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

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.

Decolorization of malachite green by cytochrome c in the mitochondria of the fungus Cunninghamella elegans

We studied the decolorization of malachite green (MG) by the fungus Cunninghamella elegans. The mitochondrial activity for MG reduction was increased with a simultaneous increase of a 9-kDa protein, called CeCyt. The presence of cytochrome c in CeCyt protein was determined by optical absorbance spectroscopy with an extinction coefficient (E(550-535)) of 19.7+/-6.3 mM(-1) cm(-1) and reduction potential of + 261 mV. When purified CeCyt was added into the mitochondria, the specific activity of CeCyt reached 440 +/- 122 micromol min(-1) mg(-1) protein. The inhibition of MG reduction by stigmatellin, but not by antimycin A, indicated a possible linkage of CeCyt activity to the Qo site of the bc1 complex. The RT-PCR results showed tight regulation of the cecyt gene expression by reactive oxygen species. We suggest that CeCyt acts as a protein reductant for MG under oxidative stress in a stationary or secondary growth stage of this fungus.

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.

Fungal metabolite gliotoxin targets flavocytochrome b558 in the activation of the human neutrophil NADPH oxidase

Fungal gliotoxin (GT) is a potent inhibitor of the O(2)(-)-generating NADPH oxidase of neutrophils. We reported that GT-treated neutrophils fail to phosphorylate p47(phox), a step essential for the enzyme activation, because GT prevents the colocalization of protein kinase C betaII with p47(phox) on the membrane. However, it remains unanswered whether GT directly affects any of NADPH oxidase components. Here, we examine the effect of GT on the NADPH oxidase components in the cell-free activation assay. The O(2)(-)-generating ability of membranes obtained from GT-treated neutrophils is 40.0 and 30.6% lower, respectively, than the untreated counterparts when assayed with two distinct electron acceptors, suggesting that flavocytochrome b(558) is affected in cells by GT. In contrast, the corresponding cytosol remains competent for activation. Next, GT addition in vitro to the assay consisting of flavocytochrome b(558) and cytosolic components (native cytosol or recombinant p67(phox), p47(phox), and Rac2) causes a striking inhibition (50% inhibitory concentration = 3.3 microM) when done prior to the stimulation with myristic acid. NADPH consumption is also prevented by GT, but the in vitro assembly of p67(phox), p47(phox), and Rac2 with flavocytochrome b(558) is normal. Posterior addition of GT to the activated enzyme is ineffective. The separate treatment of membranes with GT also causes a marked loss of flavocytochrome b(558)'s ability to reconstitute O(2)(-) generation, supporting the conclusion at the cellular level. The flavocytochrome b(558) heme spectrum of the GT-treated membranes stays, however, unchanged, showing that hemes remain intact. These results suggest that GT directly harms site(s) crucial for electron transport in flavocytochrome b(558), which is accessible only before oxidase activation.

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.

Inhibition of vascular NADH/NADPH oxidase activity by thiol reagents: lack of correlation with cellular glutathione redox status

Vascular NAD(P)H oxidase activity contributes to oxidative stress. Thiol oxidants inhibit leukocyte NADPH oxidase. To assess the role of reactive thiols on vascular oxidase, rabbit iliac/carotid artery homogenates were incubated with distinct thiol reagents. NAD(P)H-driven enzyme activity, assessed by lucigenin (5 or 250 microM) luminescence, was nearly completely (> 97%) inhibited by the oxidant diamide (1mM) or the alkylator p-chloromercuryphenylsulfonate (pCMPS, 0.5mM). Analogous inhibition was also shown with EPR spectroscopy using DMPO as a spin trap. The oxidant dithionitrobenzoic acid (0.5mM) inhibited NADPH-driven signals by 92% but had no effect on NADH-driven signals. In contrast, the vicinal dithiol ligand phenylarsine oxide (PAO, 1 microM) induced minor nonsignificant inhibition of NADPH-driven activity, but significant stimulation of NADH-triggered signals. The alkylator N-ethyl maleimide (NEM, 0.5mM) or glutathione disulfide (GSSG, 3mM) had no effect with each substrate. Coincubation of N-acetylcysteine (NAC, 3mM) with diamide or pCMPS reversed their inhibitory effects by 30-60%, whereas NAC alone inhibited the oxidase by 52%. Incubation of intact arterial rings with the above reagents disclosed similar results, except that PAO became inhibitor and NAC stimulator of NADH-driven signals. Notably, the cell-impermeant reagent pCMPS was also inhibitory in whole rings, suggesting that reactive thiol(s) affecting oxidase activity are highly accessible. Since lack of oxidase inhibition by NEM or GSSG occurred despite significant cellular glutathione depletion, change in intracellular redox status is not sufficient to account for oxidase inhibition. Moreover, the observed differences between NADPH and NADH-driven oxidase activity point to complex or multiple enzyme forms.

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.

Despite structural similarities between gp91phox and FRE1, flavocytochrome b558 does not mediate iron uptake by myeloid cells

Superoxide (O2-) generated by the phagocyte reduced nicotinamide adenine dinucleotide phosphate oxidase is dependent on electron transfer by flavocytochrome b558 (flavocytochrome b), a transmembrane heterodimer that forms the redox center of the oxidase at the plasma or phagosomal membrane. The larger of its two subunits, gp91phox, is homologous to the yeast iron reductase subunit FRE1, and these two proteins share many structural and functional characteristics. Because FRE1 is required for iron uptake in yeast, we hypothesized that flavocytochrome b might serve a similar function in human phagocytes and thus provide a mechanism for the transferrin-independent iron acquisition observed in myeloid cells. To determine whether flavocytochrome b was required for iron uptake, we compared iron acquisition by polymorphonuclear neutrophils (PMNs) or Epstein-Barr virus (EBV)-transformed B lymphocytes derived from individuals with X-linked chronic granulomatous disease (CGD) with iron acquisition by normal cells. Our results indicate that all cells acquired iron to the same extent and that uptake could be significantly enhanced in the presence of the trivalent metal gallium. The gallium enhancement of iron uptake observed in PMNs or in EBV-transformed B lymphocytes derived from healthy individuals was mirrored by those derived from individuals deficient in flavocytochrome b. Furthermore, both normal and CGD-derived EBV-transformed B lymphocytes had similar iron reductase activity, suggesting that flavocytochrome b is not a biologically significant iron reductase. In contrast to previously suggested hypotheses, these results show conclusively that flavocytochrome b is not necessary for cellular iron acquisition, despite structural and functional similarities between yeast iron reductases and flavocytochrome b.

Dithiothreitol enhances arsenic trioxide-induced apoptosis in NB4 cells

Recently, arsenic trioxide (As2O3) was reported to induce clinical remission in patients with acute promyelocytic leukemia. Modulation of protein phosphorylation by binding to the vicinal thiols has been suggested as a possible mechanism. We found that phenylarsine oxide, a strong vicinal thiol-binding agent, neither induced nuclear fragmentation or DNA laddering nor increased caspase activity in NB4 cells; however, As2O3 and a weak thiol-binding agent, dimethylarsinic acid, did increase activity. Dithiothreitol (DTT) effectively suppressed the phenylarsine oxide-inhibited cellular reductive capacity, but unexpectedly, enhanced As2O3-induced apoptosis in NB4 cells. As2O3-induced and As2O3-plus-DTT-induced apoptosis in NB4 cells was modulated by oxidant modifiers, but not by nitric oxide synthase inhibitors. These results demonstrate that DTT, a dithiol agent and known antidote for trivalent inorganic arsenic, enhances the toxicity of As2O3, thereby opening a new research direction for the mechanisms of arsenic toxicity and perhaps also helping in the development of new therapeutic strategies for treating leukemias.

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.