Radical-Generating Activity, Phagocytosis, and Mechanical Properties of Four Phenotypes of Human Macrophages

Macrophages are the major players and orchestrators of inflammatory response. Expressed proteins and secreted cytokines have been well studied for two polar macrophage phenotypes-pro-inflammatory M1 and anti-inflammatory regenerative M2, but little is known about how the polarization modulates macrophage functions. In this study, we used biochemical and biophysical methods to compare the functional activity and mechanical properties of activated human macrophages differentiated from monocyte with GM-CSF (M0_GM) and M-CSF (M0_M) and polarized into M1 and M2 phenotypes, respectively. Unlike GM-CSF, which generates dormant cells with low activity, M-CSF confers functional activity on macrophages. M0_M and M2 macrophages had very similar functional characteristics-high reactive oxygen species (ROS) production level, and higher phagocytosis and survival compared to M1, while M1 macrophages showed the highest radical-generating activity but the lowest phagocytosis and survival among all phenotypes. All phenotypes decreased their height upon activation, but only M1 and M2 cells increased in stiffness, which can indicate a decrease in the migration ability of these cells and changes in their interactions with other cells. Our results demonstrated that while mechanical properties differ between M0 and polarized cells, all four phenotypes of monocyte-derived macrophages differ in their functional activities, namely in cytokine secretion, ROS production, and phagocytosis. Within the broad continuum of human macrophages obtained in experimental models and existing in vivo, there is a diversity of phenotypes with varying combinations of both markers and functional activities.

MSCs' conditioned media cytokine and growth factor profiles and their impact on macrophage polarization

BACKGROUND

There is a growing body of evidence that multipotent mesenchymal stromal cells' (MSCs') remarkable therapeutic potential is attributed not only to their differentiation and regenerative capacity, but also to the paracrine effect, underlying their immunomodulatory properties. MSCs' secretome (i.e., cytokines, growth factors, and extracellular vesicles) is therefore increasingly discussed in the context of their ability to modulate inflammatory response and promote regeneration. There is evidence that 2D or 3D culturing conditions have an impact on the cells' secretome, and here we aimed to compare the secretion of cytokines and growth factors in human MSCs from different sources cultured in 2D and 3D conditions and assess their effect on human macrophages polarization in vitro.

METHODS

MSCs were derived from human adipose tissue, bone marrow, gingiva, placenta, and umbilical cord, cultured as monolayers or as cell spheroids. Their cytokine profiles were analyzed, and data standardization was carried out using a z-score. Human peripheral blood mononuclear cells-derived macrophages were then treated with umbilical cord-derived MSCs' conditioned media and their effect on macrophages polarization was assessed.

RESULTS

Our findings suggest that umbilical cord-derived MSCs' conditioned media demonstrated the highest cytokine and growth factor levels and despite mostly pro-inflammatory cytokine profile were able to promote anti-inflammatory macrophage polarization.

CONCLUSIONS

Umbilical cord-derived MSCs' conditioned media hold great potential for therapeutic use, demonstrating significant anti-inflammatory effect on human macrophages.

Detection of HOCl-driven degradation of the pericardium scaffolds by label-free multiphoton fluorescence lifetime imaging

Artificial biomaterials can significantly increase the rate of tissue regeneration. However, implantation of scaffolds leads not only to accelerated tissue healing but also to an immune response of the organism, which results in the degradation of the biomaterial. The synergy of the immune response and scaffold degradation processes largely determines the efficiency of tissue regeneration. Still, methods suitable for fast, accurate and non-invasive characterization of the degradation degree of biomaterial are highly demandable. Here we show the possibility of monitoring the degradation of decellularized bovine pericardium scaffolds under conditions mimicking the immune response and oxidation processes using multiphoton tomography combined with fluorescence lifetime imaging (MPT-FLIM). We found that the fluorescence lifetimes of genipin-induced cross-links in collagen and oxidation products of collagen are prominent markers of oxidative degradation of scaffolds. This was verified in model experiments, where the oxidation was induced with hypochlorous acid or by exposure to activated neutrophils. The fluorescence decay parameters also correlated with the changes of micromechanical properties of the scaffolds as assessed using atomic force microscopy (AFM). Our results suggest that FLIM can be used for quantitative assessments of the properties and degradation of the scaffolds essential for the wound healing processes in vivo.

Redox phospholipidomics of enzymatically generated oxygenated phospholipids as specific signals of programmed cell death

High fidelity and effective adaptive changes of the cell and tissue metabolism to changing environments require strict coordination of numerous biological processes. Multicellular organisms developed sophisticated signaling systems of monitoring and responding to these different contexts. Among these systems, oxygenated lipids play a significant role realized via a variety of re-programming mechanisms. Some of them are enacted as a part of pro-survival pathways that eliminate harmful or unnecessary molecules or organelles by a variety of degradation/hydrolytic reactions or specialized autophageal processes. When these "partial" intracellular measures are insufficient, the programs of cells death are triggered with the aim to remove irreparably damaged members of the multicellular community. These regulated cell death mechanisms are believed to heavily rely on signaling by a highly diversified group of molecules, oxygenated phospholipids (PLox). Out of thousands of detectable individual PLox species, redox phospholipidomics deciphered several specific molecules that seem to be diagnostic of specialized death programs. Oxygenated cardiolipins (CLs) and phosphatidylethanolamines (PEs) have been identified as predictive biomarkers of apoptosis and ferroptosis, respectively. This has led to decoding of the enzymatic mechanisms of their formation involving mitochondrial oxidation of CLs by cytochrome c and endoplasmic reticulum-associated oxidation of PE by lipoxygenases. Understanding of the specific biochemical radical-mediated mechanisms of these oxidative reactions opens new avenues for the design and search of highly specific regulators of cell death programs. This review emphasizes the usefulness of such selective lipid peroxidation mechanisms in contrast to the concept of random poorly controlled free radical reactions as instruments of non-specific damage of cells and their membranes. Detailed analysis of two specific examples of phospholipid oxidative signaling in apoptosis and ferroptosis along with their molecular mechanisms and roles in reprogramming has been presented.

Redox Epiphospholipidome in Programmed Cell Death Signaling: Catalytic Mechanisms and Regulation

A huge diversification of phospholipids, forming the aqueous interfaces of all biomembranes, cannot be accommodated within a simple concept of their role as membrane building blocks. Indeed, a number of signaling functions of (phospho)lipid molecules has been discovered. Among these signaling lipids, a particular group of oxygenated polyunsaturated fatty acids (PUFA), so called lipid mediators, has been thoroughly investigated over several decades. This group includes oxygenated octadecanoids, eicosanoids, and docosanoids and includes several hundreds of individual species. Oxygenation of PUFA can occur when they are esterified into major classes of phospholipids. Initially, these events have been associated with non-specific oxidative injury of biomembranes. An alternative concept is that these post-synthetically oxidatively modified phospholipids and their adducts with proteins are a part of a redox epiphospholipidome that represents a rich and versatile language for intra- and inter-cellular communications. The redox epiphospholipidome may include hundreds of thousands of individual molecular species acting as meaningful biological signals. This review describes the signaling role of oxygenated phospholipids in programs of regulated cell death. Although phospholipid peroxidation has been associated with almost all known cell death programs, we chose to discuss enzymatic pathways activated during apoptosis and ferroptosis and leading to peroxidation of two phospholipid classes, cardiolipins (CLs) and phosphatidylethanolamines (PEs). This is based on the available LC-MS identification and quantitative information on the respective peroxidation products of CLs and PEs. We focused on molecular mechanisms through which two proteins, a mitochondrial hemoprotein cytochrome c (cyt c), and non-heme Fe lipoxygenase (LOX), change their catalytic properties to fulfill new functions of generating oxygenated CL and PE species. Given the high selectivity and specificity of CL and PE peroxidation we argue that enzymatic reactions catalyzed by cyt c/CL complexes and 15-lipoxygenase/phosphatidylethanolamine binding protein 1 (15LOX/PEBP1) complexes dominate, at least during the initiation stage of peroxidation, in apoptosis and ferroptosis. We contrast cell-autonomous nature of CLox signaling in apoptosis correlating with its anti-inflammatory functions vs. non-cell-autonomous ferroptotic signaling facilitating pro-inflammatory (necro-inflammatory) responses. Finally, we propose that small molecule mechanism-based regulators of enzymatic phospholipid peroxidation may lead to highly specific anti-apoptotic and anti-ferroptotic therapeutic modalities.

Redox (phospho)lipidomics of signaling in inflammation and programmed cell death

In addition to the known prominent role of polyunsaturated (phospho)lipids as structural blocks of biomembranes, there is an emerging understanding of another important function of these molecules as a highly diversified signaling language utilized for intra- and extracellular communications. Technological developments in high-resolution mass spectrometry facilitated the development of a new branch of metabolomics, redox lipidomics. Analysis of lipid peroxidation reactions has already identified specific enzymatic mechanisms responsible for the biosynthesis of several unique signals in response to inflammation and regulated cell death programs. Obtaining comprehensive information about millions of signals encoded by oxidized phospholipids, represented by thousands of interactive reactions and pleiotropic (patho)physiological effects, is a daunting task. However, there is still reasonable hope that significant discoveries, of at least some of the important contributors to the overall overwhelmingly complex network of interactions triggered by inflammation, will lead to the discovery of new small molecule regulators and therapeutic modalities. For example, suppression of the production of AA-derived pro-inflammatory mediators, HXA3 and LTB4, by an iPLA2 γ inhibitor, R-BEL, mitigated injury associated with the activation of pro-inflammatory processes in animals exposed to whole-body irradiation. Further, technological developments promise to make redox lipidomics a powerful approach in the arsenal of diagnostic and therapeutic instruments for personalized medicine of inflammatory diseases and conditions.

[Neutrophils as a source of factors that increase the length of the inflammatory phase of wound healing in patients with type 2 diabetes mellitus]

Oxidative stress and neutrophil activation leading to an increase in myeloperoxidase (MPO), elastase and neutrophil extracellular trap (NET) levels in blood are considered as pathogenic mechanisms responsible for the development of extremity damage in people with type 2 diabetes mellitus (T2DM). The aim of this study was to analyze the relationship between factors, associated with neutrophil activation, and the length of the initial phase of wound healing (the inflammatory phase) in T2DM patients. Patients were divided retrospectively into three groups depending on the damage extent: group 1 (wound on toe) < group 2 (wound on foot) < group 3 (wound on lower leg). Compared to the control group (healthy volunteers), T2DM patients at admission to hospital had significantly (p<0.05) increased levels of blood glucose and glycated hemoglobin (groups 1-3), ESR (groups 1 and 3), blood neutrophil count (groups 2 and 3), plasma MPO concentration (groups 1-3) and blood NET concentration (group 3) and decreased levels of plasma thiols (groups 1-3) and erythrocyte glutathione peroxidase activity (groups 2 and 3). The length of hospital stay after surgical procedures corresponded to the length of the inflammatory phase of the wound healing process and correlated with the number of blood neutrophils in patients before surgery (r=0.72, p<0.05). Leukocytic intoxication index depended on wound area (r=0.59, p<0.05), and it was significantly higher for groups 2 and 3 compared to the control group and group 1. The neutrophil count before surgery in T2DM patients with damage in the lower extremities correlated with the length of the inflammatory phase of wound healing. The correlation found can be attributed to an increase in extracellular MPO and NETs, which, in its turn, results from the activation and degranulation of neutrophils and netosis. Thus, the duration of the inflammatory phase of wound healing depends on specific aspects of systemic inflammation increasing oxidative/halogenative stress and intoxication.

Peroxidase Activity of Human Hemoproteins: Keeping the Fire under Control

The heme in the active center of peroxidases reacts with hydrogen peroxide to form highly reactive intermediates, which then oxidize simple substances called peroxidase substrates. Human peroxidases can be divided into two groups: (1) True peroxidases are enzymes whose main function is to generate free radicals in the peroxidase cycle and (pseudo)hypohalous acids in the halogenation cycle. The major true peroxidases are myeloperoxidase, eosinophil peroxidase and lactoperoxidase. (2) Pseudo-peroxidases perform various important functions in the body, but under the influence of external conditions they can display peroxidase-like activity. As oxidative intermediates, these peroxidases produce not only active heme compounds, but also protein-based tyrosyl radicals. Hemoglobin, myoglobin, cytochrome c/cardiolipin complexes and cytoglobin are considered as pseudo-peroxidases. Рeroxidases play an important role in innate immunity and in a number of physiologically important processes like apoptosis and cell signaling. Unfavorable excessive peroxidase activity is implicated in oxidative damage of cells and tissues, thereby initiating the variety of human diseases. Hence, regulation of peroxidase activity is of considerable importance. Since peroxidases differ in structure, properties and location, the mechanisms controlling peroxidase activity and the biological effects of peroxidase products are specific for each hemoprotein. This review summarizes the knowledge about the properties, activities, regulations and biological effects of true and pseudo-peroxidases in order to better understand the mechanisms underlying beneficial and adverse effects of this class of enzymes.

Wall Thickness of Industrial Multi-Walled Carbon Nanotubes Is Not a Crucial Factor for Their Degradation by Sodium Hypochlorite

The propensity of multi-walled carbon nanotubes (MWCNTs) for biodegradation is important for their safe use in medical and technological applications. Here, we compared the oxidative degradation of two samples of industrial-grade MWCNTs—we called them MWCNT-d and MWCNT-t—upon their treatment with sodium hypochlorite (NaOCl). The MWCNTs had a similar inner diameter but they differed about 2-fold in the outer diameter. Electron microscopy combined with morphometric analysis revealed the different degradation of the two types of MWCNTs after their incubation with NaOCl—the thicker MWCNT-d were damaged more significantly than the thinner MWCNT-t. The both types of MWCNTs degraded at the inner side, but only MWCNT-d lost a significant number of the outer graphitic layers. Raman spectroscopy demonstrated that both MWCNTs had a similar high defectiveness. Using energy-dispersive X-ray spectroscopy, we have shown that the more degradable MWCNT-d contained the same level of oxygen as MWCNT-t, but more metal impurities. The obtained results suggest that the biodegradability of MWCNTs depends not only on the wall thickness but also on the defects and impurities. Thus, the biodegradability can be regulated by the synthesis conditions or the post-synthesis modifications. Such degradation flexibility may be important for both medical and industrial applications.

Extremely high-frequency electromagnetic radiation enhances neutrophil response to particulate agonists

The growing use of extremely high-frequency electromagnetic radiation (EHF EMR) in information and communication technology and in biomedical applications has raised concerns regarding the potential biological impact of millimeter waves (MMWs). Here, we elucidated the effects of MMW radiation on neutrophil activation induced by opsonized zymosan or E. coli in whole blood ex vivo. After agonist addition to blood, two samples were prepared. A control sample was incubated at ambient conditions without any treatment, and a test sample was exposed to EHF EMR (32.9-39.6 GHz, 100 W/m² ). We used methods that allowed us to assess the functional status of neutrophils immediately after exposure: oxidant production levels were measured by luminol-dependent chemiluminescence, and morphofunctional changes to neutrophils were observed in blood smears. Results revealed that the response of neutrophils to both agonists was intensified if blood was exposed to MMW radiation for 15 min. Neutrophils were intact in both the control and irradiated samples if no agonist was added to blood before incubation. Similarly, exposing suspensions of isolated neutrophils in plasma to MMW radiation enhanced cell response to both zymosan and E. coli. Heating blood samples was shown to be the primary mechanism underlying enhanced EHF EMR-induced oxidant production by neutrophils in response to particulate agonists. Bioelectromagnetics. 39:144-155, 2018. © 2017 Wiley Periodicals, Inc.

Enzymatic oxidative biodegradation of nanoparticles: Mechanisms, significance and applications

Biopersistence of carbon nanotubes, graphene oxide (GO) and several other types of carbonaceous nanomaterials is an essential determinant of their health effects. Successful biodegradation is one of the major factors defining the life span and biological responses to nanoparticles. Here, we review the role and contribution of different oxidative enzymes of inflammatory cells - myeloperoxidase, eosinophil peroxidase, lactoperoxidase, hemoglobin, and xanthine oxidase - to the reactions of nanoparticle biodegradation. We further focus on interactions of nanomaterials with hemoproteins dependent on the specific features of their physico-chemical and structural characteristics. Mechanistically, we highlight the significance of immobilized peroxidase reactive intermediates vs diffusible small molecule oxidants (hypochlorous and hypobromous acids) for the overall oxidative biodegradation process in neutrophils and eosinophils. We also accentuate the importance of peroxynitrite-driven pathways realized in macrophages via the engagement of NADPH oxidase- and NO synthase-triggered oxidative mechanisms. We consider possible involvement of oxidative machinery of other professional phagocytes such as microglial cells, myeloid-derived suppressor cells, in the context of biodegradation relevant to targeted drug delivery. We evaluate the importance of genetic factors and their manipulations for the enzymatic biodegradation in vivo. Finally, we emphasize a novel type of biodegradation realized via the activation of the "dormant" peroxidase activity of hemoproteins by the nano-surface. This is exemplified by the binding of GO to cyt c causing the unfolding and 'unmasking' of the peroxidase activity of the latter. We conclude with the strategies leading to safe by design carbonaceous nanoparticles with optimized characteristics for mechanism-based targeted delivery and regulatable life-span of drugs in circulation.

Peroxidase activation of cytoglobin by anionic phospholipids: Mechanisms and consequences

Cytoglobin (Cygb) is a hexa-coordinated hemoprotein with yet to be defined physiological functions. The iron coordination and spin state of the Cygb heme group are sensitive to oxidation of two cysteine residues (Cys38/Cys83) and/or the binding of free fatty acids. However, the roles of redox vs lipid regulators of Cygb's structural rearrangements in the context of the protein peroxidase competence are not known. Searching for physiologically relevant lipid regulators of Cygb, here we report that anionic phospholipids, particularly phosphatidylinositolphosphates, affect structural organization of the protein and modulate its iron state and peroxidase activity both conjointly and/or independently of cysteine oxidation. Thus, different anionic lipids can operate in cysteine-dependent and cysteine-independent ways as inducers of the peroxidase activity. We establish that Cygb's peroxidase activity can be utilized for the catalysis of peroxidation of anionic phospholipids (including phosphatidylinositolphosphates) yielding mono-oxygenated molecular species. Combined with the computational simulations we propose a bipartite lipid binding model that rationalizes the modes of interactions with phospholipids, the effects on structural re-arrangements and the peroxidase activity of the hemoprotein.

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.

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.

[Myeloperoxidase-induced biodegradation of single-walled carbon nanotubes is mediated by hypochlorite]

Broad prospects for the use of single-walled carbon nanotubes (SWNTs) in medicine and biotechnology raise the concerns about both their toxicity, and the mechanisms of biodegradation and excretion from the body. SWNTs biodegradation as a result of catalytic activity of myeloperoxidase (MPO) was shown in the isolated MPO system as well as in the suspension of neutrophils [Kagan V.E., et al., 2010]. In the present study we analyzed the ability of different MPO-produced oxidants to participate in the modification and degradation of SWNTs. The comparison of the ability of various peroxidases to degrade SWNTs in vitro revealed that myeloperoxidase, due to its ability to produce hypochlorite, and lactoperoxidase, due to its ability to produce hypobromite, are extremely efficient in the degradation of carbon nanotubes. The biodegradation of SWNTs in the model system can also be caused by free radicals generated as a result of heme degradation and, to a lesser extent, by active oxoferryl intermediates of peroxidases. Our experiments showed that in the presence of blood plasma, peroxidase intermediates or free radical products of heme degradation were unable to initiate biodegradation of carbon nanotubes, only the generation of hypochlorite by MPO can cause the biodegradation of carbon nanotubes in vivo. Titration of SWNTs suspension containing plasma with hypochlorite at high concentrations resulted in the decrease in the optical absorbance of the suspension indicating the degradation of nanotubes. Our results clearly indicate that hypochlorite can serve as a main oxidizing agent which is able to modify and degrade nanotubes in the sites of inflammation and in the phagosomes.

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.

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.

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.

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 H(2)O(2), 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 H(2)O(2) 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 H(2)O(2), 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.

Peroxidase mechanism of lipid-dependent cross-linking of synuclein with cytochrome C: protection against apoptosis versus delayed oxidative stress in Parkinson disease

Damage of presynaptic mitochondria could result in release of proapoptotic factors that threaten the integrity of the entire neuron. We discovered that alpha-synuclein (Syn) forms a triple complex with anionic lipids (such as cardiolipin) and cytochrome c, which exerts a peroxidase activity. The latter catalyzes covalent hetero-oligomerization of Syn with cytochrome c into high molecular weight aggregates. Syn is a preferred substrate of this reaction and is oxidized more readily than cardiolipin, dopamine, and other phenolic substrates. Co-localization of Syn with cytochrome c was detected in aggregates formed upon proapoptotic stimulation of SH-SY5Y and HeLa cells and in dopaminergic substantia nigra neurons of rotenone-treated rats. Syn-cardiolipin exerted protection against cytochrome c-induced caspase-3 activation in a cell-free system, particularly in the presence of H(2)O(2). Direct delivery of Syn into mouse embryonic cells conferred resistance to proapoptotic caspase-3 activation. Conversely, small interfering RNA depletion of Syn in HeLa cells made them more sensitive to dopamine-induced apoptosis. In human Parkinson disease substantia nigra neurons, two-thirds of co-localized Syn-cytochrome c complexes occurred in Lewy neurites. Taken together, these results indicate that Syn may prevent execution of apoptosis in neurons through covalent hetero-oligomerization of cytochrome c. This immediate protective function of Syn is associated with the formation of the peroxidase complex representing a source of oxidative stress and postponed damage.

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.

[A study of the effect of ceruloplasmin and lactoferrin on the chlorination activity of leukocytic myeloperoxidase using the chemiluminescence method]

The chlorination activity of free myeloperoxidase and myeloperoxidase bound with ceruloplasmin or with both ceruloplasmin and lactoferrin has been studied by luminal-dependent chemiluminescence. It was shown that the addition of hydrogen peroxide to the "myeloperoxidase + Cl- + luminal" system is accompanied by a fast flash of light emission. In the absence of myeloperoxidase or Cl-, the flash intensity was considerably reduced. The inhibitor of myeloperoxidase NaN3, the HOCl scavengers taurine and methionine, and guaiacol, a substrate for peroxidation cycle of myeloperoxidase, prevented luminescence. These results suggest that the generation of luminescence was due to the halogenating activity of myeloperoxidase, and hence, the flash light sum may serve as a measure of chlorination activity of myeloperoxidase. The activity of myeloperoxidase was suppressed by ceruloplasmin. Lactoferrin exhibited no significant influence on the myeloperoxidase activity, nor did it prevent the inhibitory effect of ceruloplasmin when they both were combined with myeloperoxidase. These data were confirmed using alternative approaches for evaluating the myeloperoxidase activity, namely, the assessment of peroxidation activity and the taurine chlorination assay. It is noteworthy that the inhibitory effect of ceruloplasmin on chlorination and peroxidation activities of myeloperoxidase is seen with the latter, traditional approaches only if ceruloplasmin is present in a large excess relative to myeloperoxidase, whereas the chemiluminescence method allows the detection of the inhibitory effect of ceruloplasmin using lower proportions of the protein with respect to myeloperoxidase, which are close to the stoichiometry of the myeloperoxidase/ceruloplasmin and the myeloperoxidase'ceruloplasmin'lactoferrin complexes.

Role of myeloperoxidase-mediated modification of human blood lipoproteins in atherosclerosis development

The mechanism of interaction of hypochlorite and hypobromite formed in myeloperoxidase catalysis with lipids of human blood low-density lipoprotein is described. Both agents react with unsaturated lipids via two mechanisms: molecular (with the formation of mainly chloro- or bromohydrins and lysophospholipids) and free-radical (paralleled by lipid peroxidation). These reactions modify physicochemical properties of low-density lipoproteins and disorder their lipid-transporting function thus initiating early stages of atherosclerosis development.

Activation of NO donors in mitochondria: peroxidase metabolism of (2-hydroxyamino-vinyl)-triphenyl-phosphonium by cytochrome c releases NO and protects cells against apoptosis

In mitochondrial apoptosis, the formation of cytochrome c-cardiolipin complex ([CL-cyt c]) with peroxidase properties is an early event in the cascade of reactions that leads to cell death. Herein, we report the synthesis of a new prodrug, (2-hydroxyamino-vinyl)-triphenyl-phosphonium (HVTP), which compartmentalizes exclusively into mitochondria, undergoes a [CL-cyt c]-catalyzed bioactivation to nitric oxide (NO), inhibits peroxidase activity, and protects cells from apoptosis.

[Determination of antibiotics using luminescent Escherichia coli and serum]

The methodical bases for detecting antibiotics using a bioluminescent assay and blood serum are briefed. Antibiotics inhibit the luminescence of a genetically engineered Escherichia coli strain. The degree of inhibition depended on the type of antibiotic, its concentration, and the time of cell incubation with antibiotic. The highest cell sensitivity was recorded towards the aminoglycoside antibiotics, which amounted to 85 +/- 10 ng/ml for gentamicin and streptomycin. The sensitivity of this system to a number of antibiotics essentially increased when the cells were previously activated with blood serum. The sensitivity of this method for gentamicin and streptomycin in the presence of blood serum amounted to 2.5 +/- 0.5 ng/ml; for tetracycline, 45 +/- 8 ng/ml. Use of the sera containing specific antibodies to the antibiotic detected provided a high sensitivity of the biosensor tested. Comparison of the luminescences of E. coli cells activated with normal and specific antisera upon incubation with an antibiotic allows the type of antibiotic and its quantitative content in the sample to be determined. Characteristic of the analysis of antibiotics with the help of recombinant E. coli are a high accuracy, sensitivity, specificity, simplicity, and a short time needed for measurement.

Cardiolipin switch in mitochondria: shutting off the reduction of cytochrome c and turning on the peroxidase activity

Upon interaction with anionic phospholipids, particularly mitochondria-specific cardiolipin (CL), cytochrome c (cyt c) loses its tertiary structure and its peroxidase activity dramatically increases. CL-induced peroxidase activity of cyt c has been found to be important for selective CL oxidation in cells undergoing programmed death. During apoptosis, the peroxidase activity and the fraction of CL-bound cyt c markedly increase, suggesting that CL may act as a switch to regulate cyt c's mitochondrial functions. Using cyclic voltammetry and equilibrium redox titrations, we show that the redox potential of cyt c shifts negatively by 350-400 mV upon binding to CL-containing membranes. Consequently, functions of cyt c as an electron transporter and cyt c reduction by Complex III are strongly inhibited. Further, CL/cyt c complexes are not effective in scavenging superoxide anions and are not effectively reduced by ascorbate. Thus, both redox properties and functions of cyt c change upon interaction with CL in the mitochondrial membrane, diminishing cyt c's electron donor/acceptor role and stimulating its peroxidase activity.

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.

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 H(2)O(2)-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.

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 H(2)O(2)-activated peroxidase intermediates resulting in their reduction. A dose-dependent quenching of H(2)O(2)-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, H(2)O(2)-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.

Cytochrome c acts as a cardiolipin oxygenase required for release of proapoptotic factors

Programmed death (apoptosis) is turned on in damaged or unwanted cells to secure their clean and safe self-elimination. The initial apoptotic events are coordinated in mitochondria, whereby several proapoptotic factors, including cytochrome c, are released into the cytosol to trigger caspase cascades. The release mechanisms include interactions of B-cell/lymphoma 2 family proteins with a mitochondria-specific phospholipid, cardiolipin, to cause permeabilization of the outer mitochondrial membrane. Using oxidative lipidomics, we showed that cardiolipin is the only phospholipid in mitochondria that undergoes early oxidation during apoptosis. The oxidation is catalyzed by a cardiolipin-specific peroxidase activity of cardiolipin-bound cytochrome c. In a previously undescribed step in apoptosis, we showed that oxidized cardiolipin is required for the release of proapoptotic factors. These results provide insight into the role of reactive oxygen species in triggering the cell-death pathway and describe an early role for cytochrome c before caspase activation.

New approach for specific determination of antibiotics by use of luminescent Escherichia coli and immune serum

This paper describes a possible application of luminescent Escherichia coli activated by blood serum for high-sensitivity and high-specificity assays of antibiotics in solutions. Antibiotics inhibited luminescence of a genetically engineered E. coli strain; the system sensitivity to some antibiotics grew notably after the cells had been preactivated by blood serum. The highest level of sensitivity (2.8 +/- 0.6 ng/ml) of luminescent cells was obtained for aminoglycoside antibiotics (gentamicin and streptomycin). It is feasible to create the specific biosensor for antibiotics on the basis of bioluminescent E. coli strains by applying sera containing antibodies against the antibiotic under assay. The presence of antibodies specific for gentamicin in serum affects inhibition of luminescent cells by gentamicin but not inhibition by other antibiotics.

[L-Glutamate oxidase from Streptomyces cremeus 510 MGU: effect of nitrogen sources on enzyme secretion]

Effect of nitrogen sources (organic complexes and mineral salts) on L-glutamate oxidase synthesis by Streptomyces cremeus 510 MGU was studied. Optimal enzyme production was not provided by any single nitrogen source. The most effective combination of nitrogen sources (soy flour, peptone, ammonium sulfate) was elaborated by the mathematical planning method. The results of experiment allowed to enhance biosynthesis of extracellular L-glutamate oxidase to 2.4-2.6 U/mL. It was shown that L-glutamate oxidase of Streptomyces cremeus 510 MGU is highly specific to substance and is stable during storage of filtrated culture with pH 6.8-9.0.

[L-Glutamate oxidase from Streptomyces cremeus 510 MGU: growth media optimization for the enhancement of the producer fermentative activity]

The investigation was devoted to culture conditions optimization aimed to maximum secretion of extracellular L-glutamate oxidase by Streptomyces cremeus 510 MGU. It was shown that Ca ions at the concentration 5-20 mM and 0.1% ammonium sulphate enhanced activity of extracellular enzyme to 4 folds. L-glutamate acid supplement had no effect on enzyme activity. Influence of some bivalent cations and aeration regimes on L-glutamate oxidase activity was investigated. Growth media optimization along with screening of active variants resulted with isolation of the strain with L-glutamate oxidase activity about 2 U/mL Rate of peroxide degradation in the presence of filtrated culture of S. cremeus was determined by chemiluminescence method.

[Binding of divalent cations with low density lipoproteins. A study by ESR]

The binding of bivalent metal ions Cu2+, Zn2+, Ca2+, Mg2+ to low-density lipoproteins (LDL) was investigated by the ESR technique. The monitoring of ESR spectra of paramagnetic Mn2+ ions in the presence of above-listed cations made it possible to evaluate the dissociation constants of their complexes with LDL. The effective dissociation constant of the complex Mn(2+)-LDL used for calculations was KD = (1.1 +/- 0.4) x 10(-4) M according to literature data. The investigated cations may be classified into two groups: 1) low dissociation constants were characteristic for Cu2+ ions [KD = (1.3 +/- 0.5) x 10(-4) M], which demonstrated a high oxidative ability, and for Zn2+ [KD = (0.95 +/- 0.45) x 10(-4) M] and Mn2+ ions, which could strongly influence the copper-induced LDL oxidation; 2) Ca2+ and Mg2+ were characterized by higher values of KD [(6 +/- 1) x 10(-4) M and (7.5 +/- 1.5) x 10(-4) M, accordingly] and slightly affected the Cu(2+)-induced oxidation of LDL. The results of the present work reinforced our earlier conjecture that cations may influence the process of lipid peroxidation, binding only to particular binding sites on the surface of LDL.

The effect of oxidatively modified low-density lipoproteins on platelet aggregability and membrane fluidity

The influence of oxidised low-density lipoproteins (oxLDL) on blood cell functions plays a role in the progression of atherosclerosis. In the present work the effects of mildly oxidised LDL (moxLDL) on platelet aggregability and plasma membrane fluidity were studied and analysed from the viewpoint of the extent of LDL oxidation. Native or oxidised LDL were incubated with platelet rich plasma (PRP) at the volume ratio 1:1. As a control, plasma was incubated with buffer. The effects on ADP-induced platelet aggregation and certain membrane characteristics are described. (1) Mildly oxidised LDL diminished the time-dependent decrease in platelet aggregability that was observed when PRP was incubated with buffer or native LDL. The higher the oxidation extent of moxLDL, the lesser (if any) decrease in platelet activity occurred. Therefore moxLDL activated platelets in PRP. Cu2+-oxidised LDL, characterised by a high extent of lipid oxidation, inhibited ADP-induced platelet aggregation. (2) Comparison of the ESR spectra of spin-labelled fatty acid (5-doxylstearate) incorporated into the plasma membrane of washed platelets indicated that the presence of moxLDL in the incubation medium resulted in a reduced fluidity of the outer membrane layer. The cholesterol:phospholipid ratio in platelets appeared to be the same after PRP incubation with native LDL, moxLDL or buffer. It may be proposed that the binding of oxLDL to the platelet surface leads to a modification of the membrane fluidity, thus mediating the activating action of LDL on platelets. Both effects were proportional to the extent of lipid oxidation in LDL. The results of this paper indicate a crucial role for mildly oxidised LDL in platelet activation.

[Effect of oxidized low density lipoproteins on the structure of platelet membrane. Use of electron paramagnetic resonance]

The effect of low-density lipoproteins on the structure of platelet plasma membrane was studied by electron paramagnetic resonance spectroscopy. Low-density lipoproteins were incubated with platelet rich plasma at a volume ratio 1:1. Plasma incubated with buffer served as a control. After incubation, the fluidity of platelet plasma membrane was determined by electron spin resonance probes 5-doxylstearate and 16-doxylstearate, which were immobilized in membranes of cells subjected to triple precipitation. Significant differences in the order parameter S, which characterizes the spectrum of the 5-doxylstearate probe, for samples incubated with the buffer and oxidized low-density lipoproteins were found. The dependence of the parameter on incubation time and the extend of oxidation of low-density lipoproteins were obtained. No significant differences in rotational correlation time of 16-doxylstearate between platelets incubated with and without oxidized low-density lipoproteins was observed within the limits of experimental error; however, the changes in the half-width of the low-field component may be considered reliable. The interaction of oxidized low-density lipoproteins with platelets leads to an increase in plasma membrane fluidity, thereby mediating the activating action on platelets.

[The effect of oxidized low density lipoproteins on ADP-induced platelet aggregation in plasma]

The investigation of the effect of oxidized lipoproteins on platelet activity is important for the understanding of the plague formation under atherosclerosis. In the present work, we examined the influence of low density lipoproteins (LDL) on ADP-induced platelet aggregation in the platelet rich plasma. In was demonstrated that mixing of plasma and LDL was accompanied by the decrease of ADP-induced aggregation parameters as compared to control (mixing with buffer). After 1 h incubation, platelet ADP-aggregation in the sample containing oxidized LDL (oxLDL) exceeded the ADP-aggregation in the control sample. The dependence of the aggregation parameters on the incubation time and on the degree of LDL oxidation were obtained. No difference in the cholesterol and phospholipid content was observed between cells incubated with buffer, native or oxidized LDL. Therefore, the possible oxLDL-induced accumulation of cholesterol in platelet membranes is excluded as a reason for the increased cell aggregation.

[Effect of metal cations on the copper induced peroxidation of the low density lipoproteins]

The effect of metal cations on copper-catalyzed lipid peroxidation (LPO) of low density lipoproteins (LDL) was examined. The presence of metal cations in the incubation media containing LDL (0.8 mg protein/ml) and CuSO4 (0-80 microM) influenced on LPO of LDL as evident by the measurement of TBARS. With the concentrations of CuSO4 less than 10 microM, the metal cations caused an increase in LDL peroxidation. Zn2+ appeared to be the most effective inductor, Mn2+ was less effective, and the influence of Ca2+ and Mg2+ was insignificant. With greater CuSO4 concentrations Mg2+ showed no effect on TBARS formation in LDL while the addition of other nontransition metal cations to the incubation mixture led to the inhibition of LDL peroxidation. The capacity for inhibition decreased in the row Mn2+ > Zn2+ > Ca2+ > Mg2+. The possible mechanism explaining these results may be in the competition of metal ions for copper binding sites on LDL. Our results allow to suggest the existence of two types of copper binding sites on LDL, tight-binding sites which are non-effective in LPO and effective weak-binding sites.

Inhibitor analysis of LDL-induced platelet aggregation

Oxidized low density lipoproteins (oxLDL) bounded to specific receptors on the platelet surface are able to activate platelets. However, the exact mechanism of signal transduction from LDL receptors into the cell still requires investigation. In the present paper inhibitors of the main enzymes of known platelet activation pathways were used to investigate the mechanism of the LDL-induced platelet aggregation. Our experiments were performed with autoxidized LDL (2-thiobarbituric acid reactive substances < 8 nmoles/mg). We demonstrated that the main enzymes of the arachidonate cycle do not play an important role in LDL-induced platelet aggregation, whereas inhibition of protein kinase C and phospholipase C--principal enzymes of the phosphoinositide cycle--resulted in the inhibition of LDL-induced platelet aggregation in a dose-dependent manner. It was also shown that transmembrane calcium transport was necessary for LDL-induced platelet activation. Thus, we conclude that the phosphoinositide cycle is the main mechanism of cellular signal transduction during LDL-induced platelet activation.

[Study of yeast phosphoglycerate kinase by electron paramagnetic resonance. 1. Modification of protein SH-group by spin labels. Conformational changes of enzyme]

The conditions for modifications of a single SH-group of phosphoglycerate kinase from yeast by maleimid spin label and the mercury-containing spin labels were defined. The time course of immobilization of mercury containing paramagnetic radicals was obtained. Based on changes in EPR spectra we calculated dissociation constants for complexes of phosphoglycerate kinase with 3-phosphoglycerate and sulfate ions. Distinct substances were shown to induce different changes in enzyme conformation. It was concluded that approach of enzyme domains is caused by the binding of 3-phosphoglycerate-a specific substrate of this enzyme.