The superoxide-generating system of human neutrophils possesses a novel diaphorase activity. Evidence for distinct regulation of electron flow within NADPH oxidase by p67-phox and p47-phox

The protein kinase A negatively regulates reactive oxygen species production by phosphorylating gp91phox/NOX2 in human neutrophils

Superoxide anion production by neutrophils is essential for host defense against microbes. Superoxide anion generates other reactive oxygen species (ROS) that are very toxic for microbes and host cells, therefore their excessive production could induce inflammatory reactions and tissue injury. Cyclic adenosine monophosphate (cAMP) elevating agents are considered to be physiological inhibitors of superoxide production by neutrophils but the mechanisms involved in this inhibitory effect are poorly understood. Superoxide is produced by the phagocyte NADPH oxidase, a complex enzyme composed of two membrane subunits, gp91^phox or NOX2 and p22^phox, and four cytosolic components p47^phox, p67^phox, p40^phox, and Rac2. Except Rac2, these proteins are known to be phosphorylated upon neutrophil stimulation. Here we show that forskolin, an activator of the adenylate cyclase-cAMP-PKA pathway, induced phosphorylation of gp91phox/NOX2 and inhibited fMLF-induced NADPH oxidase activation in human neutrophils. H89, a PKA inhibitor prevented the forskolin-induced phosphorylation of gp91phox and restored NADPH oxidase activation. Furthermore, PKA phosphorylated the recombinant gp91phox/NOX2-cytosolic C-terminal region in vitro only on a few specific peptides containing serine residues, as compared to PKC. Interestingly, phosphorylation of NOX2-Cter by PKA alone did not induce interaction with the cytosolic components p47^phox, p67^phox and Rac2, however it induced inhibition of PKC-induced interaction. Furthermore, PKA alone did not induce NOX2 electron transfer activity, however it inhibited PKC-induced activation. These results suggest that PKA phosphorylates NOX2 in human neutrophils, a process essential to limit ROS production and inflammation under physiological conditions. Our data identify the cAMP-PKA-NOX2-axis as a critical gatekeeper of neutrophil ROS production.

Cell-Free NADPH Oxidase Activation Assays: A Triumph of Reductionism

The superoxide (O₂^·-)-generating NADPH oxidase complex of phagocytes comprises a membrane-associated heterodimeric flavocytochrome, known as cytochrome b ₅₅₈ (consisting of NOX2 and p22^phox) and four cytosolic regulatory proteins, p47^phox, p67^phox, p40^phox, and the small GTPase Rac. Under physiological conditions, in the resting phagocyte, O₂^·- generation is initiated by engagement of membrane receptors by a variety of stimuli, followed by signal transduction sequences leading to the translocation of the cytosolic components to the membrane and their association with the cytochrome, a process known as NADPH oxidase assembly. A consequent conformational change in NOX2 initiates the electron flow along a redox gradient, from NADPH to molecular oxygen (O₂), leading to the one-electron reduction of O₂ to O₂^·-. Historically, methodological difficulties in the study of the assembled complex derived from stimulated cells, due to its lack of stability, led to the design of "cell-free" systems (also known as "broken cells" or in vitro systems). In a major paradigm shift, the cell-free systems have as their starting point NADPH oxidase components derived from resting (unstimulated) phagocytes, or as in the predominant method at present, recombinant proteins representing the components of the NADPH oxidase complex. In cell-free systems, membrane receptor stimulation and the signal transduction sequence are absent, the accent being placed on the actual process of assembly, all of which takes place in vitro. Thus, a mixture of the individual components of the NADPH oxidase is exposed in vitro to an activating agent, the most common being anionic amphiphiles, resulting in the formation of a complex between cytochrome b ₅₅₈ and the cytosolic components and O₂^·- generation in the presence of NADPH. Alternative activating pathways require posttranslational modification of oxidase components or modifying the phospholipid milieu surrounding cytochrome b ₅₅₈. Activation is commonly quantified by measuring the primary product of the reaction, O₂^·-, trapped immediately after its generation by an appropriate acceptor in a kinetic assay, permitting the calculation of rates of O₂^·- production, but numerous variations exist, based on the assessment of reaction products or the consumption of substrates. Cell-free assays played a paramount role in the identification and characterization of the components of the NADPH oxidase complex, the performance of structure-function studies, the deciphering of the mechanisms of assembly, the search for inhibitory drugs, and the diagnosis of various forms of chronic granulomatous disease (CGD).

Down-regulation of NOX2 activity in phagocytes mediated by ATM-kinase dependent phosphorylation

NADPH oxidases (NOX) have many biological roles, but their regulation to control production of potentially toxic ROS molecules remains unclear. A previously identified insertion sequence of 21 residues (called NIS) influences NOX activity, and its predicted flexibility makes it a good candidate for providing a dynamic switch controlling the NOX active site. We constructed NOX2 chimeras in which NIS had been deleted or exchanged with those from other NOXs (NIS1, 3 and 4). All contained functional heme and were expressed normally at the plasma membrane of differentiated PLB-985 cells. However, NOX2-ΔNIS and NOX2-NIS1 had neither NADPH-oxidase nor reductase activity and exhibited abnormal translocation of p47^phox and p67^phox to the phagosomal membrane. This suggested a functional role of NIS. Interestingly after activation, NOX2-NIS3 cells exhibited superoxide overproduction compared with wild-type cells. Paradoxically, the V_max of purified unstimulated NOX2-NIS3 was only one-third of that of WT-NOX2. We therefore hypothesized that post-translational events regulate NOX2 activity and differ between NOX2-NIS3 and WT-NOX2. We demonstrated that Ser486, a phosphorylation target of ataxia telangiectasia mutated kinase (ATM kinase) located in the NIS of NOX2 (NOX2-NIS), was phosphorylated in purified cytochrome b₅₅₈ after stimulation with phorbol 12-myristate-13-acetate (PMA). Moreover, ATM kinase inhibition and a NOX2 Ser486Ala mutation enhanced NOX activity whereas a Ser486Glu mutation inhibited it. Thus, the absence of Ser486 in NIS3 could explain the superoxide overproduction in the NOX2-NIS3 mutant. These results suggest that PMA-stimulated NOX2-NIS phosphorylation by ATM kinase causes a dynamic switch that deactivates NOX2 activity. We hypothesize that this downregulation is defective in NOX2-NIS3 mutant because of the absence of Ser486.

Cell-free NADPH oxidase activation assays: "in vitro veritas"

The superoxide (O2 (∙-))-generating NADPH oxidase complex of phagocytes comprises a membrane-imbedded heterodimeric flavocytochrome, known as cytochrome b 558 (consisting of Nox2 and p22 (phox) ) and four cytosolic regulatory proteins, p47 (phox) , p67 (phox) , p40 (phox) , and the small GTPase Rac. Under physiological conditions, in the resting phagocyte, O2 (∙-) generation is initiated by engagement of membrane receptors by a variety of stimuli, followed by specific signal transduction sequences leading to the translocation of the cytosolic components to the membrane and their association with the cytochrome. A consequent conformational change in Nox2 initiates the electron "flow" along a redox gradient, from NADPH to oxygen, leading to the one-electron reduction of molecular oxygen to O2 (∙-). Methodological difficulties in the dissection of this complex mechanism led to the design "cell-free" systems (also known as "broken cells" or in vitro systems). In these, membrane receptor stimulation and all or part of the signal transduction sequence are missing, the accent being placed on the actual process of "NADPH oxidase assembly," thus on the formation of the complex between cytochrome b 558 and the cytosolic components and the resulting O2 (∙-) generation. Cell-free assays consist of a mixture of the individual components of the NADPH oxidase complex, derived from resting phagocytes or in the form of purified recombinant proteins, exposed in vitro to an activating agent (distinct from and unrelated to whole cell stimulants), in the presence of NADPH and oxygen. Activation is commonly quantified by measuring the primary product of the reaction, O2 (∙-), trapped immediately after its generation by an appropriate acceptor in a kinetic assay, permitting the calculation of the linear rate of O2 (∙-) production, but numerous variations exist, based on the assessment of reaction products or the consumption of substrates. Cell-free assays played a paramount role in the identification and characterization of the components of the NADPH oxidase complex, the deciphering of the mechanisms of assembly, the search for inhibitory drugs, and the diagnosis of various forms of chronic granulomatous disease (CGD).

Inhibition of NADPH oxidase activation by peptides mapping within the dehydrogenase region of Nox2-A "peptide walking" study

In this study, the "peptide walking" approach was applied to the DH region of Nox2 (residues 288-570) with the purpose of identifying domains of functional importance in the assembly and/or catalytic function of the NADPH oxidase complex of phagocytes. Ninety-one overlapping 15-mer peptides were synthesized to cover the full length of the Nox2 DH region, and these were tested for the ability to interfere with the activation of the oxidase in vitro in two semi-recombinant cell-free systems. The first consisted of phagocyte membranes p47(phox), p67(phox), and Rac1 and an amphiphile; the second was p47(phox)- and amphiphile-free and contained prenylated Rac1. We identified 10 clusters of inhibitory peptides with IC(50) values of 10 μM, all of which were inhibitory, also in the absence of p47(phox). Based on the identification of residues shared by peptides in a particular cluster, we defined 10 functional domains in the Nox2 DH region. One domain corresponded to one FAD-binding subdomain, and four domains overlapped parts of three NADPH-binding subdomains. As expected, most inhibitory peptides acted only when added prior to the completion of oxidase assembly, but peptides associated with two NADPH-binding subdomains were also active after assembly. Kinetic analysis demonstrated that inhibition by peptides was not explained by competition for substrates (FAD, NADPH) but was of a more complex nature: noncompetitive with respect to FAD and uncompetitive with respect to NADPH. We conclude that oxidase-inhibitory peptides, in five out of 10 clusters identified, act by interfering with FAD- and NADPH-related redox reactions.

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.

Intracellular generation of superoxide by the phagocyte NADPH oxidase: how, where, and what for?

Professional phagocytes increase their consumption of molecular oxygen during the phagocytosis of microbes or when encountering a variety of nonparticulate stimuli. In these circumstances, oxygen is reduced by the phagocyte NADPH oxidase, and reactive oxygen species (ROS), which are important for the microbicidal activity of the cells, are generated. The structure and function of the NADPH oxidase have been resolved in part by studying cells from patients with chronic granulomatous disease (CGD), a condition characterized by the inability of phagocytes to assemble a functional NADPH oxidase and thus to produce ROS. As a result, patients with CGD have a predisposition to infections as well as a variety of inflammatory symptoms. A long-standing paradigm has been that NADPH oxidase assembly occurs exclusively in the plasma membrane or invaginations thereof (phagosomes). A growing body of evidence points to the possibility that phagocytes are capable of NADPH oxidase assembly in nonphagosomal intracellular membranes, resulting in ROS generation within intracellular organelles also in the absence of phagocytosis. The exact nature of these ROS-producing organelles is yet to be determined, but granules are prime suspects. Recent clinical findings indicate that the generation of intracellular ROS by NADPH oxidase activation is important for limiting inflammatory reactions and that intracellular and extracellular ROS production are regulated differently. Here we discuss the accumulating knowledge of intracellular ROS production in phagocytes and speculate on the precise role of these oxidants in regulating the inflammatory process.

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.

Characterization of superoxide overproduction by the D-Loop(Nox4)-Nox2 cytochrome b(558) in phagocytes-Differential sensitivity to calcium and phosphorylation events

NADPH oxidase is a crucial element of phagocytes involved in microbicidal mechanisms. It becomes active when membrane-bound cytochrome b(558), the redox core, is assembled with cytosolic p47(phox), p67(phox), p40(phox), and rac proteins to produce superoxide, the precursor for generation of toxic reactive oxygen species. In a previous study, we demonstrated that the potential second intracellular loop of Nox2 was essential to maintaining NADPH oxidase activity by controlling electron transfer from FAD to O(2). Moreover, replacement of this loop by the Nox4-D-loop (D-loop(Nox4)-Nox2) in PLB-985 cells induced superoxide overproduction. In the present investigation, we demonstrated that both soluble and particulate stimuli were able to induce this superoxide overproduction. Superoxide overproduction was also observed after phosphatidic acid activation in a purified cell-free-system assay. The highest oxidase activity was obtained after ionomycin and fMLF stimulation. In addition, enhanced sensitivity to Ca(2+) influx was shown by thapsigargin, EDTA, or BTP2 treatment before fMLF activation. Mutated cytochrome b(558) was less dependent on phosphorylation triggered by ERK1/2 during fMLF or PMA stimulation and by PI3K during OpZ stimulation. The superoxide overproduction of the D-loop(Nox4)-Nox2 mutant may come from a change of responsiveness to intracellular Ca(2+) level and to phosphorylation events during oxidase activation. Finally the D-loop(Nox4)-Nox2-PLB-985 cells were more effective against an attenuated strain of Pseudomonas aeruginosa compared to WT-Nox2 cells. The killing mechanism was biphasic, an early step of ROS production that was directly bactericidal, and a second oxidase-independent step related to the amount of ROS produced in the first step.

Constitutive NADPH-dependent electron transferase activity of the Nox4 dehydrogenase domain

NADPH oxidase 4 (Nox4) is constitutively active, while Nox2 requires the cytosolic regulatory subunits p47(phox) and p67(phox) and activated Rac with activation by phorbol 12-myristate 13-acetate (PMA). This study was undertaken to identify the domain on Nox4 that confers constitutive activity. Lysates from Nox4-expressing cells exhibited constitutive NADPH- but not NADH-dependent hydrogen peroxide production with a K(m) for NADPH of 55 +/- 10 microM. The concentration of Nox4 in cell lysates was estimated using Western blotting and allowed calculation of a turnover of approximately 200 mol of H(2)O(2) min(-1) (mol of Nox4)(-1). A chimeric protein (Nox2/4) consisting of the Nox2 transmembrane (TM) domain and the Nox4 dehydrogenase (DH) domain showed H(2)O(2) production in the absence of cytosolic regulatory subunits. In contrast, chimera Nox4/2, consisting of the Nox4 TM and Nox2 DH domains, exhibited PMA-dependent activation that required coexpression of regulatory subunits. Nox DH domains from several Nox isoforms were purified and evaluated for their electron transferase activities. Nox1 DH, Nox2 DH, and Nox5 DH domains exhibited barely detectable activities toward artificial electron acceptors, while the Nox4 DH domain exhibited significant rates of reduction of cytochrome c (160 min(-1), largely superoxide dismutase-independent), ferricyanide (470 min(-1)), and other electron acceptors (artificial dyes and cytochrome b(5)). Rates were similar to those observed for H(2)O(2) production by the Nox4 holoenzyme in cell lysates. The activity required added FAD and was seen with NADPH but not NADH. These results indicate that the Nox4 DH domain exists in an intrinsically activated state and that electron transfer from NADPH to FAD is likely to be rate-limiting in the NADPH-dependent reduction of oxygen by holo-Nox4.

Regulation of the phagocyte NADPH oxidase activity: phosphorylation of gp91phox/NOX2 by protein kinase C enhances its diaphorase activity and binding to Rac2, p67phox, and p47phox

Neutrophils generate microbicidal oxidants through activation of a multicomponent enzyme called NADPH oxidase. During activation, the cytosolic NADPH oxidase components (p47(phox), p67(phox), p40(phox), and Rac2) translocate to the membranes, where they associate with flavocytochrome b(558), which is composed of gp91(phox)/NOX2 and p22(phox), to form the active system. During neutrophil stimulation, p47(phox), p67(phox), p40(phox), and p22(phox) are phosphorylated; however, the phosphorylation of gp91(phox)/NOX2 and its potential role have not been defined. In this study, we show that gp91(phox) is phosphorylated in stimulated neutrophils. The gp91(phox) phosphoprotein is absent in neutrophils from chronic granulomatous disease patients deficient in gp91(phox), which confirms that this phosphoprotein is gp91(phox). The protein kinase C inhibitor GF109203X inhibited phorbol 12-myristate 13-acetate-induced phosphorylation of gp91(phox), and protein kinase C (PKC) phosphorylated the recombinant gp91(phox)- cytosolic carboxy-terminal flavoprotein domain. Two-dimensional tryptic peptide mapping analysis showed that PKC phosphorylated the gp91(phox)-cytosolic tail on the same peptides that were phosphorylated on gp91(phox) in intact cells. In addition, PKC phosphorylation increased diaphorase activity of the gp91(phox) flavoprotein cytosolic domain and its binding to Rac2, p67(phox), and p47(phox). These results demonstrate that gp91(phox) is phosphorylated in human neutrophils by PKC to enhance its catalytic activity and assembly of the complex. Phosphorylation of gp91(phox)/NOX2 is a novel mechanism of NADPH oxidase regulation.

Increased myeloperoxidase activity and protein nitration are indicators of inflammation in patients with Chagas' disease

In this study, we investigated whether inflammatory responses contribute to oxidative/nitrosative stress in patients with Chagas' disease. We used three tests (enzyme-linked immunosorbent assay, immuno-flow cytometry, and STAT-PAK immunochromatography) to screen human serum samples (n = 1,481) originating from Chiapas, Mexico, for Trypanosoma cruzi-specific antibodies. We identified 121 subjects who were seropositive for T. cruzi-specific antibodies, a finding indicative of an 8.5% seroprevalence in the rural population from Chiapas. Seropositive and seronegative subjects were examined for plasma levels of biomarkers of inflammation, i.e., myeloperoxidase (MPO), inducible nitric oxide synthase (iNOS), and xanthine oxidase (XOD), as well as for oxidative (advanced oxidation protein products [AOPPs]) and nitrosative (3-nitrotyrosine [3NT]) biomarkers. The seropositive subjects exhibited a significant increase in MPO activity and protein level, the indicator of neutrophil activation. Subsequently, a corresponding increase in AOPP contents, formed by MPO-dependent hypochlorous acid and chloramine formation, was noted in seropositive subjects. The plasma level of 3NT was significantly increased in seropositive subjects, yet we observed no change in XOD activity (O(2)(-) source) and nitrate/nitrite contents (denotes iNOS activation and NO production), which implied that direct peroxynitrite formation does not contribute to increased nitrosative damage in chagasic subjects. Instead, a positive correlation between increased MPO activity and protein 3NT formation was observed, which suggested to us that MPO-dependent formation of nitrylchloride that occurs in the presence of physiological NO and O(2)(-) concentrations contributes to protein nitration. Overall, our data demonstrate that T. cruzi-induced neutrophil activation is pathological and contributes to MPO-mediated collateral protein oxidative and nitrosative damage in human patients with Chagas' disease. Therapies capable of suppressing MPO activity may be useful in controlling the inflammation and oxidative/nitrosative pathology in chagasic cardiomyopathy.

A fluorescently tagged C-terminal fragment of p47phox detects NADPH oxidase dynamics during phagocytosis

The assembly of cytosolic p47(phox) and p67(phox) with flavocytochrome b(558) at the membrane is crucial for activating the leukocyte NADPH oxidase that generates superoxide for microbial killing. p47(phox) and p67(phox) are linked via a high-affinity, tail-to-tail interaction involving a proline-rich region (PRR) and a C-terminal SH3 domain (SH3b), respectively, in their C-termini. This interaction mediates p67(phox) translocation in neutrophils, but is not required for oxidase activity in model systems. Here we examined phagocytosis-induced NADPH oxidase assembly, showing the sequential recruitment of YFP-tagged p67(phox) to the phagosomal cup, and, after phagosome internalization, a probe for PI(3)P followed by a YFP-tagged fragment derived from the p47(phox) PRR. This fragment was recruited in a flavocytochrome b(558)-dependent, p67(phox)-specific, and PI(3)P-independent manner. These findings indicate that p47PRR fragment probes the status of the p67(phox) SH3b domain and suggest that the p47(phox)/p67(phox) tail-to-tail interaction is disrupted after oxidase assembly such that the p67(phox)-SH3b domain becomes accessible. Superoxide generation was sustained within phagosomes, indicating that this change does not correlate with loss of enzyme activity. This study defines a sequence of events during phagocytosis-induced NADPH oxidase assembly and provides experimental evidence that intermolecular interactions within this complex are dynamic and modulated after assembly on phagosomes.

Enhanced nitrosative stress during Trypanosoma cruzi infection causes nitrotyrosine modification of host proteins: implications in Chagas' disease

Oxidative/nitrosative stress may be important in the pathology of Chagas' disease. Experimental animals infected by Trypanosoma cruzi showed an early rise in myocardial and peripheral protein-3-nitrotyrosine (3NT) and protein-carbonyl formation that persisted during the chronic stage of disease. In comparison, experimental chronic ethanol-induced cardiomyopathy was slow to develop and presented with a moderate increase in oxidative stress and minimal to no nitrosative stress after long-term alcohol feeding of animals. The oxidative stress in both chagasic animals and animals with ethanol-induced cardiomyopathy correlated with the persistence of reactive oxygen species-producing inflammatory intermediates. Protein-3NT formation in T. cruzi-infected animals was associated with enhanced nitric oxide expression (inferred by nitrite/nitrate levels) and myeloperoxidase activity, suggesting that both peroxynitrite- and myeloperoxidase-mediated pathways contribute to increased protein nitration in Chagas' disease. We used one- and two-dimensional gel electrophoresis and Western blot analysis to identify disease-specific plasma proteins that were 3NT-modified in T. cruzi-infected animals. Nitrated protein spots (56 in total) were sequenced by matrix-assisted laser desorption ionization/time of flight mass spectrometry and liquid chromatography-tandem mass spectrometry and identified by a homology search of public databases. Clustering of 3NT-modified proteins according to their functional characteristics revealed that the nitration of immunoglobulins, apolipoprotein isoforms, and other proteins might perturb their functions and be important in the pathology of Chagas' disease. We also showed that nitrated peptides derived from titin and alpha-actin were released into the plasma of patients with Chagas' disease. Such modified proteins may be useful biomarkers of Chagas' disease.

Hydrogen sulfide enhances reducing activity in neurons: neurotrophic role of H2S in the brain?

Hydrogen sulfide (H2S) can enzymatically be produced from cysteine in the brain. H2S functions as a synaptic modulator as well as a neuroprotectant from oxidative stress in the brain. Here we show that H2S specifically enhances the reducing activity in neurons and mouse neuroblastoma Neuro2a cells. An inhibitor of protein tyrosine kinase, genistein, suppresses the effect of H2S, suggesting that tyrosine kinase may be involved in the enhancement of reducing activity by H2S. The H2S-specific enhancement of the reducing activity in neurons may lead to a neurotrophic role in the brain.

Liposome-mediated cellular delivery of active gp91(phox)

Background

Gp91^phox is a transmembrane protein and the catalytic core of the NADPH oxidase complex of neutrophils. Lack of this protein causes chronic granulomatous disease (CGD), a rare genetic disorder characterized by severe and recurrent infections due to the incapacity of phagocytes to kill microorganisms.

Methodology

Here we optimize a prokaryotic cell-free expression system to produce integral mammalian membrane proteins.

Conclusions

Using this system, we over-express truncated forms of the gp91^phox protein under soluble form in the presence of detergents or lipids resulting in active proteins with a “native-like” conformation. All the proteins exhibit diaphorase activity in the presence of cytosolic factors (p67^phox, p47^phox, p40^phox and Rac) and arachidonic acid. We also produce proteoliposomes containing gp91^phox protein and demonstrate that these proteins exhibit activities similar to their cellular counterpart. The proteoliposomes induce rapid cellular delivery and relocation of recombinant gp91^phox proteins to the plasma membrane. Our data support the concept of cell-free expression technology for producing recombinant proteoliposomes and their use for functional and structural studies or protein therapy by complementing deficient cells in gp91^phox protein.

An overview of chagasic cardiomyopathy: pathogenic importance of oxidative stress

There is growing evidence to suggest that chagasic myocardia are exposed to sustained oxidative stress-induced injuries that may contribute to disease progression. Pathogen invasion- and replication-mediated cellular injuries and immune-mediated cytotoxic reactions are the common source of reactive oxygen species (ROS) in infectious etiologies. However, our understanding of the source and role of oxidative stress in chagasic cardiomyopathy (CCM) remains incomplete. In this review, we discuss the evidence for increased oxidative stress in chagasic disease, with emphasis on mitochondrial abnormalities, electron transport chain dysfunction and its role in sustaining oxidative stress in myocardium. We discuss the literature reporting the consequences of sustained oxidative stress in CCM pathogenesis.

Crucial role of two potential cytosolic regions of Nox2, 191TSSTKTIRRS200 and 484DESQANHFAVHHDEEKD500, on NADPH oxidase activation

Assembly of cytosolic factors p67(phox) and p47(phox) with cytochrome b(558) is one of the crucial keys for NADPH oxidase activation. Certain sequences of Nox2 appear to be involved in cytosolic factor interaction. The role of the D-loop (191)TSSTKTIRRS(200) and the C-terminal (484)DESQANHFAVHHDEEKD(500) of Nox2 on oxidase activity and assembly was investigated. Charged amino acids were mutated to neutral or reverse charge by directed mutagenesis to generate 21 mutants. Recombinant wild-type or mutant Nox2 were expressed in the X-CGD PLB-985 cell model. K195A/E, R198E, R199E, and RR198199QQ/AA mutations in the D-loop of Nox2 totally abolished oxidase activity. However, these D-loop mutants demonstrated normal p47(phox) translocation and iodonitrotetrazolium (INT) reductase activity, suggesting that charged amino acids of this region are essential for electron transfer from FAD to oxygen. Replacement of Nox2 D-loop with its homolog of Nox1, Nox3, or Nox4 was fully functional. In addition, fMLP (formylmethionylleucylphenylalanine)-activated R199Q-Nox2 and D-loop(Nox4)-Nox2 mutants exhibited four to eight times the NADPH oxidase activity of control cells, suggesting that these mutations lead to a more efficient oxidase activation process. In contrast, the D484T and D500A/R/G mutants of the alpha-helical loop of Nox2 exhibited no NADPH oxidase and INT reductase activities associated with a defective p47(phox) membrane translocation. This suggests that the alpha-helical loop of the C-terminal of Nox2 is probably involved in the correct assembly of the NADPH oxidase complex occurring during activation, permitting cytosolic factor translocation and electron transfer from NADPH to FAD.

Activation of the flavoprotein domain of gp91phox upon interaction with N-terminal p67phox (1-210) and the Rac complex

A series of truncated forms of His(6)-tagged gp91phox were expressed, solubilized, and purified in the presence of 30 microM FAD. The truncated gp91phox with the longest sequence in the C-terminal region (221-570) (gp91C) showed the highest activity (turnover rate, 0.92) for NADPH diaphorase in the presence of either 0.3% Triton X-100 or 0.5% Genapol X-80. Activity was not inhibited by superoxide dismutase but was blocked by an inhibitor of the respiratory burst oxidase, diphenylene iodonium. The flavinated gp91C contained approximately 0.9 mol of FAD/mol of protein (MW 46 kDa) and 12% alpha-helix content. In the absence of p47phox, p67phox showed considerable activation of gp91C in the presence of Rac. Carboxyl-terminal truncated p67phox (1-210) (p67N), which is the minimal active fragment, was fused with Rac or Q61LRac. The fusion protein p67N-Rac (or p67N-Q61LRac) showed a 2-fold higher stimulatory effect on NBT reductase activity of gp91C than the combination of the individual cytosolic p67N and Rac proteins. In contrast, Rac-p67N, a fusion with the opposite orientation, showed a smaller significant effect on the enzyme activity. The EC(50) values for p67phox, p67N, p67N-Rac, and Rac-p67N were 8.00. 4.35, 2.56, and 15.2 microM, respectively, while the K(m) value for NADPH in the presence and absence of the cytosolic components was almost the same (40-55 microM). In the presence of Rac, p67N or p67phox bound to gp91C with a molar ratio of approximately 1:1 but neither p67N nor Rac alone showed significant binding.

Inhibition of the yeast metal reductase heme protein fre1 by nitric oxide (NO): a model for inhibition of NADPH oxidase by NO

Nitric oxide (NO) has been found to inhibit the actions of the transmembrane metal reductase Fre1 in the yeast Saccharomyces cerevisiae. This membrane-spanning heme protein is homologous to the gp91(PHOX) protein of the NADPH oxidase enzyme complex and is responsible for reducing extracellular oxidized metals (i.e., ferric and cupric ions) before high-affinity uptake. Consistent with its role in metal metabolism, inhibition of Fre1 by NO also inhibited yeast growth in low-iron medium. Inhibition by NO was found to be O(2)-dependent and irreversible. Further examination of the chemistry responsible for activity loss shows that the generation of N(2)O(3) via NO-O(2) chemistry was responsible for the activity loss, possibly via nitrosation of the protein followed by loss of the heme prosthetic group.

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.

Functional epitope on human neutrophil flavocytochrome b558

mAb NL7 was raised against purified flavocytochrome b(558), important in host defense and inflammation. NL7 recognized the gp91(phox) flavocytochrome b(558) subunit by immunoblot and bound to permeabilized neutrophils and neutrophil membranes. Epitope mapping by phage display analysis indicated that NL7 binds the (498)EKDVITGLK(506) region of gp91(phox). In a cell-free assay, NL7 inhibited in vitro activation of the NADPH oxidase in a concentration-dependent manner, and had marginal effects on the oxidase substrate Michaelis constant (K(m)). mAb NL7 did not inhibit translocation of p47(phox), p67(phox), or Rac to the plasma membrane, and bound its epitope on gp91(phox) independently of cytosolic factor translocation. However, after assembly of the NADPH oxidase complex, mAb NL7 bound the epitope but did not inhibit the generation of superoxide. Three-dimensional modeling of the C-terminal domain of gp91(phox) on a corn nitrate reductase template suggests close proximity of the NL7 epitope to the proposed NADPH binding site, but significant separation from the proposed p47(phox) binding sites. We conclude that the (498)EKDVITGLK(506) segment resides on the cytosolic surface of gp91(phox) and represents a region important for oxidase function, but not substrate or cytosolic component binding.

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.

Exploration of the diaphorase activity of neutrophil NADPH oxidase

In the O2- generating flavocytochrome b, the membrane-bound component of the neutrophil NADPH oxidase, electrons are transported from NADPH to O2 in the following sequence: NADPH --> FAD --> heme b -->O2. Although p-iodonitrotetrazolium (INT) has frequently been used as a probe of the diaphorase activity of the neutrophil flavocytochrome b, the propensity of its radical to interact reversibly with O2 led us to question its specificity. This study was undertaken to reexamine the interaction of INT with the redox components of the neutrophil flavocytochrome b. Two series of inhibitors were used, namely the flavin analog 5-deaza FAD and the heme inhibitors bipyridyl and benzylimidazole. The following results indicate that INT reacts preferentially with the hemes rather than with the FAD redox center of flavocytochrome b and is not therefore a specific probe of the diaphorase activity of flavocytochrome b. First, in anaerobiosis, reduced heme b in activated membranes was reoxidized by INT as efficiently as by O2 even in the presence of concentrations of 5-deaza FAD which fully inhibited the NADPH oxidase activity. Second, the titration curve of dithionite-reduced heme b in neutrophil membranes obtained by oxidation with increasing amounts of INT was strictly superimposable on that of dithionite-reduced hemin. Third, INT competitively inhibited the O2 uptake by the activated NADPH oxidase in a cell-free system. Finally, the heme inhibitor bipyridyl competitively inhibited the reduction of INT in anaerobiosis, and the oxygen uptake in aerobiosis.

Essential requirement of cytosolic phospholipase A(2) for stimulation of NADPH oxidase-associated diaphorase activity in granulocyte-like cells

We have previously established a model of cytosolic phospholipase A(2) (cPLA(2))-deficient differentiated PLB-985 cells (PLB-D cells) and demonstrated that cPLA(2)-generated arachidonic acid (AA) is essential for NADPH oxidase activation. In this study we used this model to investigate the physiological role of cPLA(2) in regulation of NADPH oxidase-associated diaphorase activity. A novel diaphorase activity assay, using 4-iodonitrotetrazolium violet as an electron acceptor, was used in permeabilized neutrophils and PLB-985 cells differentiated toward the granulocytic or monocytic phenotypes. Phorbol 12-myristate 13-acetate, guanosine 5'-3-O- (thio)triphosphate (GTP gamma S), or FMLP stimulated a similar diphenylene iodonium-sensitive diaphorase activity pattern in neutrophils and in differentiated parent PLB-985 cells. This diaphorase activity was not detected in undifferentiated cells, but developed during differentiation. Furthermore, diaphorase activity could not be stimulated in permeabilized neutrophils from X-linked CGD patients and in differentiated gp91(phox)-targeted PLB-985 cells that lacked normal expression of gp91(phox), but was restored to these cells following transduction with retrovirus encoding gp91(phox). The differentiated PLB-D cells showed no diaphorase activity when stimulated by either GTP gamma S or FMLP, and only partial activation when stimulated with phorbol 12-myristate 13-acetate. Diaphorase activity in response to either agonists was fully restored by the addition of 10 microm free AA. The permeabilized cell 4-iodonitrotetrazolium violet reduction assay offers a unique tool for the evaluation of NADPH oxidase-associated diaphorase activity in stimulated whole cells. These results establish an essential and specific physiological requirement of cPLA(2)-generated AA in activation of electron transfer through the FAD reduction center of NADPH oxidase.

Molecular basis for Rac2 regulation of phagocyte NADPH oxidase

A Rac GTPase-regulated multiprotein NADPH oxidase is critical for the formation of reactive oxygen species (ROS) in phagocytic leukocytes and other nonphagocytic cells. NADPH oxidase reduces molecular oxygen to form superoxide anion in a two-step process. Electrons are initially transferred from NADPH to cytochrome b-associated FAD, then to cytochrome b heme and finally to molecular oxygen. We show here that Rac is required for both electron-transfer reactions. Mutational and biophysical analysis shows that Rac and p67phox independently regulate cytochrome b to catalyze the transfer of electrons from NADPH to FAD. However, they must interact with each other to induce the subsequent transfer of electrons from FAD to cytochrome b heme and molecular oxygen. This two-step model of regulation by Rac GTPase may provide a means of more effectively controlling the inflammatory responses of phagocytic leukocytes.

The NADPH oxidase diaphorase activity in permeabilized human neutrophils and granulocytic like PLB-985 cells

The phagocyte NADPH oxidase is a multicomponent transport chain that generates superoxide, a precursor of microbicidal oxidants, important for host defense. This transport chain is contained mainly in the large membrane subunit of the oxidase (gp91phox), and transfers electrons from cytosolic NADPH, through FAD binding and heme centers, to molecular oxygen (Babior, 1999; Fujii and Kakinuma, 1991; Rotrosen et al., 1992; Segal and Abo, 1993). Cross et al. have recently described a novel NADPH oxidase diaphorase activity present in the membrane fraction of activated neutrophils, using a cell free model (Cross et al., 1994). This diaphorase activity is measured by the artificial electron acceptor 4-iodonitrotetrazolium violet (INT) and is attributed to the reduction of the flavin center of the flavocytochrome (Cross et al., 1994; Li and Guillory, 1997). In the present study we establish a system for detecting diaphorase activity in intact cells. Neutrophils and PLB-985 cells, that were differentiated using 1.25% dimethyl sulfoxide (DMSO) to granulocyte phenotype, were permeabilized by electroporation, and diaphorase activity was determined using INT. Neutrophils and differentiated PLB-985 cells stimulated by PMA or GTP gamma S showed a diaphorase activity that was not present in unstimulated differentiated cells. The diaphorase activity could not be detected in undifferentiated cells and was developed during differentiation. The pattern of diaphorase activity in stimulated parent differentiated PLB cells was similar to that observed in stimulated human neutrophils. The permeabilized-INT cell system offers a unique tool for the evaluation of NADPH oxidase diaphorase activity, in whole cells.

Fungal gliotoxin targets the onset of superoxide-generating NADPH oxidase of human neutrophils

Gliotoxin from Aspergillus, bearing a S&bond;S bond in its structure, prevented the onset of O(-)(2) generation by the human neutrophil NADPH oxidase in response to phorbol myristate acetate (PMA). Gliotoxin affected the activation process harder than the activated oxidase, as shown by its stronger inhibition when added to neutrophils prior to, than post-PMA at maximum enzyme turnover. Decreased O(-)(2) generation persisted even if cells treated with gliotoxin were subsequently washed, with half-inhibition concentrations (IC(50)) of 5.3, and 3.5 microM for treatments of 15 and 30 min, respectively. In addition, gliotoxin made neutrophils reduce cytochrome c regardless of absence of PMA, through its reaction with intracellular reductants in an oxygen-dependent process, named redox cycling. Thus, we next tested whether preincubation of neutrophils with gliotoxin under hypoxic conditions would relieve the inhibition of NADPH oxidase. Instead, this prevention of redox cycling significantly favored damage to the NADPH oxidase with an IC(50) of 0.009 microM. Moreover, conversion of gliotoxin to its dithiol derivative by addition of reduced dithiothreitol during incubation protected cells from losing oxidase activity. These findings support that the disulfide form of gliotoxin targets NADPH oxidase activation.

Functional Analysis of NADPH Oxidase in Granulocytic Cells Expressing a ▵488-497 gp91phox Deletion Mutant

Chronic granulomatous disease (CGD) is a group of inherited disorders in which phagocytes are unable to generate superoxide (O2−) due to genetic defects in any 1 of 4 essential NADPH oxidase components. Mutations in the X-linked gene for gp91phox, the large subunit of the flavocytochromeb558 heterodimer, account for the majority of CGD. An X-CGD patient in which a splice junction mutation results in an in-frame deletion of 30 nucleotides encoding amino acids 488 to 497 of gp91phox (▵488-497 gp91phox) has previously been reported. In this study, we generated myeloid PLB-985 cells expressing the mutant ▵488-497 gp91phox to further characterize its functional properties. These cells mimicked the phenotype of the patient’s neutrophils with normal expression of a nonfunctional ▵488-497 gp91phox flavocytochrome. Translocation of p47phox and p67phox to ▵488-497 gp91phox PLB-985 plasma membranes was not affected, as determined both in activated intact cells and in the cell-free system. Furthermore, a synthetic peptide corresponding to residues 488-497 of gp91phox was relatively ineffective in inhibiting O2− production in the cell-free oxidase assay (IC50, ∼500 μmol/L), suggesting that residues 488-497 of gp91phox are not directly involved in oxidase assembly. Mutant ▵488-497 gp91phox flavocytochrome failed to support iodonitrotetrazolium (INT) reduction, showing a disruption of electron transfer from NADPH to the FAD center of gp91phox. However, the FAD binding capacity of the mutant flavocytochrome was normal, as measured by equilibrium dialysis. Taken together, these results suggest that the ▵488-497 deletion in gp91phox disrupts electron transfer to FAD, either due to a defect in NADPH binding or to impaired delivery of electrons from NADPH.

A novel protein kinase target for the lipid second messenger phosphatidic acid

Activation of phospholipase D occurs in response to a wide variety of hormones, growth factors, and other extracellular signals. The initial product of phospholipase D, phosphatidic acid (PA), is thought to serve a signaling function, but the intracellular targets for this lipid second messenger are not clearly identified. The production of PA in human neutrophils is closely correlated with the activation of NADPH oxidase, the enzyme responsible for the respiratory burst. We have developed a cell-free system, in which the activation of NADPH oxidase is induced by the addition of PA. Characterization of this system revealed that a multi-functional cytosolic protein kinase was a target for PA, and that two NADPH oxidase components were substrates for the enzyme. Partial purification of the PA-activated protein kinase separated the enzyme from known protein kinase targets of PA. The partially purified enzyme was selectively activated by PA, compared to other phospholipids, and phosphorylated the oxidase component p47-phox on both serine and tyrosine residues. PA-activated protein kinase activity was present in a variety of hematopoietic cells and cell lines and in rat brain, suggesting it has widespread distribution. We conclude that this protein kinase may be a novel target for the second messenger function of PA.

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.

NADPH dehydrogenase activity of p67PHOX, a cytosolic subunit of the leukocyte NADPH oxidase

The leukocyte NADPH oxidase catalyzes the one-electron reduction of oxygen to O2- at the expense of NADPH. It is a multicomponent enzyme comprising a membrane-bound flavocytochrome (cytochrome b558) and at least four cytosolic components: p47PHOX, p67PHOX, p40PHOX, and Rac, a small GTPase. All the oxidase components except p40PHOX are required for enzyme activity. Many aspects of their function, however, are unclear. Using the electron acceptor ferricyanide, we found that recombinant p67PHOX from baculovirus-infected Sf9 cells could mediate the dehydrogenation of NADPH. NADPH dehydrogenation was not dependent on FAD and was insensitive to superoxide dismutase. Several control experiments showed that NADPH dehydrogenation was accomplished by p67PHOX, not by a trace contaminant in the p67PHOX preparation. The NADPH dehydrogenase activity of p67PHOX was proportional to enzyme concentration, and showed saturation kinetics with NADPH (Km 92 +/- 5 microM), but was inhibited at high concentrations of ferricyanide. NADH was also used as a substrate by p67PHOX (Km 123 +/- 38 microM). Taken together, these results show that p67PHOX is able to mediate pyridine nucleotide dehydrogenation. These findings raise the possibility that p67PHOX might participate directly in electron transfer between NADPH and the oxidase flavin.

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.

Metalloregulation of FRE1 and FRE2 homologs in Saccharomyces cerevisiae

The high affinity uptake systems for iron and copper ions in Saccharomyces cerevisiae involve metal-specific permeases and two known cell surface Cu(II) and Fe(III) metalloreductases, Fre1 and Fre2. Five novel genes found in the S. cerevisiae genome exhibit marked sequence similarity to Fre1 and Fre2, suggesting that the homologs are part of a family of proteins related to Fre1 and Fre2. The homologs are expressed genes in S. cerevisiae, and their expression is metalloregulated as is true with FRE1 and FRE2. Four of the homologs (FRE3-FRE6) are specifically iron-regulated through the Aft1 transcription factor. These genes are expressed either in cells limited for iron ion uptake by treatment with a chelator or in cells lacking the high affinity iron uptake system. Expression of FRE3-FRE6 is elevated in AFT1-1 cells and attenuated in aft1 null cells, showing that iron modulation occurs through the Aft1 transcriptional activator. The fifth homolog FRE7 is specifically copper-metalloregulated. FRE7 is expressed in cells limited in copper ion uptake by a Cu(I)-specific chelator or in cells lacking the high affinity Cu(I) permeases. The constitutive expression of FRE7 in MAC1 cells and the lack of expression in mac1-1 cells are consistent with Mac1 being the critical transcriptional activator of FRE7 expression. The 5' promoter sequence of FRE7 contains three copper-responsive promoter elements. Two elements are critical for Mac1-dependent FRE7 expression. Combinations of either the distal and central elements or the central and proximal elements result in copper-regulated FRE7 expression. Spacing between Mac1-responsive sites is important as shown by the attenuated expression of FRE7 and CTR1 when two elements are separated by over 100 base pairs. From the three Mac1-responsive elements in FRE7, a new consensus sequence for Mac1 binding can be established as TTTGC(T/G)C(A/G).

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).

About the pKa of the active-site histidine in flavocytochrome b2 (yeast L-lactate dehydrogenase)

Flavocytochrome b2 or L-lactate dehydrogenase from yeasts catalyzes the oxidation of L-lactate at the expense of monoelectronic acceptors such as cytochrome c, its physiological partner. When incubated in the presence of both L-lactate and a keto acid, the enzyme catalyzes a transhydrogenation reaction wherein only the flavin is involved. During this reaction, the substrate alpha-hydrogen is transferred not only to the solvent but also in part to the keto acid, which acts as reverse substrate. Thus, when bound to the reduced enzyme, this hydrogen is sticky. In the context of a carbanion mechanism, it resides on Nepsilon of His373, the active site base. We have shown before that a correlation between the amount of intermolecular hydrogen transfer from [2-3H] lactate and the keto acid reverse substrate concentration enables the determination of the first-order rate constant, kHe, for exchange of the substrate-derived protein-bound hydrogen with bulk solvent (Urban P, Lederer F, 1985, J Biol Chem 260:11115-11122). In this work, we show that the exchange with the solvent appears to be independent of the phosphate buffer concentration in the range from 40 to 500 mM. It is thus probable that exchange occurs directly with water molecules. The second-order rate constant for exchange is then 0.16 (+/-0.03) M(-1) s(-1). Using the Eigen equation, this figure yields a pKa of 9.1+/-0.1 for His373 in the reduced enzyme, compared to a probable value of 6.0 or less in the oxidized enzyme (Suzuki H, Ogura YC, 1970, J Biochem 67:291-295). The mechanistic significance of these results is discussed.

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.

Papain proteolysis releases a soluble NADPH dependent diaphorase activity from bovine neutrophil membranes

An NADPH dependent cytochrome c reductase has been purified from resting bovine neutrophil membranes. A high degree of purification, approaching homogeneity, is indicated by the presence of a single 75 kDa protein band on silver stained SDS-PAGE (10%). The purified protein catalyzes as well an NADPH dependent reduction of iodonitrotetrazolium violet (INT). Limited papain digestion of the purified preparation produces a 65 kDa product which retains both enzymatic activities. In a similar fashion papain digestion of the plasma membrane bound protein generates a fully active soluble NADPH dependent INT and cytochrome c reductase preparation (65 kDa). Proteolytic cleavage would appear to occur at a protein-membrane anchor remote from the proteins catalytic site. The cytochrome c reductase acts independently of the O2-generating cytochrome b558, a leukocyte plasma membrane protein which also catalyzes an NADPH dependent INT reduction.

Keratinocyte superoxide generation

We have demonstrated using the reduction of cytochrome c, that the keratinocyte cell line H357 generates superoxide at significant rates (8.36 nmol/h/10[6] cells). The rate of superoxide release decreased as the cells reached confluence. Superoxide production was increased more than twofold following preincubation with IL-1beta, or by the addition of the Ca2+ ionophore, Ionomycin. Other stimuli known to activate the NADPH oxidase of phagocytes were ineffective, but the regulatory cytokine IFNgamma lowered the rate of release. Inhibitors of lipoxygenase function decreased the rate of superoxide production, whereas inhibitors of cyclo-oxygenase, xanthine oxidase, or NADPH oxidase failed to inhibit. The addition of NADH or NADPH to whole cells increased the rate threefold.

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.

Investigation of the inhibition by acetylshikonin of the respiratory burst in rat neutrophils

1. The ability of acetylshikonin to inhibit the respiratory burst in rat neutrophils was characterized and the underlying mechanism of action was also assessed in the present study. 2. Acetylshikonin caused an irreversible and a concentration-dependent inhibition of formylmethionylleucyl-phenylalanine (fMLP) plus dihydrocytochalasin B (CB)- and phorbol 12-myristate 13-acetate (PMA)-induced superoxide anion (O2.-) generation with IC50 values of 0.48 +/- 0.03 and 0.39 +/- 0.03 microM, respectively. Acetylshikonin also inhibited the O2 consumption in neutrophils in response to fMLP/CB as well as to PMA. 3. Acetylshikonin did not scavenge the generated O2.- in the xanthine-xanthine oxidase system or during dihydroxyfumaric acid (DHF) autoxidation but, on the contrary, acetylshikonin enhanced the O2.- generation in these cell-free oxygen radical generating systems. 4. Acetylshikonin inhibited the formation of inositol trisphosphate (IP3) (39.0 +/- 7.8% inhibition at 10 microM, P < 0.05) in neutrophils in response to fMLP. 5. Both the neutrophil cytosolic protein kinase C (PKC) activity and the PMA-induced PKC associated with the membrane were unaffected by acetylshikonin. 6. Acetylshikonin did not affect the porcine heart protein kinase A (PKA) activity. Upon exposure to acetylshikonin, the cellular cyclic AMP level was decreased in neutrophils in response to fMLP. 7. The cellular formation of phosphatidic acid (PA) and, in the presence of ethanol, phosphatidylethanol (PEt) induced by fMLP/CB were inhibited by acetylshikonin (60.1 +/- 7.3 and 63.2 +/- 10.5% inhibition, respectively, at 10 microM, both P < 0.05). Moreover, acetylshikonin attenuated the fMLP/CB-induced protein tyrosine phosphorylation (about 90% inhibition at 1 microM). 8. In PMA-activated neutrophil particulate NADPH oxidase preparations, acetylshikonin did not inhibit, but enhanced, the O2.- generation in the presence of NADPH. However, acetylshikonin decreased the membrane associated p47phox in PMA-activated neutrophils (about 60% inhibition at 1 microM). 9. Collectively, these results suggest that the attenuation of protein tyrosine phosphorylation and a failure in the assembly of a functional NADPH oxidase complex probably contribute predominantly to the inhibition of respiratory burst in neutrophils by acetylshikonin. In contrast, the blockade of phospholipase C (PLC) and phospholipase D (PLD) pathways play only a minor role in this respect.

Cellular localization of the inhibitory action of abruquinone A against respiratory burst in rat neutrophils

1. The possible mechanisms of action of the inhibitory effect of abruquinone A on the respiratory burst in rat neutrophils in vitro was investigated. 2. Abruquinone A caused an irreversible and a concentration-dependent inhibition of formylmethionylleucyl-phenylalanine (fMLP) plus dihydrocytochalasin B (CB)- and phorbol 12-myristate 13-acetate (PMA)-induced superoxide anion (O2.-) generation with IC50 values of 0.33 +/- 0.05 microgram ml-1 and 0.49 +/- 0.04 microgram ml-1, respectively. 3. Abruquinone A also inhibited O2 consumption in neutrophils in response to fMLP/CB and PMA. However, abruquinone A did not scavenge the generated O2.- in xanthine-xanthine oxidase system and during dihydroxyfumaric acid (DHF) autoxidation. 4. Abruquinone A inhibited both the transient elevation of [Ca2+]i in the absence of [Ca2+]o (IC50 7.8 +/- 0.2 micrograms ml-1) and the generation of inositol trisphosphate (IP3) (IC50 10.6 +/- 2.0 micrograms ml-1) in response to fMLP. 5. Abruquinone A did not affect the enzyme activaties of neutrophil cytosolic protein kinase C (PKC) and porcine heart protein kinase A (PKA). 6. Abruquinone A had no effect on intracellular guanosine 3':5'-cyclic monophosphate (cyclic GMP) levels but decreased the adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels. 7. The cellular formation of phosphatidic acid (PA) and phosphatidylethanol (PEt) induced by fMLP/ CB was inhibited by abruquinone A with IC50 values of 2.2 +/- 0.6 micrograms ml-1 and 2.5 +/- 0.3 micrograms ml-1, respectively. Abruquinone A did not inhibit the fMLP/CB-induced protein tyrosine phosphorylation but induced additional phosphotyrosine accumulation on proteins of 73-78 kDa in activated neutrophils. 8. Abruquinone A inhibited both the O2.- generation in PMA-activated neutrophil particulate NADPH oxidase (IC50 0.6 +/- 0.1 microgram ml-1) and the iodonitrotetrazolium violet (INT) reduction in arachidonic acid (AA)-activated cell-free system (IC50 1.5 +/- 0.2 micrograms ml-1) 9. Collectively, these results indicate that the inhibition of respiratory burst in rat neutrophils by abruquinone A is mediated partly by the blockade of phospholipase C (PLC) and phospholipase D (PLD) pathways, and by suppressing the function of NADPH oxidase through the interruption of electron transport.