Irina I. Vlasova

Carbon nanotubes degraded by neutrophil myeloperoxidase induce less pulmonary inflammation

We have shown previously that single-walled carbon nanotubes can be catalytically biodegraded over several weeks by the plant-derived enzyme, horseradish peroxidase. However, whether peroxidase intermediates generated inside human cells or biofluids are involved in the biodegradation of carbon nanotubes has not been explored. Here, we show that hypochlorite and reactive radical intermediates of the human neutrophil enzyme myeloperoxidase catalyse the biodegradation of single-walled carbon nanotubes in vitro, in neutrophils and to a lesser degree in macrophages. Molecular modelling suggests that interactions of basic amino acids of the enzyme with the carboxyls on the carbon nanotubes position the nanotubes near the catalytic site. Importantly, the biodegraded nanotubes do not generate an inflammatory response when aspirated into the lungs of mice. Our findings suggest that the extent to which carbon nanotubes are biodegraded may be a major determinant of the scale and severity of the associated inflammatory responses in exposed individuals.

Biodegradation of single-walled carbon nanotubes through enzymatic catalysis.

We show here the biodegradation of single-walled carbon nanotubes through natural, enzymatic catalysis. By incubating nanotubes with a natural horseradish peroxidase (HRP) and low concentrations of H2O2 (approximately 40 microM) at 4 degrees C over 12 weeks under static conditions, we show the increased degradation of nanotube structure. This reaction was monitored via multiple characterization methods, including transmission electron microscopy (TEM), dynamic light scattering (DLS), gel electrophoresis, mass spectrometry, and ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy. These results mark a promising possibility for carbon nanotubes to be degraded by HRP in environmentally relevant settings. This is also tempting for future studies involving biotechnological and natural (plant peroxidases) ways for degradation of carbon nanotubes in the environment.

Nitric Oxide Inhibits Peroxidase Activity of Cytochrome c· Cardiolipin Complex and Blocks Cardiolipin Oxidation

The increased production of NO during the early stages of apoptosis indicates its potential involvement in the regulation of programmed cell death through yet to be identified mechanisms. Recently, an important role for catalytically competent peroxidase form of pentacoordinate cytochrome c (cyt c) in a complex with a mitochondria-specific phospholipid, cardiolipin (CL), has been demonstrated during execution of the apoptotic program. Because the cyt c·CL complex acts as CL oxygenase and selectively oxidizes CL in apoptotic cells in a reaction dependent on the generation of protein-derived (tyrosyl) radicals, we hypothesized that binding and nitrosylation of cyt c regulates CL oxidation. Here we demonstrate by low temperature electron paramagnetic resonance spectroscopy that CL facilitated interactions of ferro- and ferri-states of cyt c with NO and NO–, respectively, to yield a mixture of penta- and hexa-coordinate nitrosylated cyt c. In the nitrosylated cyt c·CL complex, NO chemically reacted with H2O2-activated peroxidase intermediates resulting in their reduction. A dose-dependent quenching of H2O2-induced protein-derived radicals by NO donors was shown using direct electron paramagnetic resonance measurements as well as immuno-spin trapping with antibodies against protein 5,5-dimethyl-1-pyrroline N-oxide-nitrone adducts. In the presence of NO donors, H2O2-induced oligomeric forms of cyt c positively stained for 3-nitrotyrosine confirming the reactivity of NO toward tyrosyl radicals of cyt c. Interaction of NO with the cyt c·CL complex inhibited its peroxidase activity with three different substrates: CL, etoposide, and 3,3′-diaminobenzidine. Given the importance of CL oxidation in apoptosis, mass spectrometry analysis was utilized to assess the effects of NO on oxidation of 1,1′2,2′-tertalinoleoyl cardiolipin. NO effectively inhibited 1,1′2,2′-tertalinoleoyl cardiolipin oxidation catalyzed by the peroxidase activity of cyt c. Thus, NO can act as a regulator of peroxidase activity of cyt c·CL complexes.

A mitochondria-targeted inhibitor of cytochrome c peroxidase mitigates radiation-induced death

The risk of radionuclide release in terrorist acts or exposure of healthy tissue during radiotherapy demand potent radioprotectants/radiomitigators. Ionizing radiation induces cell death by initiating the selective peroxidation of cardiolipin in mitochondria by the peroxidase activity of its complex with cytochrome c leading to release of hemoprotein into the cytosol and commitment to the apoptotic program. Here we design and synthesize mitochondria-targeted triphenylphosphonium-conjugated imidazole-substituted oleic and stearic acids which blocked peroxidase activity of cytochrome c/cardiolipin complex by specifically binding to its heme-iron. We show that both compounds inhibit pro-apoptotic oxidative events, suppress cyt c release, prevent cell death, and protect mice against lethal doses of irradiation. Significant radioprotective/radiomitigative effects of imidazole-substituted oleic acid are observed after pretreatment of mice from 1 hr before through 24 hrs after the irradiation.

Mechanisms of cardiolipin oxidation by cytochrome c : Relevance to Pro- and antiapoptotic functions of etoposide

Execution of apoptotic program in mitochondria is associated with accumulation of cardiolipin peroxidation products required for the release of proapoptotic factors into the cytosol. This suggests that lipid antioxidants capable of inhibiting cardiolipin peroxidation may act as antiapoptotic agents. Etoposide, a widely used antitumor drug and a topoisomerase II inhibitor, is a prototypical inducer of apoptosis and, at the same time, an effective lipid radical scavenger and lipid antioxidant. Here, we demonstrate that cardiolipin oxidation during apoptosis is realized not via a random cardiolipin peroxidation mechanism but rather proceeds as a result of peroxidase reaction in a tight cytochrome c/cardiolipin complex that restrains interactions of etoposide with radical intermediates generated in the course of the reaction. Using low-temperature and ambient-temperature electron paramagnetic resonance spectroscopy of H2O2-induced protein-derived (tyrosyl) radicals and etoposide phenoxyl radicals, respectively, we established that cardiolipin peroxidation and etoposide oxidation by cytochrome c/cardiolipin complex takes place predominantly on protein-derived radicals of cytochrome c. We further show that etoposide can inhibit cytochrome c-catalyzed oxidation of cardiolipin competing with it as a peroxidase substrate. Peroxidase reaction of cytochrome c/cardiolipin complexes causes cross-linking and oligomerization of cytochrome c. With nonoxidizable tetraoleoyl-cardiolipin, the cross-linking occurs via dityrosine formation, whereas bifunctional lipid oxidation products generated from tetralinoleoyl-cardiolipin participate in the production of high molecular weight protein aggregates. Protein aggregation is effectively inhibited by etoposide. The inhibition of cardiolipin peroxidation by etoposide, however, is realized at far higher concentrations than those at which it induces apoptotic cell death. Thus, oxidation of cardiolipin by the cytochrome c/cardiolipin peroxidase complex, which is essential for apoptosis, is not inhibited by proapoptotic concentrations of the drug.

Lung macrophages "digest" carbon nanotubes using a superoxide/peroxynitrite oxidative pathway.

In contrast to short-lived neutrophils, macrophages display persistent presence in the lung of animals after pulmonary exposure to carbon nanotubes. While effective in the clearance of bacterial pathogens and injured host cells, the ability of macrophages to “digest” carbonaceous nanoparticles has not been documented. Here, we used chemical, biochemical, and cell and animal models and demonstrated oxidative biodegradation of oxidatively functionalized single-walled carbon nanotubes via superoxide/NO* → peroxynitrite-driven oxidative pathways of activated macrophages facilitating clearance of nanoparticles from the lung.

Peroxidase Activity of Hemoglobin·Haptoglobin Complexes: COVALENT AGGREGATION AND OXIDATIVE STRESS IN PLASMA AND MACROPHAGES*

As a hemoprotein, hemoglobin (Hb) can, in the presence of H2O2, act as a peroxidase. In red blood cells, this activity is regulated by the reducing environment. For stroma-free Hb this regulation is lost, and the potential for Hb to become a peroxidase is high and further increased by inflammatory cells generating superoxide. The latter can be converted into H2O2 and feed Hb peroxidase activity. Haptoglobins (Hp) bind with extracellular Hb and reportedly weaken Hb peroxidase activity. Here we demonstrate that: (i) Hb peroxidase activity is retained upon binding with Hp; (ii) in the presence of H2O2, Hb·Hp peroxidase complexes undergo covalent cross-linking; (iii) peroxidase activity of Hb·Hp complexes and aggregates consumes reductants such as ascorbate and nitric oxide; (iv) cross-linked Hb·Hp aggregates are taken up by macrophages at rates exceeding those for noncovalently cross-linked Hb·Hp complexes; (v) the engulfed Hb·Hp aggregates activate superoxide production and induce intracellular oxidative stress (deplete endogenous glutathione and stimulate lipid peroxidation); (vi) Hb·Hp aggregates cause cytotoxicity to macrophages; and (vii) Hb·Hp aggregates are present in septic plasma. Overall, our data suggest that under conditions of severe inflammation and oxidative stress, peroxidase activity of Hb·Hp covalent aggregates may cause macrophage dysfunction and microvascular vasoconstriction, which are commonly seen in severe sepsis and hemolytic diseases.

PEGylated single-walled carbon nanotubes activate neutrophils to increase production of hypochlorous acid, the oxidant capable of degrading nanotubes

Perspectives for the use of carbon nanotubes in biomedical applications depend largely on their ability to degrade in the body into products that can be easily cleared out. Carboxylated single-walled carbon nanotubes (c-SWCNTs) were shown to be degraded by oxidants generated by peroxidases in the presence of hydrogen peroxide. In the present study we demonstrated that conjugation of poly(ethylene glycol) (PEG) to c-SWCNTs does not interfere with their degradation by peroxidase/H(2)O(2) system or by hypochlorite. Comparison of different heme-containing proteins for their ability to degrade PEG-SWCNTs has led us to conclude that the myeloperoxidase (MPO) product hypochlorous acid (HOCl) is the major oxidant that may be responsible for biodegradation of PEG-SWCNTs in vivo. MPO is secreted mainly by neutrophils upon activation. We hypothesize that SWCNTs may enhance neutrophil activation and therefore stimulate their own biodegradation due to MPO-generated HOCl. PEG-SWCNTs at concentrations similar to those commonly used in in vivo studies were found to activate isolated human neutrophils to produce HOCl. Both PEG-SWCNTs and c-SWCNTs enhanced HOCl generation from isolated neutrophils upon serum-opsonized zymosan stimulation. Both types of nanotubes were also found to activate neutrophils in whole blood samples. Intraperitoneal injection of a low dose of PEG-SWCNTs into mice induced an increase in percentage of circulating neutrophils and activation of neutrophils and macrophages in the peritoneal cavity, suggesting the evolution of an inflammatory response. Activated neutrophils can produce high local concentrations of HOCl, thereby creating the conditions favorable for degradation of the nanotubes.

pH-dependent regulation of myeloperoxidase activity.

The balance between peroxidase and chlorinating activities of myeloperoxidase (MPO) is very important for the enhancement of antimicrobial action and prevention of damage caused by hypochlorite. In the present paper, the peroxidase and chlorinating activities have been studied at various pH values. The possibility of using neutrophil protein solution for the evaluation of MPO activity has been demonstrated. It is shown that at neutral pH MPO had higher affinity to peroxidase substrate guaiacol: at pH 7.4, chloride ions did not compete with guaiacol up to the concentration of 150 mM. At acidic pH, chlorinating activity of MPO dominates: only hypochlorite production can be detected at equal chloride and guaiacol concentrations of 15 mM. However, horseradish peroxidase does not exhibit any difference in activity in the presence of chloride ions even at acidic pH values. It was demonstrated by MALDI-TOF mass-spectrometry that the amount of hypochlorite produced is sufficient to modify phospholipids (with formation of Cl-and Br-hydrins and lyso-derivatives) only at acidic pH (5.0). Thus, in the presence of phenolic peroxidase substrate, MPO chlorinating activity can be displayed at acidic pH only. It can lead to elimination of hypochlorite production in normal tissues at neutral pH (7.4) and its enhancement in phagosomes where the pH range is 4.7–6.0.

Myeloperoxidase-dependent oxidation of etoposide in human myeloid progenitor CD34+ cells.

Etoposide is a widely used anticancer drug successfully used for the treatment of many types of cancer in children and adults. Its use, however, is associated with an increased risk of development of secondary acute myelogenous leukemia involving the mixed-lineage leukemia (MLL) gene (11q23) translocations. Previous studies demonstrated that the phenoxyl radical of etoposide can be produced by action of myeloperoxidase (MPO), an enzyme found in developing myeloid progenitor cells, the likely origin for myeloid leukemias. We hypothesized, therefore, that one-electron oxidation of etoposide by MPO to its phenoxyl radical is important for converting this anticancer drug to genotoxic and carcinogenic species in human CD34+ myeloid progenitor cells. In the present study, using electron paramagnetic resonance spectroscopy, we provide conclusive evidence for MPO-dependent formation of etoposide phenoxyl radicals in growth factor-mobilized CD34+ cells isolated from human umbilical cord blood and demonstrate that MPO-induced oxidation of etoposide is amplified in the presence of phenol. Formation of etoposide radicals resulted in the oxidation of endogenous thiols, thus providing evidence for etoposide-mediated MPO-catalyzed redox cycling that may play a role in enhanced etoposide genotoxicity. In separate studies, etoposide-induced DNA damage and MLL gene rearrangements were demonstrated to be dependent in part on MPO activity in CD34+ cells. Together, our results are consistent with the idea that MPO-dependent oxidation of etoposide in human hematopoietic CD34+ cells makes these cells especially prone to the induction of etoposide-related acute myeloid leukemia.