Chapter 2 The Interaction of Reactive Oxygen and Nitrogen Species with Membranes

Generating Concentration Gradients across Membranes for Molecular Dynamics Simulations of Periodic Systems

The plasma membrane forms the boundary between a living entity and its environment and acts as a barrier to permeation and flow of substances. Several computational means of calculating permeability have been implemented for molecular dynamics (MD) simulations-based approaches. Except for double bilayer systems, most permeability studies have been performed under equilibrium conditions, in large part due to the challenges associated with creating concentration gradients in simulations utilizing periodic boundary conditions. To enhance the scientific understanding of permeation and complement the existing computational means of characterizing membrane permeability, we developed a non-equilibrium method that enables the generation and maintenance of steady-state gradients in MD simulations. We utilize PBCs advantageously by imposing a directional bias to the motion of permeants so that their crossing of the boundary replenishes the gradient, like a previous study on ions. Under these conditions, a net flow of permeants across membranes may be observed to determine bulk permeability by a direct application of J=PΔc. In the present study, we explore the results of its application to an exemplary O2 and POPC bilayer system, demonstrating accurate and precise permeability measurements. In addition, we illustrate the impact of permeant concentration and the choice of thermostat on the permeability. Moreover, we demonstrate that energetics of permeation can be closely examined by the dissipation of the gradient across the membrane to gain nuanced insights into the thermodynamics of permeability.

Mitochondria Play Essential Roles in Intracellular Protection against Oxidative Stress-Which Molecules among the ROS Generated in the Mitochondria Can Escape the Mitochondria and Contribute to Signal Activation in Cytosol?

Questions about which reactive oxygen species (ROS) or reactive nitrogen species (RNS) can escape from the mitochondria and activate signals must be addressed. In this study, two parameters, the calculated dipole moment (debye, D) and permeability coefficient (Pm) (cm s-1), are listed for hydrogen peroxide (H2O2), hydroxyl radical (•OH), superoxide (O2•-), hydroperoxyl radical (HO2•), nitric oxide (•NO), nitrogen dioxide (•NO2), peroxynitrite (ONOO-), and peroxynitrous acid (ONOOH) in comparison to those for water (H2O). O2•- is generated from the mitochondrial electron transport chain (ETC), and several other ROS and RNS can be generated subsequently. The candidates which pass through the mitochondrial membrane include ROS with a small number of dipoles, i.e., H2O2, HO2•, ONOOH, •OH, and •NO. The results show that the dipole moment of •NO2 is 0.35 D, indicating permeability; however, •NO2 can be eliminated quickly. The dipole moments of •OH (1.67 D) and ONOOH (1.77 D) indicate that they might be permeable. This study also suggests that the mitochondria play a central role in protecting against further oxidative stress in cells. The amounts, the long half-life, the diffusion distance, the Pm, the one-electron reduction potential, the pKa, and the rate constants for the reaction with ascorbate and glutathione are listed for various ROS/RNS, •OH, singlet oxygen (1O2), H2O2, O2•-, HO2•, •NO, •NO2, ONOO-, and ONOOH, and compared with those for H2O and oxygen (O2). Molecules with negative electrical charges cannot directly diffuse through the phospholipid bilayer of the mitochondrial membranes. Short-lived molecules, such as •OH, would be difficult to contribute to intracellular signaling. Finally, HO2• and ONOOH were selected as candidates for the ROS/RNS that pass through the mitochondrial membrane.

How living cells are affected during the cold atmospheric pressure plasma treatment

When the electric discharge process is limited by high voltage electrodes shielding, the ionization measure would be controlled to less than one percent and the temperature to less than 37 °C even at atmospheric pressure, so-called cold atmospheric pressure plasma (CAP). CAP has been found to have profound medical applications in association with its reactive oxygen and nitrogen species (ROS/RNS). In this way that during plasma exposure, the subjected medium (e.g. cell cytoplasmic membrane in plasma therapy) interacts with ROS/RNS. Accordingly, a precise study of the mentioned interactions and their consequences on the cells' behavior changes, is necessary. The results lead to the reduction of possible risks and provide the opportunity of optimizing the efficacy of CAP before the development of CAP applications in the field of plasma medicine. In this report molecular dynamic (MD) simulation is used to investigate the mentioned interactions and a proper and compatible comparison with the experimental results is presented. Based on this, the effects of H2O2, NO and O2 on the living cell's membrane are investigated in biological conditions. Our results show that: i) The hydration of phospholipid polar heads would be enhanced associated with the H2O2 presence. ii) A new definition of the surface area assigned to each phospholipid (APL), more reliable and compatible with the physical expectations, is introduced. iii) The long-term behavior of NO and O2 is their penetration into the lipid bilayer and sometimes passing through the membrane into the cell. The latter would be an indication of internal cells' pathways activation leading to modification of cells' function.

Unraveling the Transport Properties of RONS across Nitro-Oxidized Membranes

The potential of cold atmospheric plasma (CAP) in biomedical applications has received significant interest, due to its ability to generate reactive oxygen and nitrogen species (RONS). Upon exposure to living cells, CAP triggers alterations in various cellular components, such as the cell membrane. However, the permeation of RONS across nitrated and oxidized membranes remains understudied. To address this gap, we conducted molecular dynamics simulations, to investigate the permeation capabilities of RONS across modified cell membranes. This computational study investigated the translocation processes of less hydrophilic and hydrophilic RONS across the phospholipid bilayer (PLB), with various degrees of oxidation and nitration, and elucidated the impact of RONS on PLB permeability. The simulation results showed that less hydrophilic species, i.e., NO, NO₂, N₂O₄, and O₃, have a higher penetration ability through nitro-oxidized PLB compared to hydrophilic RONS, i.e., HNO₃, s-cis-HONO, s-trans-HONO, H₂O₂, HO₂, and OH. In particular, nitro-oxidation of PLB, induced by, e.g., cold atmospheric plasma, has minimal impact on the penetration of free energy barriers of less hydrophilic species, while it lowers these barriers for hydrophilic RONS, thereby enhancing their translocation across nitro-oxidized PLB. This research contributes to a better understanding of the translocation abilities of RONS in the field of plasma biomedical applications and highlights the need for further analysis of their role in intracellular signaling pathways.

PCYT2-regulated lipid biosynthesis is critical to muscle health and ageing

Muscle degeneration is the most prevalent cause for frailty and dependency in inherited diseases and ageing. Elucidation of pathophysiological mechanisms, as well as effective treatments for muscle diseases, represents an important goal in improving human health. Here, we show that the lipid synthesis enzyme phosphatidylethanolamine cytidyltransferase (PCYT2/ECT) is critical to muscle health. Human deficiency in PCYT2 causes a severe disease with failure to thrive and progressive weakness. pcyt2-mutant zebrafish and muscle-specific Pcyt2-knockout mice recapitulate the participant phenotypes, with failure to thrive, progressive muscle weakness and accelerated ageing. Mechanistically, muscle Pcyt2 deficiency affects cellular bioenergetics and membrane lipid bilayer structure and stability. PCYT2 activity declines in ageing muscles of mice and humans, and adeno-associated virus-based delivery of PCYT2 ameliorates muscle weakness in Pcyt2-knockout and old mice, offering a therapy for individuals with a rare disease and muscle ageing. Thus, PCYT2 plays a fundamental and conserved role in vertebrate muscle health, linking PCYT2 and PCYT2-synthesized lipids to severe muscle dystrophy and ageing.

Liposomes embedded with PEGylated iron oxide nanoparticles enable ferroptosis and combination therapy in cancer

Ferroptosis, an iron-dependent regulated cell death process driven by excessive lipid peroxides, can enhance cancer vulnerability to chemotherapy, targeted therapy and immunotherapy. As an essential upstream process for ferroptosis activation, lipid peroxidation of biological membranes is expected to be primarily induced by intrabilayer reactive oxygen species (ROS), indicating a promising strategy to initiate peroxidation by improving the local content of diffusion-limited ROS in the lipid bilayer. Herein, liposomes embedded with PEG-coated 3 nm γ-Fe₂O₃ nanoparticles in the bilayer (abbreviated as Lp-IO) were constructed to promote the intrabilayer generation of hydroxyl radicals (•OH) from hydrogen peroxide (H₂O₂), and the integration of amphiphilic PEG moieties with liposomal bilayer improved lipid membrane permeability to H₂O₂ and •OH, resulting in efficient initiation of lipid peroxidation and thus ferroptosis in cancer cells. Additionally, Lp-IO enabled traceable magnetic resonance imaging and pH/ROS dual-responsive drug delivery. Synergistic antineoplastic effects of chemotherapy and ferroptosis, and alleviated chemotherapeutic toxicity, were achieved by delivering doxorubicin (capable of xCT and glutathione peroxidase inhibition) with Lp-IO. This work provides an efficient alternative for triggering therapeutic lipid peroxidation and a ferroptosis-activating drug delivery vehicle for combination cancer therapies.

Homeostasis inside Single Activated Phagolysosomes: Quantitative and Selective Measurements of Submillisecond Dynamics of Reactive Oxygen and Nitrogen Species Production with a Nanoelectrochemical Sensor

Reactive oxygen and nitrogen species (ROS/RNS) are generated by macrophages inside their phagolysosomes. This production is essential for phagocytosis of damaged cells and pathogens, i.e., protecting the organism and maintaining immune homeostasis. The ability to quantitatively and individually monitor the four primary ROS/RNS (ONOO^-, H₂O₂, NO, and NO₂^-) with submillisecond resolution is clearly warranted to elucidate the still unclear mechanisms of their rapid generation and to track their concentration variations over time inside phagolysosomes, in particular, to document the origin of ROS/RNS homeostasis during phagocytosis. A novel nanowire electrode has been specifically developed for this purpose. It consisted of wrapping a SiC nanowire with a mat of 3 nm platinum nanoparticles whose high electrocatalytic performances allow the characterization and individual measurements of each of the four primary ROS/RNS. This allowed, for the first time, a quantitative, selective, and statistically robust determination of the individual amounts of ROS/RNS present in single dormant phagolysosomes. Additionally, the submillisecond resolution of the nanosensor allowed confirmation and measurement of the rapid ability of phagolysosomes to differentially mobilize their enzyme pools of NADPH oxidases and inducible nitric oxide synthases to finely regulate their homeostasis. This reveals an essential key to immune responses and immunotherapies and rationalizes its biomolecular origin.

Electroanalysis at a Single Giant Vesicle Generating Enzymatically a Reactive Oxygen Species

In the framework of artificial or synthetic cell development, giant liposomes are common basic structures. Their enclosed membrane allows encapsulating proteins, DNA, reactants, etc., while its phospholipid nature allows some exchanges with the surrounding medium. Biochemical reactions induced inside giant liposomes or vesicles are often monitored or imaged by fluorescence microscopy techniques. Here, we show that electrochemistry performed with ultramicroelectrodes is perfectly suitable to monitor an enzymatic reaction occurring in a single giant unilamellar vesicle. Glucose oxidase (GOx) was microinjected inside individual vesicles containing 1 mM glucose. H₂O₂ was thus generated in the vesicle and progressively diffused across the membrane toward the surrounding environment. An ultramicroelectrode sensitive to H₂O₂ (black platinum-modified carbon surface) was placed next to the membrane and provided a direct detection of the hydrogen peroxide flux generated by the enzyme activity. Electrochemistry offered a highly sensitive (in situ detection), selective (potential applied at the electrode), time-resolved analysis (chronoamperometry) of the GOx activity over an hour duration, without modifying the internal giant unilamellar vesicles (GUV) medium. These results demonstrate that electroanalysis with microsensors is well adapted and complementary to fluorescence microscopy to sense enzymatic activities, for instance, generating reactive oxygen species, at single vesicles further used to develop artificial cells.

Sampling efficiency of the counting method for permeability calculations estimated with the inhomogeneous solubility-diffusion model

Permeability is a key property in various fields such as membrane technology for chemical separation and transport of substances through cell membranes. At the molecular scale, the counting method uses the number of membrane crossings in a conventional unbiased molecular dynamics simulation to predict the permeability. This contribution investigates under which conditions the counting method has insufficient statistics. An equation is derived for a compartmental model based on the inhomogeneous solubility-diffusion (Smoluchowski) model, giving insight into how the flux correlates with the solubility of permeants. This equation shows that a membrane crossing is a rare event not only when the membrane forms a large free energy barrier but also when the membrane forms a deep free energy well that traps permeants. Such a permeant trap has a high permeability; yet, the counting method suffers from poor statistics. To illustrate this, coarse-grained MD was run for 16 systems of dipalmitoylphosphatidylcholine bilayer membranes with different permeant types. The composition rule for permeability is shown to also hold for fluxes, and it is highlighted that the considered thickness of the membrane causes uncertainty in the permeability calculation of highly permeable membranes. In conclusion, a high permeability in itself is not an effective indicator of the sampling efficiency of the counting method, and caution should be taken for permeants whose solubility varies greatly over the simulation box. A practical consequence relevant in, e.g., drug design is that a drug with high membrane permeability might get trapped by membranes thus reducing its efficacy.

Diffusion and Transport of Reactive Species Across Cell Membranes

This chapter includes an overview of the structure of cell membranes and a review of the permeability of membranes to biologically relevant oxygen and nitrogen reactive species, namely oxygen, singlet oxygen, superoxide, hydrogen peroxide, hydroxyl radical, nitric oxide, nitrogen dioxide, peroxynitrite and also hydrogen sulfide. Physical interactions of these species with cellular membranes are discussed extensively, but also their relevance to chemical reactions such as lipid peroxidation. Most of these species are involved in different cellular redox processes ranging from physiological pathways to damaging reactions against biomolecules. Cell membranes separate and compartmentalize different processes, inside or outside cells, and in different organelles within cells. The permeability of these membranes to reactive species varies according to the physicochemical properties of each molecule. Some of them, such as nitric oxide and oxygen, are small and hydrophobic and can traverse cellular membranes virtually unhindered. Nitrogen dioxide and hydrogen sulfide find a slightly higher barrier to permeation, but still their diffusion is largely unimpeded by cellular membranes. In contrast, the permeability of cellular membranes to the more polar hydrogen peroxide, is up to five orders of magnitude lower, allowing the formation of concentration gradients, directionality and effective compartmentalization of its actions which can be further regulated by specific aquaporins that facilitate its diffusion through membranes. The compartmentalizing effect on anionic species such as superoxide and peroxynitrite is even more accentuated because of the large energetic barrier that the hydrophobic interior of membranes presents to ions that may be overcome by protonation or the use of anion channels. The large difference in cell membrane permeability for different reactive species indicates that compartmentalization is possible for some but not all of them.

Molecular dynamics simulations of membrane properties affected by plasma ROS based on the GROMOS force field

Cold atmospheric plasma (CAP) has attracted substantial attention in the field of medical disinfection because its main components, reactive oxygen species (ROS), have a strong destructive effect on various cell components. The cell membrane plays an important role in maintaining proper cellular function by blocking harmful substances such as ROS. In this paper, we used molecular dynamics simulations to study the behaviour of different ROS at the membrane-water interface. The results showed that the cell membrane presented a weak barrier to hydrophobic ROS (O₂) but effectively prevented hydrophilic ROS (OH, HO₂, H₂O₂) from entering the cell. The plasma treatment significantly enhanced the permeability of the cell membrane to HO₂, while the energetic barrier to other types of ROS changed only slightly. O₂ very likely stopped in the centre of the lipid bilayer when crossing the membrane and there attacked the unsaturated region of the phospholipid. Cholesterol was most likely oxidized by HO₂, causing a condensing effect that destroyed the integrity and fluidity of the cell membrane. The study also found that large amounts of ROS decreased the thickness of the cell membrane, and the phospholipid arrangement became disordered.

Permeabilities of CO2, H2S and CH4 through Choline-Based Ionic Liquids: Atomistic-Scale Simulations

Molecular dynamics simulations are used to study the transport of CO2, H2S and CH4 molecules across environmentally friendly choline-benzoate and choline-lactate ionic liquids (ILs). The permeability coefficients of the considered molecules are calculated using the free energy and diffusion rate profiles. Both systems show the largest resistance to CH4, whereas more than 5 orders of magnitude larger permeability coefficients are obtained for the other two gas molecules. The CO2/CH4 and H2S/CH4 selectivity was estimated to be more than 104 and 105, respectively. These results indicate the great potential of the considered ILs for greenhouse gas control.

Insight from Molecular dynamic simulation of reactive oxygen species in oxidized skin membrane

Non-enzymatic lipid peroxidation of the skin-lipid bilayer causes perturbations that affect the biomembrane structure, function, and permeability of reactive oxygen species (ROS). In the present study, we employed molecular dynamics simulations to study the effect of lipid peroxidation on the bilayer structural properties and permeability of various ROS. The oxidized skin-lipid bilayer was composed of ceramide, cholesterol, free fatty acid, and 5α-hydroperoxycholesterol (5α-CH). The simulation showed that, upon oxidation, the oxidized group (-OOH) of 5α-CH migrates towards the aqueous phase and the backbone of 5α-CH tilts, which causes the membrane to expand laterally. Measurements of the permeability of all ROS along the oxidized skin-lipid bilayer revealed a decreased breaching barrier for all the species as the degree of peroxidation increased, with a resulting easy passage across the membrane. The insights from the simulations indicate that lipid peroxidation might perturb the membrane barrier, thereby inflicting oxidative stress that leads to apoptosis. This study helps to understand oxidative stress at the atomic level. To our knowledge, this is the first reported molecular dynamics simulation study on oxidized skin-lipid bilayer and permeability of ROS.

Effect of head group and lipid tail oxidation in the cell membrane revealed through integrated simulations and experiments

We report on multi-level atomistic simulations for the interaction of reactive oxygen species (ROS) with the head groups of the phospholipid bilayer, and the subsequent effect of head group and lipid tail oxidation on the structural and dynamic properties of the cell membrane. Our simulations are validated by experiments using a cold atmospheric plasma as external ROS source. We found that plasma treatment leads to a slight initial rise in membrane rigidity, followed by a strong and persistent increase in fluidity, indicating a drop in lipid order. The latter is also revealed by our simulations. This study is important for cancer treatment by therapies producing (extracellular) ROS, such as plasma treatment. These ROS will interact with the cell membrane, first oxidizing the head groups, followed by the lipid tails. A drop in lipid order might allow them to penetrate into the cell interior (e.g., through pores created due to oxidation of the lipid tails) and cause intracellular oxidative damage, eventually leading to cell death. This work in general elucidates the underlying mechanisms of ROS interaction with the cell membrane at the atomic level.

Position-Dependent Diffusion Tensors in Anisotropic Media from Simulation: Oxygen Transport in and through Membranes

A Bayesian-based methodology is developed to estimate diffusion tensors from molecular dynamics simulations of permeants in anisotropic media, and is applied to oxygen in lipid bilayers. By a separation of variables in the Smoluchowski diffusion equation, the multidimensional diffusion is reduced to coupled one-dimensional diffusion problems that are treated by discretization. The resulting diffusivity profiles characterize the membrane transport dynamics as a function of the position across the membrane, discriminating between diffusion normal and parallel to the membrane. The methodology is first validated with neat water, neat hexadecane, and a hexadecane slab surrounded by water, the latter being a simple model for a lipid membrane. Next, a bilayer consisting of pure 1-palmitoyl 2-oleoylphosphatidylcholine (POPC), and a bilayer mimicking the lipid composition of the inner mitochondrial membrane, including cardiolipin, are investigated. We analyze the detailed time evolution of oxygen molecules, in terms of both normal diffusion through and radial diffusion inside the membrane. Diffusion is fast in the more loosely packed interleaflet region, and anisotropic, with oxygen spreading more rapidly in the membrane plane than normal to it. Visualization of the propagator shows that oxygen enters the membrane rapidly, reaching its thermodynamically favored center in about 1 ns, despite the free energy barrier at the headgroup region. Oxygen transport is quantified by computing the oxygen permeability of the membranes and the average radial diffusivity, which confirm the anisotropy of the diffusion. The position-dependent diffusion constants and free energies are used to construct compartmental models and test assumptions used in estimating permeability, including Overton's rule. In particular, a hexadecane slab surrounded by water is found to be a poor model of oxygen transport in membranes because the relevant energy barriers differ substantially.

Hampering Effect of Cholesterol on the Permeation of Reactive Oxygen Species through Phospholipids Bilayer: Possible Explanation for Plasma Cancer Selectivity

In recent years, the ability of cold atmospheric pressure plasmas (CAPS) to selectively induce cell death in cancer cells has been widely established. This selectivity has been assigned to the reactive oxygen and nitrogen species (RONS) created in CAPs. To provide new insights in the search for an explanation for the observed selectivity, we calculate the transfer free energy of multiple ROS across membranes containing a varying amount of cholesterol. The cholesterol fraction is investigated as a selectivity parameter because membranes of cancer cells are known to contain lower fractions of cholesterol compared to healthy cells. We find that cholesterol has a significant effect on the permeation of reactive species across a membrane. Indeed, depending on the specific reactive species, an increasing cholesterol fraction can lead to (i) an increase of the transfer free energy barrier height and width, (ii) the formation of a local free energy minimum in the center of the membrane and (iii) the creation of extra free energy barriers due to the bulky sterol rings. In the context of plasma oncology, these observations suggest that the increased ingress of RONS in cancer cells can be explained by the decreased cholesterol fraction of their cell membrane.

Reactive oxygen species at phospholipid bilayers: distribution, mobility and permeation

Reactive oxygen species (ROS) are involved in biochemical processes such as redox signaling, aging, carcinogenesis and neurodegeneration. Although biomembranes are targets for reactive oxygen species attack, little is known about the role of their specific interactions. Here, molecular dynamics simulations were employed to determine the distribution, mobility and residence times of various reactive oxygen species at the membrane-water interface. Simulations showed that molecular oxygen (O2) accumulated at the membrane interior. The applicability of this result to singlet oxygen ((1)O2) was discussed. Conversely, superoxide (O2(-)) radicals and hydrogen peroxide (H2O2) remained at the aqueous phase. Both hydroxyl (HO) and hydroperoxyl (HO2) radicals were able to penetrate deep into the lipid headgroups region. Due to membrane fluidity and disorder, these radicals had access to potential peroxidation sites along the lipid hydrocarbon chains, without having to overcome the permeation free energy barrier. Strikingly, HO2 radicals were an order of magnitude more concentrated in the headgroups region than in water, implying a large shift in the acid-base equilibrium between HO2 and O2(-). In comparison with O2, both HO and HO2 radicals had lower lateral mobility at the membrane. Simulations revealed that there were intermittent interruptions in the H-bond network around the HO radicals at the headgroups region. This effect is expected to be unfavorable for the H-transfer mechanism involved in HO diffusion. The implications for lipid peroxidation and for the effectiveness of membrane antioxidants were evaluated.

Intramicroparticle nitrogen dioxide is a bubble nucleation site leading to decompression-induced neutrophil activation and vascular injury

Inert gases diffuse into tissues in proportion to ambient pressure, and when pressure is reduced, gas efflux forms bubbles due to the presence of gas cavitation nuclei that are predicted based on theory but have never been characterized. Decompression stress triggers elevations in number and diameter of circulating annexin V-coated microparticles (MPs) derived from vascular cells. Here we show that ∼10% MPs from wild-type (WT) but not inflammatory nitric oxide synthase-2 (iNOS) knockout (KO) mice increase in size when exposed to elevated air pressure ex vivo. This response is abrogated by a preceding exposure to hydrostatic pressure, demonstrating the presence of a preformed gas phase. These MPs have lower density than most particles, 10-fold enrichment in iNOS, and generate commensurately more reactive nitrogen species (RNS). Surprisingly, RNS only slowly diffuse from within MPs unless particles are subjected to osmotic stress or membrane cholesterol is removed. WT mice treated with iNOS inhibitor and KO mice exhibit less decompression-induced neutrophil activation and vascular leak. Contrary to injecting naïve mice with MPs from wild-type decompressed mice, injecting KO MPs triggers fewer proinflammatory events. We conclude that nitrogen dioxide is a nascent gas nucleation site synthesized in some MPs and is responsible for initiating postdecompression inflammatory injuries.

iNOS-selective inhibitors for cancer prevention: promise and progress

Nitric oxide (NO) is involved in various physiological functions and its role in tumorigenesis has been well studied. A large majority of human and experimental tumors appear to progress owing to NO resulting from iNOS, further stimulated by proinflammatory cytokines. Conversely, in some cases, NO is associated with induction of apoptosis and tumor regression. This dichotomy of NO is largely explained by the complexity of signaling pathways in tumor cells, which respond to NO very differently depending on its concentration. In addition, NO alters many signaling pathways through chemical modifications, such as the addition of S-nitrosothiols and nitrosotyrosine to target proteins altering various biological pathways. Hence, iNOS inhibitors are designed and developed to inhibit various organ site cancers including the colon. Here, we review iNOS expression, generation of NO, involvement of NO in altering signaling pathways, and iNOS select inhibitors and their possible use for the prevention and treatment of various cancers.

Antioxidant, antihyperuricemic and xanthine oxidase inhibitory activities of Hyoscyamus reticulatus

CONTEXT

 Xanthine oxidase (XO) is a key enzyme in the pathophysiological homeostasis of hyperuricemia. It catalyzes the oxidation of hypoxanthine to xanthine and then to uric acid, the reaction involves the formation of free radical intermediates and superoxide byproducts.

OBJECTIVES

 This study was undertaken to investigate the antioxidant, antihyperuricemic, and xanthine oxidase inhibitory potentials of Hyoscyamus reticulatus L. (Solanaceae) extract.

MATERIALS AND METHODS

 The antioxidant potency was measured using the ABTS•+ scavenging capacity system, which includes Trolox as a standard. The xanthine oxidase inhibitory activity of the extract was quantitated in vitro by measuring the decline in the catalytic rate of xanthine oxidase following incubations with the plant extracts and using xanthine as a substrate. The hypouricemic potential of the extract was evaluated using an in vivo model for hyperuricemia. We tested three different doses of the extract and allopurinol was used as standard antihyperuricemic positive control.

RESULTS

 H. reticulatus aqueous extract exhibited significant antioxidant scavenging properties (533.26 μmol TE/g dry extract weight) and inhibitory effect on xanthine oxidase activity (IC₅₀ 12.8 μg/mL). Furthermore, oral administration of the aqueous extract significantly reduced serum urate levels in oxonate-induced hyperuricemic mice in a dose-dependent manner.

DISCUSSION AND CONCLUSION

 Our results suggest that the aqueous extract of H. reticulatus aerial parts might have great potential as an antioxidant and a hypouricemic agent. Our lab is currently identifying the active compounds in the extract to which the biological activities could be attributed.

The need for monitoring the actual nitric oxide concentration in tumors

The significance of the role of nitric oxide (NO) in cancer is evident from 1,100 publications on the subject; its triggering of apoptosis at high concentrations is documented in 300 publications. While aspects of the rate of generation of NO in tumors have been extensively studied, the rate of its removal from tumors has not been considered, even though it is the difference between the two rates that determines the all important steady-state NO concentration, and thus the likelihood of apoptosis-triggering. The rate of transport of NO scales with its concentration gradient at the interface between a neoplasm and the phase to which it diffuses, which can be air, fat, or blood. Diffusional loss of NO to air would explain the initial two-dimensionality of neoplasms of the skin and lung. The greater solubility of NO in lipids than in aqueous phases should cause its extraction by nearby fat, and would account for the positive correlation between obesity and the incidence of some cancers, such as cancers of the breast. And the rapid consumption of NO by red blood cells implies depletion of excess NO in tumors after they are vascularized: angiogenesis should blunt any apoptosis-triggering NO attack of the immune system. Thus, cancer research and the practice of oncology may benefit of in-tumor monitoring of the actual NO concentration. Miniature NO monitoring electrodes, that might serve the purpose, are reviewed.