Lactoferrin-catalysed hydroxyl radical production. Additional requirement for a chelating agent

Superoxide: The enigmatic chemical chameleon in neutrophil biology

The burst of superoxide produced when neutrophils phagocytose bacteria is the defining biochemical feature of these abundant immune cells. But 50 years since this discovery, the vital role superoxide plays in host defense has yet to be defined. Superoxide is neither bactericidal nor is it just a source of hydrogen peroxide. This simple free radical does, however, have remarkable chemical dexterity. Depending on its environment and reaction partners, superoxide can act as an oxidant, a reductant, a nucleophile, or an enzyme substrate. We outline the evidence that inside phagosomes where neutrophils trap, kill, and digest bacteria, superoxide will react preferentially with the enzyme myeloperoxidase, not the bacterium. By acting as a cofactor, superoxide will sustain hypochlorous acid production by myeloperoxidase. As a substrate, superoxide may give rise to other forms of reactive oxygen. We contend that these interactions hold the key to understanding the precise role superoxide plays in neutrophil biology. State-of-the-art techniques in mass spectrometry, oxidant-specific fluorescent probes, and microscopy focused on individual phagosomes are needed to identify bactericidal mechanisms driven by superoxide. This work will undoubtably lead to fascinating discoveries in host defense and give a richer understanding of superoxide's varied biology.

Staphylococcus aureus, phagocyte NADPH oxidase and chronic granulomatous disease

Dysfunction of phagocytes is a relevant risk factor for staphylococcal infection. The most common hereditary phagocyte dysfunction is chronic granulomatous disease (CGD), characterized by impaired generation of reactive oxygen species (ROS) due to loss of function mutations within the phagocyte NADPH oxidase NOX2. Phagocytes ROS generation is fundamental to eliminate pathogens and to regulate the inflammatory response to infection. CGD is characterized by recurrent and severe bacterial and fungal infections, with Staphylococcus aureus as the most frequent pathogen, and skin and lung abscesses as the most common clinical entities. Staphylococcus aureus infection may occur in virtually any human host, presumably because of the many virulence factors of the bacterium. However, in the presence of functional NOX2, staphylococcal infections remain rare and are mainly linked to breaches of the skin barrier. In contrast, in patients with CGD, S. aureus readily survives and frequently causes clinically apparent disease. Astonishingly, little is known why S. aureus, which possesses a wide range of antioxidant enzymes (e.g. catalase, SOD), is particularly sensitive to control through NOX2. In this review, we will evaluate the discovery of CGD and our present knowledge of the role of NOX2 in S. aureus infection.

The function of the NADPH oxidase of phagocytes and its relationship to other NOXs in plants, invertebrates, and mammals

The NADPH (nicotinamide adenine dinucleotide phosphate) oxidase (NOX) of ‘professional’ phagocytic cells transfers electrons across the wall of the phagocytic vacuole, forming superoxide in the lumen. It is generally accepted that this system promotes microbial killing through the generation of reactive oxygen species and through the activity of myeloperoxidase. An alternative scenario exists in which the passage of electrons across the membrane alters the pH and generates a charge that drives ions into, and out of, the vacuole. It is proposed that the primary function of the oxidase is to produce these pH changes and ion fluxes, and the issues surrounding these processes are considered. The neutrophil oxidase is the prototype of a whole family of NOXs that exist throughout biology, from plants to man, which might function, at least in part, in a similar fashion. Some examples of how these other NOXs might influence ion fluxes are examined.

Antimicrobial proteins and polypeptides in pulmonary innate defence

Inspired air contains a myriad of potential pathogens, pollutants and inflammatory stimuli. In the normal lung, these pathogens are rarely problematic. This is because the epithelial lining fluid in the lung is rich in many innate immunity proteins and peptides that provide a powerful anti-microbial screen. These defensive proteins have anti-bacterial, anti- viral and in some cases, even anti-fungal properties. Their antimicrobial effects are as diverse as inhibition of biofilm formation and prevention of viral replication. The innate immunity proteins and peptides also play key immunomodulatory roles. They are involved in many key processes such as opsonisation facilitating phagocytosis of bacteria and viruses by macrophages and monocytes. They act as important mediators in inflammatory pathways and are capable of binding bacterial endotoxins and CPG motifs. They can also influence expression of adhesion molecules as well as acting as powerful anti-oxidants and anti-proteases. Exciting new antimicrobial and immunomodulatory functions are being elucidated for existing proteins that were previously thought to be of lesser importance. The potential therapeutic applications of these proteins and peptides in combating infection and preventing inflammation are the subject of ongoing research that holds much promise for the future.

How neutrophils kill microbes

Neutrophils provide the first line of defense of the innate immune system by phagocytosing, killing, and digesting bacteria and fungi. Killing was previously believed to be accomplished by oxygen free radicals and other reactive oxygen species generated by the NADPH oxidase, and by oxidized halides produced by myeloperoxidase. We now know this is incorrect. The oxidase pumps electrons into the phagocytic vacuole, thereby inducing a charge across the membrane that must be compensated. The movement of compensating ions produces conditions in the vacuole conducive to microbial killing and digestion by enzymes released into the vacuole from the cytoplasmic granules.

Mass spectrometric characterization of transferrins and their fragments derived by reduction of disulfide bonds

Mass spectrometry, proteomics, and protein chemistry methods are used to characterize the cleavage products of 79 kDa transferrin proteins induced by iron-catalyzed oxidation, including a novel C-terminal polypeptide released upon disulfide reduction. Top-down electrospray ionization tandem mass spectrometry (ESI-MS/MS) of intact multiply-charged transferrin from a variety of species (human, bovine, rabbit, chicken) performed on a quadrupole time-of-flight mass spectrometer yields multiply-charged b(n)-products originating near residues 56-69 from the N-terminal region, in addition to their complementary y(n)-products. Incubation of transferrin with reductants, such as dithiothreitol (DTT) or tris(2-carboxyethyl)-phosphine (TCEP), yields an increase in multiple charging observed by ESI-MS and an increase in molecular weight consistent with disulfide reduction. However, mammalian transferrins release a 6-8 kDa fragment upon disulfide reduction. Protein acetylation and MS/MS sequencing demonstrate that the fragment originates from the C-terminus of the protein, and that it is a separate polypeptide linked via three disulfide bonds to the main transferrin chain. The existence of a separate C-terminal chain is not annotated in protein sequence databases and, to date, has not been reported in the literature. Iron-catalyzed cleavage induces fragments originating from both the N- and C-terminus of transferrin.

Lactoferrin and host defense

Lactoferrin is a multifunctional member of the transferrin family of nonheme iron-binding glycoproteins. Lactoferrin is found at the mucosal surface where it functions as a prominent component of the first line of host defense against infection and inflammation. The protein is also an abundant component of the specific granules of neutrophils and can be released into the serum upon neutrophil degranulation. While the iron-binding properties were originally believed to be solely responsible for the host defense properties ascribed to lactoferrin, it is now known that other mechanisms contribute to the broad spectrum anti-infective and anti-inflammatory roles of this protein. In this article, current information on the functions and mechanism of action of lactoferrin are reviewed, with particular emphasis on the activities that contribute to this protein's role in host defense. In addition, studies demonstrating that lactoferrin inhibits allergen-induced skin inflammation in both mice and humans, most likely secondary to TNF-alpha (tumor necrosis factor alpha) production, are summarized. Collectively, these results suggest that lactoferrin functions as a key component of mammalian host defense at the mucosal surface.

Assessment of structural features of the pseudomonas siderophore pyochelin required for its ability to promote oxidant-mediated endothelial cell injury

We previously showed that iron chelated to the Pseudomonas aeruginosa siderophore pyochelin enhances oxidant-mediated injury to pulmonary artery endothelial cells by catalyzing hydroxyl radical (HO(*)) formation. Therefore, we examined pyochelin structural/chemical features that may be important in this process. Five pyochelin analogues were examined for (i) capacity to accentuate oxidant-mediated endothelial cell injury, (ii) HO(*) catalytic ability, (iii) iron transfer to endothelial cells, and (iv) hydrophobicity. All compounds catalyzed similar HO(*) production, but only the hydrophobic ones containing a thiazolidine ring enhanced cell injury. Transfer of iron to endothelial cells did not correlate with cytotoxicity. Finally, binding of Fe(3+) by pyochelin led to Fe(2+) formation, perhaps explaining how Fe(3+)-pyochelin augments H(2)O(2)-mediated cell injury via HO(*) formation. The ability to bind iron in a catalytic form and the molecule's thiazolidine ring, which increases its hydrophobicity, are key to pyochelin's cytotoxicity. Reduction of Fe(3+) to Fe(2+) may also be important.

Lactoferrin binds CpG-containing oligonucleotides and inhibits their immunostimulatory effects on human B cells

Unmethylated CpG dinucleotide motifs in bacterial DNA, as well as oligodeoxynucleotides (ODN) containing these motifs, are potent stimuli for many host immunological responses. These CpG motifs may enhance host responses to bacterial infection and are being examined as immune activators for therapeutic applications in cancer, allergy/asthma, and infectious diseases. However, little attention has been given to processes that down-modulate this response. The iron-binding protein lactoferrin is present at mucosal surfaces and at sites of infection. Since lactoferrin is known to bind DNA, we tested the hypothesis that lactoferrin will bind CpG-containing ODN and modulate their biological activity. Physiological concentrations of lactoferrin (regardless of iron content) rapidly bound CpG ODN. The related iron-binding protein transferrin lacked this capacity. ODN binding by lactoferrin did not require the presence of CpG motifs and was calcium independent. The process was inhibited by high salt, and the highly cationic N-terminal sequence of lactoferrin (lactoferricin B) was equivalent to lactoferrin in its ODN-binding ability, suggesting that ODN binding by lactoferrin occurs via charge-charge interaction. Heparin and bacterial LPS, known to bind to the lactoferricin component of lactoferrin, also inhibited ODN binding. Lactoferrin and lactoferricin B, but not transferrin, inhibited CpG ODN stimulation of CD86 expression in the human Ramos B cell line and decreased cellular uptake of ODN, a process required for CpG bioactivity. Lactoferrin binding of CpG-containing ODN may serve to modulate and terminate host response to these potent immunostimulatory molecules at mucosal surfaces and sites of bacterial infection.

gamma-Glutamyltransferase dependent generation of reactive oxygen species from a glutathione/transferrin system

In the presence of molecular oxygen and iron or copper ions, a number of antioxidants paradoxically generate reactive oxygen species (ROS) leading to free radical damage of nucleic acids and oxidative modification of lipids and proteins. The present work demonstrates that the combination of three components, which are often considered as part of an antioxidant protection system, can generate ROS. Purified human gamma-glutamyltransferase (GGT) in the presence of 2 mM glutathione (GSH) and 80 microM transferrin, as an iron source, at pH 7.4 generates ROS, as measured by chemiluminescence of luminol. Initiated by the addition of purified GGT, generation of ROS reached a maximal rate in the first 6 min. Intensity of the chemiluminescence was only slightly enhanced by addition of 200 microM hydrogen peroxide. Generation of ROS was also investigated in transfected V79 cells expressing human GGT. In comparison with GGT negative V79 cells, only recombinant cells expressing a high level of GGT on the cell membrane were able to generate ROS. Generation of ROS in these cells reached a maximum within 2 min and was enhanced by 200 microM hydrogen peroxide. We further confirmed the hypothesis that cysteinylglycine (CysGly), a product of GGT/GSH reaction, identified by high-performance liquid chromatography, but not GSH, was responsible for ROS formation initiated by the reductive release of iron from transferrin. These data clearly indicate that under physiological conditions, GGT is directly involved in ROS generation.

Binding of iron and inhibition of iron-dependent oxidative cell injury by the "calcium chelator" 1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid (BAPTA)

A role for increases in intracellular calcium (Ca2+) has been suggested in the pathophysiology of various forms of oxidant-mediated cell injury. In recent studies, we found that iron bound to the Pseudomonas aeruginosa siderophore, pyochelin, augments oxidant-mediated endothelial cell injury by catalyzing the formation of hydroxyl radical (HO.). To investigate the role of Ca2+ in this process, the effects of two Ca2+ chelating agents, Fura-2 and 1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid (BAPTA), were assessed. BAPTA, but not Fura-2, was protective against H2O2/ferripyochelin-mediated injury. Subsequent data suggested that chelation of iron rather than Ca2+ by BAPTA was most likely responsible. Spectrophotometry demonstrated that both ferrous (Fe2+) and ferric (Fe3+) iron formed a complex with BAPTA. The affinity of BAPTA for the metals was Fe3+ > Ca2+ > Fe2+. BAPTA was found to decrease markedly iron-catalyzed production of HO. and/or ferryl species when analyzed by spin trapping. Although our results do not definitively prove that BAPTA protects endothelial cells from ferripyochelin-associated damage by chelating iron, these data indicate that caution must be exercised in utilizing protective effects of intracellular "Ca2+ chelating agents" as evidence for a role of alterations in cellular Ca2+ levels in experimental conditions in which iron-mediated oxidant production is also occurring.

Augmentation of oxidant injury to human pulmonary epithelial cells by the Pseudomonas aeruginosa siderophore pyochelin

Pseudomonas aeruginosa causes acute and chronic infections of the human lung, with resultant tissue injury. We have previously shown that iron bound to pyochelin, a siderophore secreted by the organism to acquire iron, is an efficient catalyst for hydroxyl radical (HO.) formation and augments injury to pulmonary artery endothelial cells resulting from their exposure to superoxide (O2.) and/or H2O2. Sources for O2-. and H2O2 included phorbol myristate acetate (PMA)-stimulated neutrophils and pyocyanin. Pyocyanin, another P. aeruginosa secretory product, undergoes cell-mediated redox, thereby forming O2-. and H2O2. In P. aeruginosa lung infections, damage to airway epithelial cells is probably more extensive than that to endothelial cells. Therefore, we examined whether ferripyochelin also augments oxidant-mediated damage to airway epithelial cells. A549 cells, a human type II alveolar epithelial cell line, was exposed to H2O2, PMA-stimulated neutrophils, or pyocyanin, and injury was determined by release of 51Cr from prelabeled cells. Ferripyochelin significantly increased (> 10-fold) oxidant-mediated cell injury regardless of whether H2O2, neutrophils, or pyocyanin was employed. Apo-pyochelin was not effective, and ferripyochelin was not toxic by itself at the concentrations employed. Spin trapping with alpha-(4-pyrridyl-1-oxide)-N-t-butyl-nitrone-ethanol confirmed the generation of HO., and injury was decreased by a variety of antioxidants, including superoxide dismutase, catalase, and dimethylthiourea. These data are consistent with the hypothesis that the presence of ferripyochelin at sites of P. aeruginosa lung infection could contribute to tissue injury through its ability to promote HO.-mediated damage to airway epithelial cells.

Role of oxidants in microbial pathophysiology

Reactive oxidant species (superoxide, hydrogen peroxide, hydroxyl radical, hypohalous acid, and nitric oxide) are involved in many of the complex interactions between the invading microorganism and its host. Regardless of the source of these compounds or whether they are produced under normal conditions or those of oxidative stress, these oxidants exhibit a broad range of toxic effects to biomolecules that are essential for cell survival. Production of these oxidants by microorganisms enables them to have a survival advantage in their environment. Host oxidant production, especially by phagocytes, is a counteractive mechanism aimed at microbial killing. However, this mechanism may be contribute to a deleterious consequence of oxidant exposure, i.e., inflammatory tissue injury. Both the host and the microorganism have evolved complex adaptive mechanisms to deflect oxidant-mediated damage, including enzymatic and nonenzymatic oxidant-scavenging systems. This review discusses the formation of reactive oxidant species in vivo and how they mediate many of the processes involved in the complex interplay between microbial invasion and host defense.

Protease cleavage of iron-transferrin augments pyocyanin-mediated endothelial cell injury via promotion of hydroxyl radical formation

Although a number of bacterium- and host-derived factors have been suggested to contribute to the pathogenesis of Pseudomonas aeruginosa-associated tissue injury, the mechanism remains unclear. We have previously shown that protease modification of iron (Fe)-transferrin generates new iron chelates capable of catalyzing hydroxyl radical (.OH) formation from superoxide and hydrogen peroxide. The latter two oxidants are generated during redox cycling of another P. aeruginosa secretory product, pyocyanin. The lung is a major site of P. aeruginosa infection, with damage to local endothelial cells contributing to the pathogenesis of such infections. Endothelial cells are highly susceptible to oxidant-mediated injury. Therefore, we examined whether pseudomonas elastase-cleaved Fe-transferrin and pyocyanin synergistically enhance pulmonary artery endothelial cell injury via .OH formation. By measuring 51Cr release from cultured endothelial cell monolayers, pseudomonas elastase-cleaved Fe-transferrin significantly augmented cell injury resulting from cellular exposure to sublethal concentrations of pyocyanin. This enhancement in injury was not protease specific, as similar results were obtained with pyocyanin in combination with trypsin- or porcine pancreatic elastase-cleaved Fe-transferrin. The association of iron with the transferrin appeared to be necessary in this process. Supporting the involvement of .OH generation via the Haber-Weiss reaction in augmenting cell injury, catalase, dimethyl thiourea, superoxide dismutase, deferoxamine, and dimethyl sulfoxide significantly inhibited cell injury resulting from exposure to pyocyanin and protease-cleaved Fe-transferrin. Furthermore, spin trapping demonstrated the production of .OH in this cellular system. We conclude that .OH formation resulting from the interaction of protease-cleaved Fe-transferrin and endothelial cell redox cycling of pyocyanin may contribute to P. aeruginosa-associated tissue injury via endothelial cell injury.

Protease-cleaved iron-transferrin augments oxidant-mediated endothelial cell injury via hydroxyl radical formation

Previous work has shown that the Pseudomonas-derived protease, pseudomonas elastase (PAE), can modify transferrin to form iron complexes capable of catalyzing the formation of hydroxyl radical (.OH) from neutrophil (PMN)-derived superoxide (.O2-) and hydrogen peroxide (H2O2). As the lung is a major site of Pseudomonas infection, the ability of these iron chelates to augment oxidant-mediated pulmonary artery endothelial cell injury via release of 51Cr from prelabeled cells was examined. Diferrictransferrin previously cleaved with PAE significantly enhanced porcine pulmonary artery endothelial cell monolayer injury from 2.3-6.3 to 15.8-17.0% of maximum, resulting from exposure to H2O2, products of the xanthine/xanthine oxidase reaction, or PMA-stimulated PMNs. Iron associated with transferrin appeared to be responsible for cell injury. Spin trapping and the formation of thiobarbituric acid-reactive 2-deoxyribose oxidation products demonstrated the production of .OH in this system. The addition of catalase, dimethyl thiourea, and the hydrophobic spin trap, alpha-phenyl-n-terbutyl-nitrone, offered significant protection from injury (27.8-58.2%). Since sites of Pseudomonas infection contain other proteases, the ability of porcine pancreatic elastase and trypsin to substitute for PAE was examined. Results were similar to those observed with PAE. We conclude .OH formation resulting from protease alteration of transferrin may serve as a mechanism of tissue injury at sites of bacterial infection and other processes characterized by increased proteolytic activity.

Inhibition of in vitro replication of the oyster parasite Perkinsus marinus by the natural iron chelators transferrin, lactoferrin, and desferrioxamine

The mammalian iron-binding proteins transferrin and lactoferrin, the bactericidal peptide lactoferricin B, and the bacterial siderophore desferrioxamine were tested for their ability to inhibit the in vitro replication of the oyster parasite Perkinsus marinus. All three chelators were effective in reducing the parasite proliferation in a dose-dependent manner. Lactoferricin B, a peptide of lactoferrin that exhibits bactericidal properties unrelated to iron chelation, had no inhibitory activity on the parasite. When the chelators were partially or completely saturated with the appropriate iron equivalents, their inhibitory effects on the parasite proliferation were diminished or abolished accordingly, confirming that this activity was related to the chelator's capacity for iron sequestration. Our results indicate that the parasite has a strong requirement for soluble iron and its growth rates are correlated with iron availability. We propose that excess iron accumulation in the host Crassostrea virginica promotes parasite proliferation. P. marinus may avoid oxidative damage that would compromise its intracellular survival by exhaustion the host's intracellular selected iron pools required for superoxide and hydroxyl radical production.

Proposed mechanisms for the involvement of lactoferrin in the hydrolysis of nucleic acids

Lactoferrin has recently been proposed to have ribonuclease activity in the absence of bound iron. We and others have demonstrated previously that lactoferrin interacts with DNA and will bind a number of transition metal ions via surface-exposed histidyl residues. In the present study, we investigated the possibility that surface-bound copper ions on lactoferrin may catalyze the production of active oxygen species responsible for the hydrolysis of nucleic acids. Purified lactoferrin (apo- and holo-forms) was incubated with CuCl2 in solution to obtain lactoferrin with surface binding sites saturated by Cu(II)ions. the lactoferrin-Cu(II) complex was purified by Bio-Gel P-6 chromatography columns and tested for hydrolytic activity against DNA and RNA as analyzed by agarose gel electrophoresis. Incubation of lactoferrin-Cu(II) complexes with supercoiled plasmid Bluescript II SK DNA led to the rapid formation of relaxed open circular or linear forms of DNA characterized by changed electrophoretic mobility. Lactoferrin with bound Cu(II) also caused extensive degradation of yeast tRNA molecules in the presence of hydrogen peroxide. Covalent modification of surface-exposed histidyl residues by carboxyethylation with diethylpyrocarbonate abolished the lactoferrin-associated hydrolytic activity. These results indicate that lactoferrin-bound Cu(II) can indeed facilitate the hydrolysis of DNA and RNA molecules. Copper-binding sites on lactoferrin appear to serve as centers for repeated production of hydroxyl radicals via a Fenton-type Haber-Weiss reaction. Enhanced nuclease activity associated with elevated local concentrations of lactoferrin would promote microbial degradation.

The role of lactoferrin as an anti-inflammatory molecule

The formation of hydroxyl radical via the iron catalyzed Haber-Weiss reaction has been implicated in phagocyte-mediated microbicidal activity and inflammatory tissue injury. The fact that neutrophils contain lactoferrin and mononuclear phagocytes have the capacity to acquire exogenous iron has suggested that iron bound to lactoferrin may influence the nature of free radical products generated by these cells. Over the years the iron-lactoferrin complex has been heralded as both a promoter and inhibitor of hydroxyl radical formation. This manuscript is intended to provide an overview of work performed to date related to this controversy and to present results of a number of preliminary studies which shed further light on the role of lactoferrin in inflammation.

Bovine mammary lactoferrin: implications from messenger ribonucleic acid (mRNA) sequence and regulation contrary to other milk proteins

The regulation of bovine mammary lactoferrin, an important component of the antimicrobial defenses of the mammary gland, is poorly understood compared with the other milk proteins. The complete sequence for bovine lactoferrin mRNA shows it to be highly homologous to other lactoferrins and transferrins. However, regional differences in the deduced AA sequence of bovine lactoferrin compared with human lactoferrin and transferrin imply functional differences between them. Steady-state levels of bovine lactoferrin mRNA (by Northern blot) in the bovine mammary gland indicate that bovine lactoferrin expression is minimal in the developing and lactating gland but is strongly induced by mammary involution. The overall regulation of bovine lactoferrin in the mammary gland appears to be contrary to that of the other milk proteins. Features identified in the mRNA of bovine mammary lactoferrin may contribute to the differences in regulation between lactoferrin and other bovine milk proteins and to differences in concentrations of lactoferrin in milk across species. Lactoferrin secretion by bovine mammary cells grown in vitro does not appear to be dependent on prolactin and shows regulation by substratum, serum, and cell population to be different from that for casein. In contrast to casein, efficient secretion of lactoferrin from the cell does not require detachment of collagen substratum.

Inflammatory response to cardiopulmonary bypass

Cardiac operations with cardiopulmonary bypass cause a systemic inflammatory response, which can lead to organ injury and postoperative morbidity. Causative factors include surgical trauma, contact of blood with the extracorporeal circuit, and lung reperfusion injury on discontinuing bypass. Advances in immunological techniques have allowed measurement of both plasma and intracellular components of this multifaceted perioperative response. This includes activation of the complement, coagulation, fibrinolytic, and kallikrein cascades, activation of neutrophils with degranulation and protease enzyme release, oxygen radical production, and the synthesis of various cytokines from mononuclear cells (including tumor necrosis factor, interleukin-1, and interleukin-6). Advances in our understanding of the interactions between these markers of cellular and humoral responses to cardiopulmonary bypass will enable more effective intervention to reduce the deleterious effects and improve the outlook for patients undergoing cardiac operations beyond the 1990s.

Interaction of the Pseudomonas aeruginosa secretory products pyocyanin and pyochelin generates hydroxyl radical and causes synergistic damage to endothelial cells. Implications for Pseudomonas-associated tissue injury

Pyocyanin, a secretory product of Pseudomonas aeruginosa, has the capacity to undergo redox cycling under aerobic conditions with resulting generation of superoxide and hydrogen peroxide. By using spin trapping techniques in conjunction with electron paramagnetic resonance spectrometry (EPR), superoxide was detected during the aerobic reduction of pyocyanin by NADH or porcine endothelial cells. No evidence of hydroxyl radical formation was detected. Chromium oxalate eliminated the EPR spectrum of the superoxide-derived spin adduct resulting from endothelial cell exposure to pyocyanin, suggesting superoxide formation close to the endothelial cell plasma membrane. We have previously reported that iron bound to the P. aeruginosa siderophore pyochelin (ferripyochelin) catalyzes the formation of hydroxyl free radical from superoxide and hydrogen peroxide via the Haber-Weiss reaction. In the present study, spin trap evidence of hydroxyl radical formation was detected when NADH and pyocyanin were allowed to react in the presence of ferripyochelin. Similarly, endothelial cell exposure to pyocyanin and ferripyochelin also resulted in hydroxyl radical production which appeared to occur in close proximity to the cell surface. As assessed by 51Cr release, endothelial cells which were treated with pyocyanin or ferripyochelin alone demonstrated minimal injury. However, endothelial cell exposure to the combination of pyochelin and pyocyanin resulted in 55% specific 51Cr release. Injury was not observed with the substitution of iron-free pyochelin and was diminished by the presence of catalase or dimethyl thiourea. These data suggest the possibility that the P. aeruginosa secretory products pyocyanin and pyochelin may act synergistically via the generation of hydroxyl radical to damage local tissues at sites of pseudomonas infection.

Ferritin as a source of iron for oxidative damage

The generation of deleterious activated oxygen species capable of damaging DNA, lipids, and proteins requires a catalyst such as iron. Once released, ferritin iron is capable of catalyzing these reactions. Thus, agents that promote iron release may lead to increased oxidative damage. The superoxide anion formed enzymatically, radiolytically, via metal-catalyzed oxidations, or by redox cycling xenobiotics reductively mobilizes ferritin iron and promotes oxidative damage. In addition, a growing list of compounds capable of undergoing single electron oxidation/reduction reactions exemplified by paraquat, adriamycin, and alloxan have been reported to release iron from ferritin. Because the rapid removal of iron from ferritin requires reduction of the iron core, it is not surprising that the reduction potential of a compound is a primary factor that determines whether a compound will mobilize ferritin iron. The reduction potential does not, however, predict the rate of iron release. Therefore, ferritin-dependent oxidative damage may be involved in the pathogenesis of diseases where increased superoxide formation occurs and the toxicity of chemicals that increase superoxide production or have an adequate reduction potential to mobilize ferritin iron.

Spin traps inhibit formation of hydrogen peroxide via the dismutation of superoxide: implications for spin trapping the hydroxyl free radical

To enhance the sensitivity of EPR spin trapping for radicals of limited reactivity, high concentrations (10-100 mM) of spin traps are routinely used. We noted that in contrast to results with other hydroxyl radical detection systems, superoxide dismutase (SOD) often increased the amount of hydroxyl radical-derived spin adducts of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) produced by the reaction of hypoxanthine, xanthine oxidase and iron. One possible explanation for these results is that high DMPO concentrations (approximately 100 mM) inhibit dismutation of superoxide (O2.-) to hydrogen peroxide (H2O2). Therefore, we examined the effect of DMPO on O2.- dismutation to H2O2. Lumazine +/- 100 mM DMPO was placed in a Clark oxygen electrode following which xanthine oxidase was added. The amount of H2O2 formed in this reaction was determined by introducing catalase and measuring the amount of generated via O2.- dismutation as compared to direct divalent O2 reduction. In the presence of 100 mM DMPO, H2O2 generation decreased 43%. DMPO did not scavenge H2O2 nor alter the rate of O2.- production. The effect of DMPO was concentration-dependent with inhibition of H2O2 production observed at [DMPO] greater than 10 mM. Inhibition of H2O2 production by DMPO was not observed if SOD was present or if the rate of O2.- formation increased. The spin trap 2-methyl-2-nitroso-propane (MNP, 10 mM) also inhibited H2O2 formation (81%). However, alpha-phenyl-N-tert-butylnitrone (PBN, 10 mM), 3,3,5,5 tetramethyl-1-pyrroline N-oxide (M4PO, 100 mM), alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN, 100 mM) had no effect. These data suggest that in experimental systems in which the rate of O2.- generation is low, formation of H2O2 and thus other H2O2-derived species (e.g., OH) may be inhibited by commonly used concentrations of some spin traps. Thus, under some experimental conditions spin traps may potentially prevent production of the very free radical species they are being used to detect.

Pseudomonas and neutrophil products modify transferrin and lactoferrin to create conditions that favor hydroxyl radical formation

In vivo most extracellular iron is bound to transferrin or lactoferrin in such a way as to be unable to catalyze the formation of hydroxyl radical from superoxide (.O2-) and hydrogen peroxide (H2O2). At sites of Pseudomonas aeruginosa infection bacterial and neutrophil products could possibly modify transferrin and/or lactoferrin forming catalytic iron complexes. To examine this possibility, diferrictransferrin and diferriclactoferrin which had been incubated with pseudomonas elastase, pseudomonas alkaline protease, human neutrophil elastase, trypsin, or the myeloperoxidase product HOCl were added to a hypoxanthine/xanthine oxidase .O2-/H2O2 generating system. Hydroxyl radical formation was only detected with pseudomonas elastase treated diferrictransferrin and, to a much lesser extent, diferriclactoferrin. This effect was enhanced by the combination of pseudomonas elastase with other proteases, most prominently neutrophil elastase. Addition of pseudomonas elastase-treated diferrictransferrin to stimulated neutrophils also resulted in hydroxyl radical generation. Incubation of pseudomonas elastase with transferrin which had been selectively iron loaded at either the NH2- or COOH-terminal binding site yielded iron chelates with similar efficacy for hydroxyl radical catalysis. Pseudomonas elastase and HOCl treatment also decreased the ability of apotransferrin to inhibit hydroxyl radical formation by a Fe-NTA supplemented hypoxanthine/xanthine oxidase system. However, apotransferrin could be protected from the effects of HOCl if bicarbonate anion was present during the incubation. Apolactoferrin inhibition of hydroxyl radical generation was unaffected by any of the four proteases or HOCl. Alteration of transferrin by enzymes and oxidants present at sites of pseudomonas and other bacterial infections may increase the potential for local hydroxyl radical generation thereby contributing to tissue injury.

Ferritin-dependent lipid peroxidation by stimulated neutrophils: inhibition by myeloperoxidase-derived hypochlorous acid but not by endogenous lactoferrin

Human neutrophils stimulated with phorbol myristate acetate or formylmethionylleucylphenylalanine caused superoxide-dependent release of iron from feritin, measured as the formation of a ferrous-ferrozine complex. The stimulated cells also caused ferritin-dependent peroxidation of phospholipid liposomes. Peroxidation was inhibited by lactoferrin, but only at concentrations considerably in excess of what could be achieved by release of endogenous lactoferrin. Peroxidation was enhanced by catalase and methionine, especially when stimulants that release myeloperoxidase were used. Peroxidation was inhibited by added myeloperoxidase. These results are explained by myeloperoxidase catalysing the formation of hypochlorous acid (HOCl) and the HOCl reacting with the lipid to inhibit peroxidation. Thus, neutrophils are able to use ferritin to promote lipid peroxidation. This may be limited under some conditions by iron binding to lactoferrin or transferrin, and more generally by reactions of the lipid with myeloperoxidase-derived HOCl. However, the latter reactions themselves may be harmful.

Prooxidant activity of transferrin and lactoferrin

Acceleration of the autoxidation of Fe2+ by apotransferrin or apolactoferrin at acid pH is indicated by the disappearance of Fe2+, the uptake of oxygen, and the binding of iron to transferrin or lactoferrin. The product(s) formed oxidize iodide to an iodinating species and are bactericidal to Escherichia coli. Toxicity to E. coli by FeSO4 (10(-5) M) and human apotransferrin (100 micrograms/ml) or human apolactoferrin (25 micrograms/ml) was optimal at acid pH (4.5-5.0) and with logarithmic phase organisms. Both the iodinating and bactericidal activities were inhibited by catalase and the hydroxyl radical (OH.) scavenger mannitol, whereas superoxide dismutase was ineffective. NaCl at 0.1 M inhibited bactericidal activity, but had little or no effect on iodination. Iodide increased the bactericidal activity of Fe2+ and apotransferrin or apolactoferrin. The formation of OH.was suggested by the formation of the OH.spin-trap adduct (5,5-dimethyl-1-pyroline N-oxide [DMPO]/OH)., with the spin trap DMPO and the formation of the methyl radical adduct on the further addition of dimethyl sulfoxide. (DMPO/OH).formation was inhibited by catalase, whereas superoxide dismutase had little or no effect. These findings suggest that Fe2+ and apotransferrin or apolactoferrin can generate OH.via an H2O2 intermediate with toxicity to microorganisms, and raise the possibility that such a mechanism may contribute to the microbicidal activity of phagocytes.

Possible role of bacterial siderophores in inflammation. Iron bound to the Pseudomonas siderophore pyochelin can function as a hydroxyl radical catalyst

Tissue injury has been linked to neutrophil associated hydroxyl radical (.OH) generation, a process that requires an exogenous transition metal catalyst such as iron. In vivo most iron is bound in a noncatalytic form. To obtain iron required for growth, many bacteria secrete iron chelators (siderophores). Since Pseudomonas aeruginosa infections are associated with considerable tissue destruction, we examined whether iron bound to the Pseudomonas siderophores pyochelin (PCH) and pyoverdin (PVD) could act as .OH catalysts. Purified PCH and PVD were iron loaded (Fe-PCH, Fe-PVD) and added to a hypoxanthine/xanthine oxidase superoxide- (.O2-) and hydrogen peroxide (H2O2)-generating system. Evidence for .OH generation was then sought using two different spin-trapping agents (5.5 dimethyl-pyrroline-1-oxide or N-t-butyl-alpha-phenylnitrone), as well as the deoxyribose oxidation assay. Regardless of methodology, .OH generation was detected in the presence of Fe-PCH but not Fe-PVD. Inhibition of the process by catalase and/or SOD suggested .OH formation with Fe-PCH occurred via the Haber-Weiss reaction. Similar results were obtained when stimulated neutrophils were used as the source of .O2- and H2O2. Addition of Fe-PCH but not Fe-PVD to stimulated neutrophils yielded .OH as detected by the above assay systems. Since PCH and PVD bind ferric (Fe3+) but not ferrous (Fe2+) iron, .OH catalysis with Fe-PCH would likely involve .O2(-)-mediated reduction of Fe3+ to Fe2+ with subsequent release of "free" Fe2+. This was confirmed by measuring formation of the Fe2(+)-ferrozine complex after exposure of Fe-PCH, but not Fe-PVD, to enzymatically generated .O2-. These data show that Fe-PCH, but not Fe-PVD, is capable of catalyzing generation of .OH. Such a process could represent as yet another mechanism of tissue injury at sites of infection with P. aeruginosa.

Inhibition of Plasmodium falciparum growth by a synthetic iron chelator

The susceptibility of the chloroquine-resistant malaria parasite Plasmodium falciparum (FCR-3) to a pyridoxal-based iron chelator was tested. 10 microM of the chelator 1[N-ethoxycarbonylmethyl-pyridoxy-lidenium]-2-[2'-pyri dyl] hydrazine bromide (code name L2-9) effectively inhibited growth in vitro of the parasites. Presaturation of the chelator with either ferric or ferrous iron partially blocked the inhibitory effect. Two hours' exposure of parasites to 20 microM L2-9 was sufficient to inhibit their growth irreversibly. Desferrioxamine blocked the inhibitory effect of L2-9. It is suggested that the chelator may be acting by generating free radicals in complexing intracellular iron.

Anti-inflammatory systems in human milk

Human milk is characterized not only by a complex host defense system that prevents the colonization and proliferation of common microbial pathogens that may pervade the alimentary tract and respiratory tract of the infant but also by a paucity of inflammatory agents and an array of anti-phlogistic factors. Clinical observations support the notion that the protection provided by human milk involves not only antimicrobial factors, but also anti-inflammatory agents. The major anti-inflammatory agents include enzymes that degrade mediators of inflammation, anti-proteases, lysozyme, lactoferrin, secretory IgA and a number of antioxidants including cysteine, ascorbate, alpha-tocopherol, and beta-carotene. It is pertinent that most of these factors are either absent or poorly represented in cow's milk or other artificial feedings that substitute for breast feeding and that the attainment of adult serum levels of some of these antioxidants in early infancy is dependent upon breast feeding. It may be that the provision of these antioxidants may help to protect the recipient's developing immunologic system which is quite susceptible to oxidant damage. The absence of breast feeding will thus deprive the infant of valuable protection against common enteric-respiratory disorders and their inflammatory consequences. It should be pointed out that the protective systems in human milk including the anti-inflammatory components may not be completely delineated, and that little is known of the in vivo fate of the factors and precisely how they protect the recipient. Those questions should form the basis of important research in the next decades.

Neutrophil degranulation inhibits potential hydroxyl-radical formation. Relative impact of myeloperoxidase and lactoferrin release on hydroxyl-radical production by iron-supplemented neutrophils assessed by spin-trapping techniques

Hydroxyl radical (.OH) formation by neutrophils in vitro requires exogenous iron. Two recent studies [Britigan, Rosen, Thompson, Chai & Cohen (1986) J. Biol. Chem. 261, 17026-17032; Winterbourn (1987) J. Clin. Invest. 78, 545-550] both reported that neutrophil degranulation could potentially inhibit the formation of .OH, but differed in their conclusions as to the responsible factor, myeloperoxidase (MPO) or lactoferrin (LF). By using a previously developed spin-trapping system which allows specific on-line detection of superoxide anion (O2-) and .OH production, the impact of MPO and LF release on neutrophil .OH production was compared. When iron-diethylenetriaminepenta-acetic acid-supplemented neutrophils were stimulated with phorbol myristate acetate or opsonized zymosan, .OH formation occurred, but terminated prematurely in spite of continued O2- generation. Inhibition of MPO by azide increased the magnitude, but not the duration, of .OH formation. No azide effect was noted when MPO-deficient neutrophils were used. Anti-LF antibody increased both the magnitude and duration of .OH generation. Pretreatment of neutrophils with cytochalasin B to prevent phagosome formation did not alter the relative impact of azide or anti-LF on neutrophil .OH production. An effect of azide or anti-LF on spin-trapped-adduct stability was eliminated as a confounding factor. These data indicate that neutrophils possess two mechanisms for limiting .OH production. Implications for neutrophil-derived oxidant damage are discussed.

Iron and oxygen: a biologically damaging mixture

Iron is a remarkably useful metal in Nature, but iron ions not safely sequestered in storage or transport proteins are hazardous because they can stimulate damaging free radical reactions. Biological examples of these are Fenton Chemistry leading to the formation of highly reactive species, such as the hydroxyl radical (.OH) and the ferryl ion (FeO2+), and lipid peroxidation. The need to conserve body iron stores has closely evolved with an essential requirement for antioxidant protection and, several 'acute-phase' proteins involved in iron metabolism such as caeruloplasmin, haptoglobins and haemopexin in collaboration with the iron binding proteins transferrin and lactoferrin contribute to our defense against oxidative damage.

The superoxide-dependent transfer of iron from ferritin to transferrin and lactoferrin

By the use of gel filtration and [59Fe]ferritin, apotransferrin and apolactoferrin were shown to take up iron released from ferritin by superoxide generated by hypoxanthine and xanthine oxidase. Apotransferrin also inhibited uptake of released iron by ferrozine. Ferritin and the xanthine oxidase system induced lipid peroxidation in phospholipid liposomes. This peroxidation was inhibited by apotransferrin or apolactoferrin. Thus, although superoxide and other free radicals can release iron from ferritin, either iron-binding protein, if present, should take up this iron and prevent its catalysing subsequent oxidative reactions.

Susceptibilities of lactoferrin and transferrin to myeloperoxidase-dependent loss of iron-binding capacity

Apolactoferrin and apotransferrin lost their ability to subsequently bind iron when exposed to an excess of either HOCl or myeloperoxidase plus H2O2 and Cl-. Apolactoferrin, however, was more resistant than apotransferrin. By oxidizing a mixture of the two proteins, then separating them by immunoprecipitation, the difference in susceptibility was shown to be due to the greater reactivity of transferrin iron-binding groups, rather than protective groups on the lactoferrin molecule. The iron-saturated proteins were much more resistant to oxidative modification than the apoproteins. The greater resistance of apolactoferrin should be advantageous for maintaining its iron binding capacity when co-released with myeloperoxidase and reactive oxygen species from stimulated neutrophils.

Do humans neutrophils form hydroxyl radical? Evaluation of an unresolved controversy

Hydroxyl radical is a potent oxidizing agent of potential importance in human pathobiology. Since neutrophilic phagocytes make superoxide and hydrogen peroxide during phagocytosis, it has been proposed that hydroxyl radical is also formed. In this paper we review the literature which supports or refutes formation of hydroxyl radical by neutrophils and the mechanism(s) by which this radical might be formed. We conclude that there is no definitive proof for hydroxyl radical formation by neutrophils. In fact, neutrophil release of lactoferrin and myeloperoxidase appears to limit formation of this radical. Future studies are likely to determine whether superoxide released by neutrophils interacts with target substrates to allow formation of hydroxyl radical.

Inhibition of heme-promoted enzymatic lipid peroxidation by desferrioxamine and EDTA

Oxyhemoglobin, methemoglobin and hematin were found to catalyze xanthine-oxidase-induced peroxidation of phospholipid liposomes, while oxy- and metmyoglobin were inactive in this respect. The peroxidation was inhibited by desferrioxamine and by EDTA. Peroxidation catalyzed by 0.4 microM oxyhemoglobin was decreased by 50% by approximately 2 microM desferrioxamine or 20 microM EDTA and completely inhibited by 10 microM desferrioxamine or 100 microM EDTA. Inhibition of hemoglobin-catalyzed peroxidation was not accompanied by any changes in the absorbance spectra of hemoglobin, indicating that the heme iron was not withdrawn by the inhibitor. Inhibition of hematin-catalyzed peroxidation by desferrioxamine may have been due to iron chelation and removal, as judged from changes in absorbance spectra. The peroxidation was apparently not dependent on hydrogen peroxide since catalase did not inhibit peroxidation but on the contrary promoted it in some cases.

The effects of dihydroxyfumarate on isolated rabbit papillary muscle function: evidence for an iron dependent non-hydroxyl radical mechanism

To delineate the active free radical species mediating the toxic effects of autoxidizing dihydroxyfumarate (DHF), isolated rabbit right ventricular papillary muscles were exposed to 4.5 mM DHF in the presence of FeCl3, ADP and bovine albumin. In the absence of free radical scavengers a 47.3 +/- 11.5% (mean +/- standard deviation) depression in contractile force was noted over 60 minutes. Neither the combination of superoxide dismutase (SOD) 3,200 u/cc and catalase (CAT) 2,950 u/cc nor mannitol 0.1 M provided statistically significant protection. Deferoxamine mesylate (DFX) 10 mg/cc (15 mM) did provide significant protection of muscle function both in the presence and absence of SOD and CAT (p less than 0.01). The degree of protection conferred by DFX alone was statistically similar to that of DFX with SOD and CAT. This data suggests the involvement of an iron-oxygen complex not dependent on superoxide or hydrogen peroxide for its formation and not readily scavenged by mannitol. The perferryl ion may be representative of such a species. Alternatively, a reactive complex similar to the 'Crypto-OH' radical proposed by Youngman may be formed by the reaction of DHF with iron and oxygen.

1018 - ACTIVATION OF OXYGEN BY METAL COMPLEXES AND ITS RELEVANCE TO AUTOXIDATIVE PROCESSES IN LIVING SYSTEMS

Aspects of the kinetics and thermodynamics of the iron-catalysed Haber-Weiss reaction are discussed with special emphasis on the potential in vivo sources of iron. In addition, the properties of the iron chelates that inhibit the Haber-Weiss reaction are considered.