Rac1 but not Rac2 contains a polybasic sequence. In their nonisoprenylated forms, Rac1 was highly effective in reconstituting NADPH oxidase activity (low EC50, high Vmax), whereas Rac2 was only minimally effective (high EC50, low Vmax). In contrast, low concentrations of isoprenylated Rac1 and Rac2 both supported high rates of superoxide generation. Like full length Rac2, truncated forms of both Rac1 and Rac2 in which the C-terminal 10 residues were eliminated were poorly activating, pointing to the C terminus of Rac1 as a determinant of activity. Mutation of single positively charged residues in the C terminus of nonisoprenylated Rac1 markedly reduced its ability to support superoxide generation, affecting both its EC50 and the Vmax. In contrast, mutation or truncation of the C terminus failed to affect the activation of PAK, a Rac-regulated protein kinase. The EC50 for Rac1 increased with increasing salt concentrations, whereas that of Rac2 was independent of salt, implicating the involvement of electrostatic forces for the former. Using flavocytochrome b558 reconstituted into phosphatidylcholine vesicles, the EC50 for Rac1 but not Rac2 decreased (increased binding) when an acidic phospholipid (phosphatidylinositol) was present, supporting a role for the Rac1 polybasic C terminus in binding to the membrane. A model in which Rac must associate simultaneously both with p67-phox and with the membrane to activate the NADPH oxidase can account for the above observations.

Intramembrane bis-heme motif for transmembrane electron transport conserved in a yeast iron reductase and the human NADPH oxidase

A plasma membrane iron reductase, required for cellular iron acquisition by Saccharomyces cerevisiae, and the human phagocytic NADPH oxidase, implicated in cellular defense, contain low potential plasma membrane b cytochromes that share elements of structure and function. Four critical histidine residues in the FRE1 protein of the iron reductase were identified by site-directed mutagenesis. Individual mutation of each histidine to alanine eliminated the entire heme spectrum without affecting expression of the apoprotein, documenting the specificity of the requirement for the histidine residues. These critical residues are predicted to coordinate a bis-heme structure between transmembrane domains of the FRE1 protein. The histidine residues are conserved in the related gp91(phox) protein of the NADPH oxidase of human granulocytes, predicting the sites of heme coordination in that protein complex. Similarly spaced histidine residues have also been implicated in heme binding by organelle b cytochromes with little overall sequence similarity to the plasma membrane b cytochromes. This bis-heme motif may play a role in transmembrane electron transport by distinct families of polytopic b cytochromes.

Electron transfer across the O2- generating flavocytochrome b of neutrophils. Evidence for a transition from a low-spin state to a high-spin state of the heme iron component

The NADPH oxidase complex of activated neutrophils consists of a membrane-bound flavocytochrome b and cytosolic activation factors. Despite its ability to react with O2, the heme b component of the flavocytochrome is insensitive to cyanide and CO2, and slowly reactive to butyl isocyanide. We report here that arachidonic acid, an anionic amphophil which elicits oxidase activation in a cell-free system induces a transition of the heme iron of the neutrophil flavocytochrome b from a low-spin hexacoordinated state to a high-spin pentacoordinated state and promotes the binding of butyl isocyanide to the heme b. Low-temperature EPR spectra of air-oxidized flavocytochrome b either purified or in its membrane-bound form showed a low-spin signal at g = 3.26 and a high-spin signal at g = 6.0. Upon addition of arachidonic acid, the g = 3.26 signal vanished; a low-spin signal at g = 2.23 appeared, and the signal at g = 6.0 progressively increased. The subsequent addition of butyl isocyanide resulted in the decrease of the g = 6.0 and g = 2.23 signals and in the appearance of a new low-spin signal at g = 2.33. Consistent with the EPR results, upon addition of arachidonic acid to oxidized flavocytochrome b, a 2.5 nm blue shift of the Soret peak was detected in low-temperature optical spectra. The subsequent addition of butyl isocyanide resulted in the emergence of a peak at 432 nm reflecting the formation of a butyl isocyanide-oxidized heme b complex. In the case of sodium dithionite-reduced flavocytochrome b, arachidonic acid promoted the binding of butyl isocyanide to the reduced heme b, as shown by the emergence of a peak at 434 nm and the decrease of the alpha band at 558 nm. The same promoting effect was encountered with sodium dodecyl sulfate, an anionic amphophil capable of eliciting oxidase activation like arachidonic acid. In contrast to arachidonic acid, arachidonic acid methyl ester was ineffective and counteracted the effect of arachidonic acid. Butyl isocyanide added to intact neutrophils was found to bind to heme b, only after the cells have been activated. These data demonstrate the transient accumulation of a pentacoordinated form of the heme iron of flavocytochrome b under in vitro and in vivo conditions; the pentacoordinated form of the reduced heme b is postulated to react with O2 to generate the superoxide anion.

Cytochrome b-558 alpha-subunit cloning and expression in rat aortic smooth muscle cells

Recent studies have shown that the NADPH oxidase participates in the generation of superoxide anion in non-phagocytic cells. Here we report the isolation and nucleotide sequence of a cDNA for the cytochrome b-558 alpha-subunit of the NADPH oxidase in rat vascular smooth muscle cells (VSMCs). The coding region of the cDNA was 93% homologous to mouse and 81% to human in nucleotide sequence and 96% homologous to mouse and 89% to human in the deduced amino acid sequence. Our results provide a tool with which to explore the mechanism of superoxide anion generation in rat VSMCs and other non-phagocytic cells.

Cytochrome b-245 of the neutrophil superoxide-generating system contains two nonidentical hemes. Potentiometric studies of a mutant form of gp91phox

Analysis of potentiometric titrations of the cytochrome b-245 from a X+ chronic granulomatous disease patient with an Arg54 --> Ser mutation in gp91phox indicates that the mutant form of the cytochrome contains two nonidentical hemes with midpoint potentials of Em7 = -220 and Em7 = -300 mV. In the light of this information, reanalysis of redox titrations of wild-type cytochrome b-245 implies that it probably also contains two separate heme centers with midpoint potentials of Em7 = -225 and Em7 = -265 mV. The effect of the Arg54 --> Ser substitution is to reduce the midpoint potential of one of the heme centers by approximately 35 mV and suggests possible interaction between Arg54 and a heme propionate side chain.

Topological mapping of neutrophil cytochrome b epitopes with phage-display libraries

Cytochrome b of human neutrophils is the central component of the microbicidal NADPH-oxidase system. However, the folding topology of this integral membrane protein remains undetermined. Two random-sequence bacteriophage peptide libraries were used to map structural features of cytochrome b by determining the epitopes of monoclonal antibodies (mAbs) 44.1 and 54.1, specific for the p22phox and gp91phox cytochrome b chains, respectively. The unique peptides of phage selected by mAb affinity purification were deduced from the phage DNA sequences. Phage selected by mAb 44.1 displayed the consensus peptide sequence GGPQVXPI, which is nearly identical to 181GGPQVNPI18 of p22phox. Phage selected by mAb 54.1 displayed the consensus sequence PKXAVDGP, which resembles 382PKIAVDGP389 of gp91phox. Western blotting demonstrated specific binding of each mAb to the respective cytochrome b subunit and selected phage peptides. In flow cytometric analysis, mAb 44.1 bound only permeabilized neutrophils, while 54.1 did not bind intact or permeabilized cells. However, mAb 54.1 immunosedimented detergent-solubilized cytochrome b in sucrose gradients. These results suggest the 181GGPQVNPI188 segment of p22phox is accessible on its intracellular surface, but the 382PKIAVDGP389 region on gp91phox is not accessible to antibody, and probably not on the protein surface.