The enzyme of granulocytes that produces superoxide and peroxide. An elusive Pimpernel

In memoriam: Filippo Rossi (1926-2022)

Description of the scientific life of Filippo Rossi, who died in October 2022.

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).

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.

Amperometric detection of superoxide dismutase at cytochrome c-immobilized electrodes: xanthine oxidase and ascorbate oxidase incorporated biopolymer membrane for in-vivo analysis

Amperometric measurement of superoxide dismutase (SOD) was carried out at cytochrome c-immobilized monolayers and ascorbate oxidase (AOD)/xanthine oxidase (XOD)/cytochrome c- and (AOD, XOD)/cytochrome c-multilayers. Cytochrome c was covalently immobilized on mercaptopropionic acid-containing self-assembled monolayers on gold. A biopolymer membrane of poly-L-lysine confining XOD and AOD was cast on the monolayer of cytochrome c. While both the cytochrome c-immobilized monolayer and multilayer electrodes show anodic current responses to the generation of superoxide radical, the sensitivity of the multilayer system for the detection of superoxide radical was high relative to that of the monolayer system. In the case of the cytochrome c-multilayer electrodes, the generation of superoxide radical near the sensing element, cytochrome c, resulted in high sensitivity for the detection of superoxide. The use of a XOD and AOD-incorporated poly-L-lysine membrane enabled the detection of the generation of superoxide radical in the presence of L-ascorbic acid. Though L-ascorbic acid could scavenge superoxide radical, the biopolymer membrane confined with AOD will oxidize any L-ascorbic acid that permeated into the membrane. By using the multilayer electrodes, one could measure the activity of SOD in the presence of L-ascorbic acid.

The oxidative inactivation of cytochrome P450 in monooxygenase reactions

Possible mechanisms of cytochrome P450 self-inactivation during catalytic turnover have been considered. Two ways of hemoprotein inactivation are so far known. The first, studied extensively by many authors, is the formation of active substrate intermediates, capable of modifying heme and apoenzyme. The second way, revealed quite recently and resulting from uncoupled cytochrome P450-catalyzed monooxygenase reactions, is yet to be clarified. Briefly, it involves formation of hydrogen peroxide in the hemoprotein active center, which interacts with the enzyme associated Fe2+, thereby generating hydroxyl radicals that bleach the heme and modify the apoenzyme. This mechanism operates with substrates and cytochrome P450 forms with partially coupled monooxygenase reactions, thus causing the formation of hydrogen peroxide as a byproduct.

Electrochemical sensors for direct reagentless measurement of superoxide production by human neutrophils

Electrochemical sensors based on immobilised cytochrome c or superoxide dismutase for the measurement of superoxide radical production by stimulated neutrophils are described. Cytochrome c was immobilised covalently at a surface-modified gold electrode and by passive adsorption to novel platinised activated carbon electrodes (PACE). The reoxidation of cytochrome c at the electrode surface upon reduction by superoxide was monitored using both xanthine/xanthine oxidase and stimulated neutrophils as sources of the free radical. In addition, bovine Cu/Zn superoxide dismutase was immobilised to PACE by passive adsorption and superoxide, generated by xanthine/xanthine oxidase, detected by oxidation of hydrogen peroxide produced by the enzymic dismutation of the superoxide radical. A biopsy needle probe electrode based on cytochrome c immobilised at PACE and suitable for continuous monitoring of free radical production was constructed and characterised.

Influence of free radical scavengers on myeloperoxidase activity and lipid peroxidation in acute skin grafts

Activated neutrophil granulocytes are an important source of oxygen free radicals in acute skin grafts. Lipid peroxidation and immigration of PMNs, indicating inflammatory mechanisms, affect each other to a variable extent. The effect of the scavengers allopurinol and alpha-D-tocopherol on both lipid peroxidation and neutrophilic infiltration was investigated. A possible mechanism for the superior effect of allopurinol compared to vitamin E was discussed.

A sensitive and specific assay for superoxide anion released by neutrophils or macrophages based on bioluminescence of polynoidin

Using the luminescent protein polynoidin, present in the bioluminescent system isolated from the marine annelid Harmothoe lunulata, we have developed a new method to measure, specifically, superoxide anion (O2-) released by macrophages or neutrophils. A small quantity of an aqueous crude extract of polynoidin is used to detect O2- released by stimulated cells. Light emission is linearly dependent on the number of cells over a wide range (10(3) to 10(7) cells), and the assay is thus more sensitive than either luminol or ferricytochrome c reduction. Luminescence is enhanced 20% by mannitol, 80% by catalase, and is totally quenched by superoxide dismutase. For the same number of cells, neutrophils showed a threefold higher release of O2- and a twofold faster first-order light decay than stimulated macrophages, in accordance with data obtained by other methods.

Application of the electrochemistry of cytochrome c to the measurement of superoxide radical production

An electrochemical sensor for the measurement of the superoxide anion produced as a result of the respiratory burst of neutrophils is described. The reduction of cytochrome c by the superoxide ion was monitored using a surface-modified gold electrode. The current produced was shown to be superoxide-specific and proportional to the number of neutrophils in both purified neutrophil preparations and plasma.

Polymorphonuclear leucocyte dysfunction during short term metabolic changes from normo- to hyperglycemia in type 1 (insulin dependent) diabetic patients

Polymorphonuclear leucocyte (PMN) ingestion of particles coated with lipopolysaccharide (LPS) from Escherichia coli was compared to other PMN functions in seven patients with insulin dependent diabetes mellitus (IDDM) during short-term controlled metabolic changes from normo- to hyperglycemia without ketoacidosis. Factors known to interfere with PMN functions were excluded. PMN ingestion of particles coated with both LPS and bovine serum albumin became reduced from normo- to hyperglycemia. PMN motility was impaired in IDDM, but did not seem to be affected by short-term changes in metabolic control. PMN metabolism did not change from normo-to hyperglycemia. Particle-uptake by diabetic PMN is impaired after short term hyperglycemia in the range normally occurring in diabetics in every-day life.

The superoxide-forming enzymatic system of phagocytes

The formation of oxygen-derived free radicals by the phagocytes (neutrophils, eosinophils, monocytes and macrophages) is catalysed by a membrane-bound NADPH oxidase which is dormant in resting cells and becomes activated during phagocytosis or following interaction of the cells with suitable soluble stimulants. This enzyme is under investigation in many laboratories but its molecular structure remains to be clarified. Possible components such as flavoproteins, cytochrome b558, and quinones have been proposed on the basis of enzyme purification studies, effects of inhibitors, kinetic properties and analysis of genetic defects of the oxidase. An extensive discussion of the evidence for the participation of these constituents is reported. On the basis of the available information on the structure and the catalytic properties of the NADPH oxidase, a series of possible models of the electron-transport chain from NADPH to O2 is presented. Finally, the triggering mechanism of the respiratory burst is discussed, with particular reference to the stimulus-response coupling and the final modification(s) of the oxidase (phosphorylation, assembly, change of lipid environment, etc.) which are involved in its activation.

Differences and similarities between human and rabbit neutrophil granulocyte responses in vitro: the effects of zymosan-activated plasma, phorbol myristate acetate and n-formyl-methionyl-leucyl-phenylalanine

The amount of reactive oxygen intermediates (ROI) generated by activated polymorphonuclear neutrophils (PMN), as well as the closeness of contact between PMN and vessel wall, may determine whether PMN activators will induce the adult respiratory distress syndrome. We examined the ROI-generating and aggregating effects of zymosan activated plasma (ZAP), phorbol myristate acetate (PMA) and n-formyl-methionyl-leucyl-phenylalanine (FMLP), on isolated human and rabbit PMN. PMA, after a short lag phase, induced a large and long-lasting increase in ROI generation. The initial peak response was higher and more rapid in human than in rabbit cells. The reaction to FMLP occurred almost instantaneously, but was much weaker than that to PMA, and ROI generation returned to near baseline in less than 10 min. No species difference was seen. ZAP caused an FMLP-like ROI response in human cells, whereas no response was observed in rabbit PMN. PMN aggregation was induced by all three activators, most markedly by PMA. No species difference was detected for PMA; FMLP gave a stronger aggregation of rabbit than of human PMN, however, while the opposite was true for ZAP. In conclusion, ZAP was a potent stimulus for PMN aggregation, but had modest (or no) effects on the production of ROI. Marked differences between human and rabbit PMN responses were observed.

Vitamin E consumption by human blood platelets activated by latex particles

Human blood platelet activation elicited by latex particles is associated to a 30% decrease in the cellular content of vitamin E. The vitamin E consumption is inhibited by the addition of catalase (500 U/ml) and azide (1 mM), but it is not affected by potassium cyanide (1 mM). It may be proposed that the challenge of platelets with particulate stimuli causes generation of oxygen reduction products, which leads to vitamin E depletion.

Inhibition of the respiratory burst of human neutrophils by the polyamine oxidase-polyamine system

The addition of the polyamines, spermine and spermidine, to human neutrophils caused a depression of the hexose-monophosphate (HMP) shunt activity of neutrophils stimulated with latex particles but not of unstimulated cells. The effect was dependent on the presence of bovine serum and was not observed when normal human serum was substituted for bovine serum. The polyamine oxidase (PAO) in bovine serum was probably responsible for generating the activity since normal human serum lacks PAO. A role for PAO was further supported by the finding that partially purified bovine PAO in the presence of polyamines similarly mediated inhibition of HMP shunt activity in stimulated neutrophils. Catalase failed to prevent the inhibitory effects of the PAO-polyamine system suggesting that H2O2 is not the responsible product. In addition, our results show that human pregnancy serum known to contain PAO activity in the presence of polyamines mediated a similar inhibition of the respiratory burst.

Opsonized-zymosan induces a respiratory burst in human blood platelets

Opsonized-zymosan-stimulated polymorphonuclear cells show a cyanide-insensitive oxygen consumption. We have investigated whether opsonized-zymosan could induce similar metabolic change in human blood platelets. Preparation of intact human blood platelets, obtained by separation through a Ficoll layer (23% w/v) were challenged with opsonized-zymosan. The polymorphonuclear cell contamination was less than 1/10(8) platelets. The opsonized-zymosan-stimulated platelets showed an increase of oxygen consumption. The mean of oxygen burst measured by a polarographic method with a Clark electrode was 11 nmole/10(9) platelets/min (S.E.M. 4; n = 15). The duration of the burst was 2 min. Unstimulated platelets did not show the oxygen burst. The inhibitors of respiratory chain and prostaglandin synthesis completely abolished the oxygen consumption by opsonized-zymosan-stimulated platelets. The simultaneous addition of NADH (1 mM) and opsonized-zymosan induced a burst of oxygen consumption, which occurred after a variable lag phase (10-12 min) from the stimulation, also in the presence of inhibitors. This burst, which lasted about 1 min, amounted to 10 nmole/10(9) platelets/min (S.E.M. 2; n = 15) and it was higher in the presence of NAN3, a catalase inhibitor. Zymosan treated with hydrazine or heated plasma (56 degrees C) did not cause increased oxygen consumption. Inulin or inulin-treated serum did not stimulate platelets. In these experimental conditions some NADH disappeared, as shown by isotachophoresis. The results demonstrated that an immunological stimulus may activate a membrane-linked cyanide-insensitive oxygen metabolizing system.

Absence of cytochrome b-245 in chronic granulomatous disease. A multicenter European evaluation of its incidence and relevance

The heme-containing protein cytochrome b-245 has been proposed as a primary component of the microbicidal oxidase system of phagocytes that normally generates superoxide-free radicals but when defective is associated with chronic granulomatous disease. We measured this cytochrome in granulocytes from 27 patients with chronic granulomatous disease and from 64 members of their families. It was undetectable in all 19 of the men in whom the defect appeared to be located on the X chromosome. Female relatives who were heterozygous carriers had reduced concentrations of the cytochrome and variable proportions of cells that were unable to generate superoxide; these two characteristics were closely related (r = 0.93 in the 16 mothers and 0.85 in all 24 carriers, P less than 0.001). In contrast, in all eight patients (seven women) with a probable autosomal recessive inheritance the cytochrome was present but nonfunctional. The properties tested, including midpoint potential, carbon monoxide binding, and organelle distribution, were normal, but the cytochrome did not undergo reduction on cellular stimulation. Thus, absence or malfunction of the cytochrome b-245 may be the causal molecular defect in chronic granulomatous disease, implicating it in the microbicidal oxidase system.

Genetic disorders of leukocyte function: what they tell us about normal antimicrobial mechanisms of human phagocytic cells

Analysis of three inherited defects of granulocyte function (Chediak-Higashi Syndrome, CHS; Chronic Granulomatous Disease, CGD; Myeloperoxidase Deficiency, MPO) has highlighted critical events for the antimicrobial function of these cells and placed others in perspective. Prompt phagosomal fusion may be more important for digestion of organisms rather than killing as indicated by the mild bactericidal defects in the CHS. The formation of O2- and H2O2 during the phagocytic respiratory burst is central for the broad antimicrobial activity of granulocytes. MPO, on the other hand, while perhaps normally participating in granulocyte antimicrobial action, appears to be essential only for the effective killing of eukaryotic organisms such as certain fungal strains. While the non-oxidative killing mechanism of neutrophils have stimulated much recent interest and were the first to be defined no specific inherited defects have been discovered which are clinically important. Genetic disorders of macrophage effector function remain to be clearly defined as do those of eosinophils. The lessons learned from the study of the granulocyte defects discussed have provided both the technology and approach to the analysis of the antimicrobial and cytocidal mechanisms of these important phagocytic cells.

The respiratory burst of phagocytic cells: facts and problems

1. The so called "soluble" oxidase(s) are not involved in the respiratory burst of guinea pig and human granulocytes and of guinea pig peritoneal resident and elicited macrophages. 2. The activation of the oxidation of NADPH by a membrane bound NAD(P)H oxidase is the main mechanism responsible for the activation of the respiration of phagocytes. 3. The oxidase is inactive in resting cells and the activated form works on the plasma membrane. 4. More than one mechanism is operative in the oxidation of NAD(P)H by cell free particles in vitro. These mechanisms vary in relation to the conditions of assay (pH and concentration of substrate). 5. Under optimal conditions in vitro the enzymatic oxidation of NADPH practically involves the univalent pathway of oxygen reduction with stoichiometry of two nanomoles of O2 formed for one nanomole of NADPH oxidized. 6. Also in intact cells all O2 is first univalently reduced to O2 and then discharged outside the cell or in the phagocytic vacuoles. 7. The main reactions involved in the O2 balance in intact cells are the univalent reduction of O2, the dismutation of O2 to H2O2 and the degradation of the peroxide through catalatic and peroxidatic mechanisms. 8. The total oxygen univalently reduced by the activated oxidase is 2-4 folds the net oxygen consumed by the cells, depending on the mechanism of H2O2 degradation. 9. All the rate of extrarespiration is accounted for by the rate of oxidation of physiological concentration of NADPH by the membrane-bound enzyme. This adequacy can be observed only under appropriate experimental conditions, because the high activity of the oxidase is not a permanent state.

Respiratory burst enzyme in human neutrophils. Evidence for multiple mechanisms of activation

Alteration of the surface of human neutrophils with the nonpenetrating, protein-inactivating agent p-diazobenzenesulfonic acid (DASA) was found to prevent activation of the respiratory burst by some stimuli, but not others. Production of superoxide anion (O2-) stimulated by concanavalin A or the chemotactic peptide formyl-methionyl-leucyl-phenylalanine FMLP was inhibited by DASA pretreatment, whereas O2- production stimulated by phorbol myristate acetate (PMA), sodium fluoride. or the ionophore A23187 was not inhibited by DASA. Pretreatment with DASA inhibited oxygen uptake stimulated by FMLP, but not oxygen uptake stimulated by PMA. DASA reproducibly inhibited activities of two known surface enzymes Mg++-ATPase and alkaline phosphatase, by 45-55% and 60-70%, respectively. The inhibition by DASA of O2- production did not appear to be caused by interference with binding of the affected stimuli, since pretreatment with DASA did not inhibit release of the lysosomal enzymes lysozyme and myeloperoxidase induced by concanavalin A or FMLP. Membrane-rich particulate fractions from neutrophils have been shown to contain NADPH-dependent oxidative activity that is presumably responsible for the phagocytosis-associated respiratory burst of intact cells. The PMA-activated enzyme was susceptible to inhibition of directly exposed to DASA in this particulate fraction. These findings suggest that more than one mechanism exists for activation of the respiratory burst oxidase in human neutrophils, and that the neutrophil possesses at least one oxidase that is not an ectoenzyme.

Oxidation-reduction properties of the cytochrome b found in the plasma-membrane fraction of human neutrophils. A possible oxidase in the respiratory burst

The oxidation-reduction midpoint potential of the cytochrome b found in the plasma membrane of human neutrophils has been determined at pH 7.0 (Em,7.0) from measurements of absorption spectra at fixed potentials. In both unstimulated and phorbol myristate acetate-stimulated cells Em,7.0 was -245 mV. Changes in pH affected the Em of the cytochrome b, with a slope of approx. 25 mV/pH unit change. The Em,7.0 of the haem group(s) of the membrane-bound myeloperoxidase of human neutrophils was found to be +34 mV. The plasma membranes contained no detectable ubiquinone, and no iron-sulphur compounds were detected by e.p.r. spectroscopy at 5-20 K. No flavins were detected by e.p.r. spectroscopy. The cytochrome b-245 was not reduced by added NADH or NADPH. Dithionite-reduced cytochrome b-245 formed a complex with CO, supplied as a saturated solution, which was dissociated with 26 microseconds illumination from a xenon flash lamp, and the recombination with CO had a half-time of approx. 6 ms. Partly (80%) reduced cytochrome b-245 was oxidized by added air-saturated buffer with a half-time faster than 1 s at 20 degrees C, a resolution limited by mixing time. These results are compatible with cytochrome b-245 acting as an oxidase.

Comparative aspects of oxidative metabolism of neutrophils from human blood and guinea pig peritonea: magnitude of the respiratory burst, dependence upon stimulating agents, and localization of the oxidases

We have compared the subcellular sites of H2O2 and presumably also superoxide-(O2-) production, and certain aspects of metabolic responses (O2 consumption, O2- production) of stimulated neutrophils from human blood and those elicited into guinea pig peritonea. Stimulation was accomplished with either opsonized zymosan or phorbol-12-myristate-13-acetate (PMA). Striking quantitative differences were observed between these cell types with regard to the increased respiration and O2- production observed during stimulation. These differences were most apparent when opsonized zymosan served as the stimulating agent. They were minimized when the soluble stimulating agent, PMA, was used. With either stimulus, the subcellular sites of H2O2 production were the same for both types of neutrophils, i.e., the plasmalemma and phagosomal membranes. No H2O2 production could be detected cytochemically in the absence of stimulation. Treatment of both unstimulated human blood and elicited guinea pig peritoneal neutrophils with the nonpenetrating, covalently linking reagent, p-diazobenzenesulfonic acid, failed to diminish O2- production upon subsequent stimulation, in contrast to a previous report. These data are discussed in terms of the possible cytological arrangements of the respiratory enzyme(s), and the different modes of stimulation of neutrophil metabolism by various agents. Ancillary data on elicited mouse peritoneal neutrophils are presented.

Leukocyte abnormalities

Certain qualitative abnormalities in neutrophils and blood monocytes are associated with frequent, severe, and recurrent bacterial infections leading to fatal sepsis, while other qualitative defects demonstrated in vitro may have few or no clinical sequelae. These qualitative defects are discussed in terms of the specific functions of locomotion, phagocytosis, degranulation, and bacterial killing.

Neutrophil dysfunction associated with states of chronic and recurrent infection

Infants, children, and young adults who suffer chronic and recurrent bacterial or fungal infection despite adequate numbers of circulating granulocytes and normal or elevated levels of immunoglobulins should be suspected of having fundamental defects in granulocyte functioning. This article considers clinical disorders for which there is evidence for associated defects of polymorphonuclear leukocytes.