Delineation of the catalytic components of the NADPH-dependent O2- generating oxidoreductase of human neutrophils

Respiratory response of phagocytes: terminal NADPH oxidase and the mechanisms of its activation

The chemical composition, properties and activation mechanism of the O2(-)-forming NADPH oxidase of phagocytes were investigated, using partially purified enzyme preparations. Highly active NADPH oxidase was extracted as an aggregate of high Mr from the membranes of neutrophils and macrophages. The enzyme complex contained phospholipids and cytochrome b-245, very little FAD and almost no quinones or NAD(P)H-dye reductase activity. The purification of a polypeptide with a relative molecular mass of 31 500 strictly paralleled the purification of NADPH oxidase, suggesting that this polypeptide is a component of the enzyme. This protein was identified as cytochrome b -245 after dissociation of the proteolipid complex and purification of the cytochrome moiety. The 31 500 Mr protein was phosphorylated in enzyme preparations from activated but not from resting cells. The results indicate that: cytochrome b-245 is a major component of NADPH oxidase; the involvement of NAD(P)H dye reductases in the O2(-)-forming activity is questionable; the cytochrome b-245: FAD ratio in the enzyme complex is much higher than that indicated in crude preparations; the Mr of pig neutrophil cytochrome b-245 is 31 500; the activation of the O-2-forming system involves a process of phosphorylation of cytochrome b-245.

Nonenzymatic NADPH-dependent reduction of 2,6-dichlorophenol-indophenol

The reduction of 2,6-dichloroindophenol (DCIP) by direct interaction with NADPH was studied. The results indicate that reduction proceeds via a direct electron transfer from NADPH to DCIP, with no oxygen consumption, and a rate constant of k = 4.69 M-1.s-1. The reduced DCIP can rapidly transfer its electrons to potassium ferricyanide (K3Fe(CN)6) or ferricytochrome c, but not to nitro blue tetrazolium. Superoxide dismutase inhibits DCIP reduction in an oxygen-dependent manner by favoring the reoxidation of the reduced DCIP. We therefore conclude DCIP is not suitable for detecting O2- when the nucleotides NADH or NADPH are present.

Studies on the anti-inflammatory activity of ebselen. Ebselen interferes with granulocyte oxidative burst by dual inhibition of NADPH oxidase and protein kinase C?

Ebselen (PZ51, 2-phenyl-1,2-benzoisoselenazol-3-(2H)-one) was shown to be an inhibitor of human granulocyte oxidative burst stimulated by phorbol myristate acetate (IC50 25 microM). Estimation of the primary oxygen metabolites of the burst was complicated by the redox chemistry of Ebselen. Ebselen inhibited NADPH-stimulated superoxide generation by a partially purified NADPH oxidase preparation with an IC50 of 0.5-1.0 microM. Ebselen was also shown to inhibit the activity of partially purified Ca2+- and phospholipid-dependent protein kinase C (IC50 ca. 0.5 microM). Phorbol ester-stimulated phosphorylation of protein in intact cells was inhibited by Ebselen (IC50 ca. 50 microM). These pharmacodynamic properties of Ebselen are discussed in terms of its anti-inflammatory activity in vivo. The findings are also discussed in terms of Ebselen's known ability to interact with sulfhydryl components of cells, particularly critical thiol components of the enzymes studied.

Leukocytic oxygen activation and microbicidal oxidative toxins

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

Electrophoretic pattern of NADPH-dependent oxidoreductive activities in K 562 and HL 60 leukemic cell lines

Morphological and functional differentiation of hemopoietic cells is accompanied by the expression of lineage-specific protein markers. NADPH-oxidoreductive enzymatic activities in HL 60 and K 562 leukemic cell lines, compared with granulocytes and erythrocytes, show a NADPH-oxidizing and a NADPH-diaphorase activity. The oxidizing activity, absent in erythrocytes, has the same electrophoretic migration in HL 60 cells and granulocytes while it is different in K 562 cells. The diaphorase, absent in HL 60 cells and granulocytes, has the same migration in erythrocytes and K 562 cells, although with a slightly different quantitative expression. K 562 cells induced to differentiation with arabinofuranosylcytosine show the appearance of a band of NADPH-oxidizing activity of granulocytic type, together with the major band found in these cells.

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.

Increased anti-tumor effect of adriamycin-loaded albumin microspheres is associated with anaerobic bioreduction of drug in tumor tissue

Anti-tumor activity and fate of adriamycin incorporated into biodegradable albumin microspheres was examined in vivo after direct intratumoral injection. Adriamycin in microspherical form displayed superior anti-tumor activity to a comparable dose of drug in solution. This was associated at later time points (40 hr, 50 hr and 72 hr after injection) with higher median parent drug concentrations in tumor tissue (4.1, 3.6, 2.6 micrograms/g respectively for microspheres and 1.6, 1.7 and 1.0 micrograms/g for solution) and the consistent detection of 7-deoxyaglycone metabolites, end products of reduction of adriamycin under anaerobic conditions (1.1, 1.0, 1.0 micrograms/g respectively for microspheres and less than 0.1 micrograms/g at all time points for solution). It is generally considered that the redox properties of anthracyclines are responsible for their toxicity to normal tissues whereas other mechanisms are responsible for antineoplastic activity. In this study we show that inducing metabolism of Adriamycin via reductive pathways is associated with increased anti-tumor effect.

Presence of cytochrome b-245 in NADPH oxidase preparations from human neutrophils

The composition of NADPH oxidase purified by Red Sepharose chromatography of extracts from human neutrophil membranes was investigated. In contrast to that was recently reported by others, the enzyme isolated according to this procedure contained a high concentration of cytochrome b-245 and little FAD. The results reinforce the belief that cytochrome b-245 is a major component of the NADPH oxidase and plays a fundamental role in the formation of O2-by neutrophils.

Purification and resolution of NADH diaphorase activity from NADPH diaphorase-linked: O2 oxidoreductase activity of human neutrophils

Intrinsic NADPH diaphorase activity is a component of the membrane-bound NAD(P)H:O2 oxidoreductase of human neutrophils. NADH-specific diaphorase activity is also present in membrane fractions rich in oxidoreductase activity. Studies were undertaken to determine whether the NADH diaphorase might also be intrinsic to the oxidoreductase. The latter diaphorase was freed from the membrane by detergent extraction and partially purified approximately 80-fold. Its apparent molecular weight following solubilization in deoxycholate and Tween-20 was 204 000 +/- 10 000. The specific activity of the partially purified diaphorase with ferricyanide as electron acceptor was 7.6 X 10(3) mU/mg protein, its pH optimum was 7.0, and its Km for NADH was 13 microM. It is completely devoid of NADPH diaphorase activity, lacks the capacity to reduce molecular oxygen, yet readily reduces ferricyanide, 2,6-dichlorophenolindophenol and ferricytochrome c. Whereas the NADH diaphorase was freed from the particulate fraction of cell lysates by extraction in 10 mM Tris-HCl buffer (pH 8.6) made up in 15% glycerol and 0.5% Tween-20, NADPH-dependent diaphorase and superoxide-generating activities also present in the membrane were not. These observations make it unlikely that the principal membrane-bound NADH diaphorase found in human neutrophils is a component of the NAD(P)H:O2 oxidoreductase, despite its common association in the same particulate fraction of cell lysates.

Studies on the nature and activation of O2(-)-forming NADPH oxidase of leukocytes. Identification of a phosphorylated component of the active enzyme

Highly active superoxide (O2-)-forming NADPH oxidase was extracted from plasmamembranes of phorbol-12-myristate-13-acetate-activated pig neutrophils and was partially purified by gel filtration chromatography. Oxidase activity copurified with cytochrome b-245 in an aggregate containing phospholipids and was almost completely separated from FAD and NAD(P)H-cytochrome c reductase. A polypeptide with molecular weight of 31,500 strictly paralleled the purification of NADPH oxidase, suggesting that it is a major component of the enzyme. The enzyme complex was then dissociated by high detergent and salt concentration and cytochrome b-245 was isolated by a further gel filtration chromatography, with a 147 fold purification with respect to the initial preparation. The cytochrome b-245 showed a 31,500 molecular weight by SDS electrophoresis, indicating that it is actually the component previously identified in the partially purified enzyme. The 31,500 protein was phosphorylated in enzyme preparations from activated but not from resting neutrophils, suggesting that phosphorylation of cytochrome b-245 is involved in the activation mechanism of the O2(-) -forming enzyme responsible for the respiratory burst in phagocytes.

Detection of NADPH diaphorase activity associated with human neutrophil NADPH-O2 oxidoreductase activity

At approximately equimolar concentrations (approximately 70 microM), and in the presence of excess catalase and superoxide dismutase, DCIP, ferricytochrome c and ferricyanide abstracted 21, 6 and 61%, respectively, of the electron equivalents given up by NADPH to the NADPH-O2 oxidoreductase complex derived from phorbol myristate acetate-stimulated human neutrophils. With a 10-fold increase in ferricyanide, all of the electron equivalents given up by NADPH to the oxidoreductase complex were shunted to ferricyanide concomitant with complete inhibition of NADPH-dependent O2 consumption. These results substantiate the existence of intrinsic diaphorase activity associated with the superoxide generating NADPH-O2 oxidoreductase of human neutrophils.

NADPH oxidase of neutrophils forms superoxide anion but does not reduce cytochrome c and dichlorophenolindophenol

Superoxide (O-2) production by partially purified NADPH oxidase from guinea pig neutrophils was markedly increased when the cells were activated by exposure to phorbol-myristate acetate. On the contrary, NADPH-dependent cytochrome c and 2,6-dichlorophenolindophenol (DCIP) reductase activities in preparations from resting and activated neutrophils were similar. The apparent Km values for NADH and NADPH of the reductase activities were different from those of the O-2 producing enzyme. The electron acceptors did not inhibit the oxygen consumption by NADPH oxidase in the presence of superoxide dismutase. Even in anaerobiosis the oxidase failed to reduce cytochrome c and DCIP. These results suggest that NAD(P)H-dependent dye reductase activities are not involved in the electron transport system responsible for the O-2 production by neutrophils.

Oxygen-radical production during inflammation may be limited by oxygen concentration

The relationship between oxygen-radical production by rat polymorphonuclear leucocytes and O2 concentration was established by the measurement of luminol-dependent chemiluminescence at defined O2 concentrations. The O2 concentration that gave 50% of the maximum stimulated oxygen-radical production was 31 +/- 9 microM for non-opsonized latex beads and 22 +/- 9 microM for chemotactic peptide. The O2 concentration in rheumatoid synovial fluid was approx. 30 microM. It is therefore proposed that radical production at an inflammatory site may be limited by O2 concentration.