Coelenterazine analogs as chemiluminescent probe for superoxide anion

Ex Tenebris Lux: Illuminating Reactive Oxygen and Nitrogen Species with Small Molecule Probes

Reactive oxygen and nitrogen species are small reactive molecules derived from elements in the air─oxygen and nitrogen. They are produced in biological systems to mediate fundamental aspects of cellular signaling but must be very tightly balanced to prevent indiscriminate damage to biological molecules. Small molecule probes can transmute the specific nature of each reactive oxygen and nitrogen species into an observable luminescent signal (or even an acoustic wave) to offer sensitive and selective imaging in living cells and whole animals. This review focuses specifically on small molecule probes for superoxide, hydrogen peroxide, hypochlorite, nitric oxide, and peroxynitrite that provide a luminescent or photoacoustic signal. Important background information on general photophysical phenomena, common probe designs, mechanisms, and imaging modalities will be provided, and then, probes for each analyte will be thoroughly evaluated. A discussion of the successes of the field will be presented, followed by recommendations for improvement and a future outlook of emerging trends. Our objectives are to provide an informative, useful, and thorough field guide to small molecule probes for reactive oxygen and nitrogen species as well as important context to compare the ecosystem of chemistries and molecular scaffolds that has manifested within the field.

Near-infrared chemiluminescence imaging of superoxide anion production in kidneys with iron3+-nitrilotriacetate-induced acute renal oxidative stress in rats

Iron-catalyzed oxidative stress generates reactive oxygen species in the kidney and induces oxidative damage including lipid, protein, and DNA modifications which induces renal injury and may lead to cancer. An analysis of oxidative stress dynamics by reactive oxygen species has not been performed non-invasively in real time in intact kidneys and is a significant challenge in biology and medicine. Here, I report that MCLA-800 is a near-infrared chemiluminescent probe that visualizes the dynamics of superoxide anion (O₂^•-) production and the upstream generation of reactive oxygen species in living rat kidneys suffering acute renal oxidative stress induced by intraperitoneal administration of iron³⁺-nitrilotriacetate (Fe³⁺-NTA) as a representative Fe³⁺ chelate. MCLA-800 was intravenously injected at 250 nmol/kg body weight and immediately transported to the kidneys with the emitting light dependent on O₂^•- production. The magnitude of O₂^•- production correlated with the Fe³⁺-NTA dose. O₂^•- was continuously produced in the blood stream following Fe³⁺-NTA injection at 0.15 mmol/kg body weight, while peak production in the renal cortex occurred at 24 h, then decreased to the background level at 72 h. This study clearly revealed the dynamics of Fe³⁺-NTA-mediated O₂^•- production in the living kidney by chemiluminescent imaging of O₂^•- production using MCLA-800.

Acridinium Ester Chemiluminescence: Methyl Substitution on the Acridine Moiety

Methyl groups were introduced on the acridine moiety in chemiluminescent acridinium esters that have electron-withdrawing groups (trifluoromethyl, cyano, nitro, ethoxycarbonyl) at the 4-position on the phenyl ester. The introduction of methyl groups at the 2-, 2,7-, and 2,3,6,7-positions on the acridine moiety shifted the optimal pH that gave relatively strong chemiluminescence intensity from neutral conditions to alkaline conditions. 4-(Ethoxycarbonyl)phenyl 2,3,6,7,10-pentamethyl-10λ⁴-acridine-9-carboxylate, trifluoromethanesulfonate salt showed long-lasting chemiluminescence under alkaline conditions. Acridinium esters to determine hydrogen peroxide concentration at pH 7-10 were newly developed.

Non-invasive and accurate readout of superoxide anion in biological systems by near-infrared light

Infectious and inflammatory diseases involve superoxide anion (O₂^•-) production. Real-time and non-invasive evaluation of O₂^•- in intact biological systems has been a significant challenge in biology and medicine. Here, I report that an advanced near-infrared chemiluminescent probe, MCLA-800, enables reliable non-invasive optical readout of O₂^•-ex vivo and in vivo. MCLA-800 allowed highly selective and sensitive monitoring of O₂^•- in undiluted human whole blood ex vivo. For the first time, the use of MCLA-800 revealed two reproducible types of O₂^•- production in response to stimulation by unopsonized zymosan particles of Saccharomyces cerevisiae, that is, slow response (S-type) and fast response (F-type), specific to each individual. O₂^•- production was synchronized with myeloperoxidase (MPO) activation in the former type but not in the latter. Moreover, as new findings, MCLA-800 chemiluminescence demonstrated that the chemiluminescence intensity-time properties of formyl-methionyl-leucyl-phenylalanine (fMLP)- or phorbol 12-myristate 13-acetate (PMA)-induced O₂^•- production and MPO activity were independent of S- and F-type zymosan-induced MCLA-800 chemiluminescence whole blood and that PMA-induced MPO activation synchronized with PMA-induced O₂^•- production in S- and F-type zymosan-induced MCLA-800 chemiluminescence whole blood, but fMLP-induced MPO activation did not synchronize with fMLP-induced O₂^•- production in both of S- and F-type blood. Furthermore, MCLA-800 spatiotemporally allowed non-invasive and clear in vivo imaging of O₂^•- in animal models of acute dermatitis and focal arthritis. Therefore, MCLA-800 could be possibly applied in various advanced diagnostic techniques.

Core-Modified Coelenterazine Luciferin Analogues: Synthesis and Chemiluminescence Properties

In this work on the design and studies of luciferins related to the blue-hued coelenterazine, the synthesis of heterocyclic analogues susceptible to produce a photon, possibly at a different wavelength, is undertaken. Here, the synthesis of O-acetylated derivatives of imidazo[1,2-b]pyridazin-3(5 H)-one, imidazo[2,1-f][1,2,4]triazin-7(1 H)-one, imidazo[1,2-a]pyridin-3-ol, imidazo[1,2-a]quinoxalin-1(5 H)-one, benzo[f]imidazo[1,2-a]quinoxalin-3(11 H)-one, imidazo[1',2':1,6]pyrazino[2,3-c]quinolin-3(11 H)-one, and 5,11-dihydro-3 H-chromeno[4,3-e]imidazo[1,2-a]pyrazin-3-one is described thanks to extensive use of the Buchwald-Hartwig N-arylation reaction. The acidic hydrolysis of these derivatives then gave solutions of the corresponding luciferin analogues, which were studied. Not too unexpectedly, even if these were "dressed" with substituents found in actual substrates of the nanoKAZ/NanoLuc luciferase, no bioluminescence was observed with these compounds. However, in a phosphate buffer, all produced a light signal, by chemiluminescence, with extensive variations in their respective intensity and this could be increased by adding a quaternary ammonium salt in the buffer. This aspect was actually instrumental to determine the emission spectra of many of these luciferin analogues.

Bioluminescence Detection of Superoxide Anion Using Aequorin

Although the superoxide anion (O2-·) is generated during normal cellular respiration and has fundamental roles in a wide range of cellular processes, such as cell proliferation, migration, apoptosis, and homeostasis, its dysregulation is associated with a variety of diseases. Regarding these prominent roles in biological systems, the development of accurate methods for quantification of superoxide anion has attracted tremendous research attention. Here, we evaluated aequorin, a calcium-dependent photoprotein, as a potential bioluminescent reporter protein of superoxide anion. The mechanism is based on the measurement of aequorin bioluminescence, where the lower the concentration of coelenterazine under the oxidation of superoxide anion, the lower the amount aequorin regeneration, leading to a decrease in bioluminescence. The bioluminescence intensity of aequorin was proportional to the concentration of superoxide anion in the range from 4 to 40 000 pM with a detection limit (S/N = 3) of 1.2 pM, which was 5000-fold lower than those of the chemiluminescence methods. The proposed method exhibited high sensitivity and has been successfully applied to the determination of superoxide anion in the plant cell samples. The results could suggest a photoprotein-based bioluminescence system as a highly sensitive, specific, and simple bioluminescent probe for in vitro detection of superoxide anion.

Small-molecule luminescent probes for the detection of cellular oxidizing and nitrating species

Reactive oxygen species (ROS) have been implicated in both pathogenic cellular damage events and physiological cellular redox signaling and regulation. To unravel the biological role of ROS, it is very important to be able to detect and identify the species involved. In this review, we introduce the reader to the methods of detection of ROS using luminescent (fluorescent, chemiluminescent, and bioluminescent) probes and discuss typical limitations of those probes. We review the most widely used probes, state-of-the-art assays, and the new, promising approaches for rigorous detection and identification of superoxide radical anion, hydrogen peroxide, and peroxynitrite. The combination of real-time monitoring of the dynamics of ROS in cells and the identification of the specific products formed from the probes will reveal the role of specific types of ROS in cellular function and dysfunction. Understanding the molecular mechanisms involving ROS may help with the development of new therapeutics for several diseases involving dysregulated cellular redox status.

Reactive Oxygen Species Imaging in a Mouse Model of Inflammatory Bowel Disease

Purpose

Reactive oxygen species (ROS) are important contributors to inflammatory bowel disease (IBD); however, there are insufficient tools for their in vivo evaluation.

Procedures

To determine if a chemiluminescent ROS reporter, coelenterazine, would be a useful tool for the detection of immune cell activation, the macrophage cell line (RAW 264.7) was treated with phorbol myristate acetate (PMA). Additionally, coelenterazine was used to monitor the changes in ROS production over time in a mouse model of IBD.

Results

In vitro, coelenterazine enabled the dynamic monitoring of the RAW 264.7 cell oxidative burst. In vivo, there were early, preclinical, changes in the localization and magnitude of coelenterazine chemiluminescent foci.

Conclusions

Coelenterazine offers a high-throughput method for assessing immune cell activation in culture and provides a means for the in vivo detection and localization of ROS during IBD disease progression.

Multimodality Imaging of Cancer Superoxide Anion Using the Small Molecule Coelenterazine

PURPOSE

We evaluated the small molecule coelenterazine as a potential reporter of cancer-associated superoxide anion in cell culture and in mice.

PROCEDURES

The superoxide anion concentrations of various cancer cell lines were quantified by coelenterazine chemiluminescence in vitro. Coelenteramide fluorescence was detected via flow cytometry and fluorescent microscopy. Coelenterazine was used for the in vivo detection of cancer-associated superoxide anion using the 4T1 breast adenocarcinoma mouse model.

RESULTS

Various cell lines in culture demonstrated different superoxide anion concentrations, with a signal range of 3.15 ± 0.06 to 11.80 ± 0.24 times that of background. In addition to chemiluminescent detection of coelenterazine, we demonstrated fluorescent detection of coelenteramide within the cytoplasm of cells. 4T1 murine mammary adenocarcinoma tumors in mice demonstrated significantly higher 2.13 ± 0.19-fold coelenterazine-based chemiluminescence than that of surrounding normal tissues.

CONCLUSIONS

Collectively, our results indicate that coelenterazine can be used to assay superoxide anion concentrations in cultured cancer cells and in tumors growing in mice.

Lighting up bioluminescence with coelenterazine: strategies and applications

Bioluminescence-based techniques, such as bioluminescence imaging, BRET and dual-luciferase reporter assay systems, have been widely used to examine a myriad of biological processes. Coelenterazine (CTZ), a luciferin or light-producing compound found in bioluminescent organisms, has sparked great curiosity and interest in searching for analogues with improved photochemical properties. This review summarizes the current development of coelenterazine analogues, their bioluminescence properties, and the rational design of caged coelenterazine towards biotargets, as well as their applications in bioassays. It should be emphasized that the design of caged luciferins can provide valuable insight into detailed molecular processes in organisms and will be a trend in the development of bioluminescent molecules.

General Bioluminescence Resonance Energy Transfer Homogeneous Immunoassay for Small Molecules Based on Quantum Dots

Here, we describe a general bioluminescence resonance energy transfer (BRET) homogeneous immunoassay based on quantum dots (QDs) as the acceptor and Renilla luciferase (Rluc) as the donor (QD-BRET) for the determination of small molecules. The ratio of the donor-acceptor that could produce energy transfer varied in the presence of different concentrations of free enrofloxacin (ENR), an important small molecule in food safety. The calculated Förster distance (R0) was 7.86 nm. Under optimized conditions, the half-maximal inhibitory concentration (IC50) for ENR was less than 1 ng/mL and the linear range covered 4 orders of magnitude (0.023 to 25.60 ng/mL). The cross-reactivities (CRs) of seven representative fluoroquinolones (FQs) were similar to the data obtained by an enzyme-linked immunosorbent assay (ELISA). The average intra- and interassay recoveries from spiked milk of were 79.8-118.0%, and the relative standard deviations (RSDs) were less than 10%, meeting the requirement of residue detection, which was a satisfactory result. Furthermore, we compared the influence of different luciferase substrates on the performance of the assay. Considering sensitivity and stability, coelenterazine-h was the most appropriate substrate. The results from this study will enable better-informed decisions on the choice of Rluc substrate for QD-BRET systems. For the future, the QD-BRET immunosensor could easily be extended to other small molecules and thus represents a versatile strategy in food safety, the environment, clinical diagnosis, and other fields.

Chemiluminescence Imaging of Superoxide Anion Detects Beta-Cell Function and Mass

Superoxide anion is produced during normal cellular respiration and plays key roles in cellular physiology with its dysregulation being associated with a variety of diseases. Superoxide anion is a short-lived molecule and, therefore, its homeostatic regulation and role in biology and disease requires dynamic quantification with fine temporal resolution. Here we validated coelenterazine as a reporter of intracellular superoxide anion concentration and used it as a dynamic measure both in vitro and in vivo. Chemiluminescence was dependent upon superoxide anion levels, including those produced during cellular respiration, and concentrations varied both kinetically and temporally in response to physiologically relevant fluctuations in glucose levels. In vivo imaging with coelenterazine revealed that beta cells of the pancreas have increased levels of superoxide anion, which acted as a measure of beta-cell function and mass and could predict the susceptibility of mice to diabetes mellitus. Glucose response and regulation are key elements of cellular physiology and organismal biology, and superoxide anion appears to play a fundamental and dynamic role in both of these processes.

Bioluminescent coelenterazine derivatives with imidazopyrazinone C-6 extended substitution

Three novel coelenterazine (CTZ) derivatives with extension at the C-6 position of the imidazopyrazinone structure show significant bioluminescence emission with known renilla luciferase variants, indicating a promising method to develop CTZ derivatives with superior optical properties compared to hitherto reported compounds.

Light-emitting channelrhodopsins for combined optogenetic and chemical-genetic control of neurons

Manipulation of neuronal activity through genetically targeted actuator molecules is a powerful approach for studying information flow in the brain. In these approaches the genetically targeted component, a receptor or a channel, is activated either by a small molecule (chemical genetics) or by light from a physical source (optogenetics). We developed a hybrid technology that allows control of the same neurons by both optogenetic and chemical genetic means. The approach is based on engineered chimeric fusions of a light-generating protein (luciferase) to a light-activated ion channel (channelrhodopsin). Ionic currents then can be activated by bioluminescence upon activation of luciferase by its substrate, coelenterazine (CTZ), as well as by external light. In cell lines, expression of the fusion of Gaussia luciferase to Channelrhodopsin-2 yielded photocurrents in response to CTZ. Larger photocurrents were produced by fusing the luciferase to Volvox Channelrhodopsin-1. This version allowed chemical modulation of neuronal activity when expressed in cultured neurons: CTZ treatment shifted neuronal responses to injected currents and sensitized neurons to fire action potentials in response to subthreshold synaptic inputs. These luminescent channelrhodopsins--or luminopsins--preserve the advantages of light-activated ion channels, while extending their capabilities. Our proof-of-principle results suggest that this novel class of tools can be improved and extended in numerous ways.

Peroxynitrite formation in nitric oxide-exposed submitochondrial particles: detection, oxidative damage and catalytic removal by Mn-porphyrins

Peroxynitrite (ONOO(-)) formation in mitochondria may be favored due to the constant supply of superoxide radical (O(2)(∙-)) by the electron transport chain plus the facile diffusion of nitric oxide ((∙)NO) to this organelle. Herein, a model system of submitochondrial particles (SMP) in the presence of succinate plus the respiratory inhibitor antimycin A (to increase O(2)(∙-) rates) and the (∙)NO-donor NOC-7 was studied to directly establish and quantitate peroxynitrite by a multiplicity of methods including chemiluminescence, fluorescence and immunochemical analysis. While all the tested probes revealed peroxynitrite at near stoichiometric levels with respect to its precursor radicals, coumarin boronic acid (a probe that directly reacts with peroxynitrite) had the more straightforward oxidation profile from O(2)(∙-)-forming SMP as a function of the (∙)NO flux. Interestingly, immunospintrapping studies verified protein radical generation in SMP by peroxynitrite. Substrate-supplemented SMP also reduced Mn(III)porphyrins (MnP) to Mn(II)P under physiologically-relevant oxygen levels (3-30 μM); then, Mn(II)P were capable to reduce peroxynitrite and protect SMP from the inhibition of complex I-dependent oxygen consumption and protein radical formation and nitration of membranes. The data directly support the formation of peroxynitrite in mitochondria and demonstrate that MnP can undergo a catalytic redox cycle to neutralize peroxynitrite-dependent mitochondrial oxidative damage.

Enzymatic activity of albumin shown by coelenterazine chemiluminescence

Bioluminescence, the emission of light from live organisms, occurs in 18 phyla and is the major communication system in the deep sea. It has appeared independently many times during evolution but its origins remain unknown. Coelenterazine bioluminescence discovered in luminous jellyfish is the most common chemistry causing bioluminescence in the sea, occurring in seven phyla. Sequence similarities between coelenterazine luciferases and photoproteins from different phyla are poor (often < 5%). The aim of this study was to examine albumin that binds organic substances as a coelenterazine luciferase to test the hypothesis that the evolutionary origin of a bioluminescent protein was the result of the formation of a solvent cage containing just a few key amino acids. The results show for the first time that bovine and human albumin catalysed coelenterazine chemiluminescence consistent with a mono-oxygenase, whereas gelatin and haemoglobin, an oxygen carrier, had very weak activity. Insulin also catalysed coelenterazine chemiluminescence and was increased by Zn(2+). Albumin chemiluminescence was heat denaturable, exhibited saturable substrate characteristics and was inhibited by cations that bound these proteins and by drugs that bind to human albumin drug site I. Molecular modelling confirmed the coelenterazine binding site and identified four basic amino acids: lys195, arg222, his242 and arg257, potentially important in binding and catalysis similar to naturally occurring coelenterazine bioluminescent proteins. These results support the 'solvent cage' hypothesis for the evolutionary origin of enzymatic coelenterazine bioluminescent proteins. They also have important consequences in diseases such as diabetes, gut disorders and food intolerance where a mono-oxygenase could affect cell surface proteins.

Modeling the effect of oxygen on the amperometric response of immobilized organoselenium-based S-nitrosothiol sensors

Amperometric detection of S-nitrosothiols (RSNOs) at submicromolar levels in blood samples is of potential importance for monitoring endothelial function and other disease states that involve changes in physiological nitric oxide (NO) production. It is shown here that the elimination of dissolved oxygen from samples is critical when covalently attached diselenocystamine-based amperometric RSNO sensors are used for practical RSNO measurements. The newest generation of RSNO sensors utilizes an amperometric NO gas sensor with a thin organoselenium modified dialysis membrane mounted at the distal sensing tip. Sample RSNOs are catalytically reduced to NO within the dialysis membrane by the immobilized organoselenium species. In the presence of oxygen, the sensitivity of these sensors for measuring low levels of RSNOs (<μM) is greatly reduced. It is demonstrated that the main scavenger of the generated nitric oxide is not the dissolved oxygen but rather superoxide anion radical generated from the reaction of the reduced organoselenium species (the reactive species in the catalytic redox cycle) and dissolved oxygen. Computer simulations of the response of the RSNO sensor using rate constants and diffusion coefficients for the reactions involved, known from the literature or estimated from fitting to the observed amperometric response curves, as well as the specific geometric dimensions of the RSNO sensor, further support that nitric oxide and superoxide anion radical quickly react resulting in near zero sensor sensitivity toward RSNO concentrations in the submicromolar concentration range. Elimination of oxygen from samples helps improve sensor detection limits to ca. 10 nM levels of RSNOs.

H2O2 inhibits BCR-dependent immediate early induction of EBV genes in Burkitt's lymphoma cells

The critical step in the Epstein-Barr virus (EBV) transition from latency to lytic replication is activation of the viral immediate early (IE) genes, BZLF1 and BRLF1. Their induction in Burkitt's lymphoma Akata cells is directly targeted by B cell receptor (BCR) signaling. On the other hand, BCR stimulation causes an outwardly directed superoxide (O(2)(*-)) burst leading to massive generation of reactive oxygen species in the cell environment. Our goal was to investigate the role of BCR-related redox changes in the IE reactivation of EBV. Production of O(2)(*-) by stimulated Akata cells was characterized using chemiluminescent dyes, lucigenin, MCLA, and coelenterazine. Expression of the EBV IE genes was analyzed by real-time PCR and Western blot assays. Catalase activity and H(2)O(2) concentration were evaluated using Amplex Red assays and by measuring light absorption at 240 nm. We show here that elevation of H(2)O(2) concentration in Akata cell suspensions inhibits the induction of the virus IE mRNA and BZLF1 protein. It was further found that Akata cells exhibit catalase-like activity that is stimulated by BCR cross-linking. The results reveal that H(2)O(2) is instrumental in the maintenance of EBV latency. Altogether they provide new evidence demonstrating the essential role of H(2)O(2) in BCR signaling.

Recombinant Metridia luciferase isoforms: expression, refolding and applicability for in vitro assay

The recombinant coelenterazine-dependent luciferases (isoforms MLuc164 and MLuc39) from the marine copepod Metridia longa were expressed as inclusion bodies in E. coli cells, dissolved in 6 M guanidinium chloride and folded in conditions developed for proteins containing intramolecular disulfide bonds. One of them (MLuc39) was obtained in an active monomeric form with a high yield. The luciferase bioluminescence is found to be initiated not only by free coelenterazine, but also by Ca2+-dependent coelenterazine-binding protein (CBP) of Renilla muelleri on Ca2+ addition. The use of CBP as a "substrate" provides higher light emission and simultaneously the lower level of background. The high purity MLuc39 can be detected down to attomol with a linear range extending over 5 orders of magnitude. The MLuc39 reveals also a high stability towards heating and chemical modification; the chemically synthesized biotinylated derivatives of the luciferase preserve 35-40% of the initial activity. The luciferase applicability as an in vitro bioluminescent reporter is demonstrated in model tandem bioluminescent solid-phase microassay combining the Ca2+-regulated photoprotein obelin and the Metridia luciferase.

Detection and measurement of reactive oxygen intermediates in mitochondria and cells

Reactive oxygen intermediates (ROIs) play a key role in a number of human diseases either by inducing cell death, cellular proliferation, or by acting as mediators in cellular signaling. Therefore, their measurement in vivo and in cell culture is desirable but technically difficult and often troublesome. To address some of the key methodological issues in examining the formation of ROI in cells and mitochondria, this chapter discusses the following: (a) the cellular sources of ROI and their enzymatic removal, (b) common methods used to determine cellular and mitochondrial ROI such as chemiluminescence, electron paramagnetic resonance spectroscopy, fluorescence, and enzymatic techniques, and (c) some common problems associated with these assays and the interpretation of data. We also provide some simple protocols for the estimation of ROI production in cells and mitochondria, and when measuring ROI in cells and mitochondria, we emphasize the need for thorough understanding of results obtained and their interpretation.

Luminescence of imidazo[1,2-a]pyrazin-3(7H)-one compounds

In this review I will discuss chemical principles of the luminescence of imidazo[1,2-a]pyrazin-3(7H)-one compounds described to date. The review is composed of two main parts, the first dealing with the bioluminescence of coelenterate luciferin "coelenterazine" and Cypridina luciferin in marine organisms and the second with the chemiluminescence of these luciferins and their analogues. In the second section, possible applications of chemiluminescence and enhanced chemiluminescence in the area of bioassay are also discussed.

Development of imidazopyrazinone red-chemiluminescent probes for detecting superoxide anions via a chemiluminescence resonance energy transfer method

During the development of useful probes for detecting superoxide anions via chemiluminescence with longer wavelengths than that of green, chemiluminescent probes that emit red light (lambda(max) 610 nm) when induced by superoxide anions were synthesized and characterized. These red-chemiluminescent probes consist of a 6-(4-methoxyphenyl)imidazo[1,2-a] pyrazin-3(7H)-one moiety, which reacts with superoxide anions to generate energy, and a sulphorhodamine 101 moiety, which accepts the energy and emits red light. Using a hypoxanthine-xanthine oxidase system for the generation of superoxide anions, it was shown that the superoxide anion-induced chemiluminescences of red-chemiluminescent probes (3 and 4) were more intense than those of the blue- and green-chemiluminescent probes 2-methyl-6-(4-methoxyphenyl)imidazo[1,2-a] pyrazin-3(7H)-one (MCLA) and 6-[4-[2-[N'-(5-fluoresceinyl)thioureido]-ethoxy]phenyl]-2-methylimidazo[1,2-a]pyrazin-3(7H)-one (FCLA), respectively, which are generally considered to be the most sensitive chemiluminescent probes. The ratio between the superoxide-dependent and background chemiluminescence intensities for 3 was comparable to those of MCLA and FCLA, but higher than that of 4. Due to its highly intense superoxide anion-induced chemiluminescence at low probe concentrations, red-chemiluminescent probe 3 is superior to MCLA and FCLA for measurement of superoxide anions.

Cyclodextrin-bound 6-(4-methoxyphenyl)imidazo[1,2-alpha+/-]pyrazin-3(7H)-ones with fluorescein as green chemiluminescent probes for superoxide anions

In providing chemiluminescent probes that have high chemiluminescence intensity and high specificity to superoxide anions, novel chemiluminescent probes involving cyclodextrins covalently bound to 6-(4-methoxyphenyl)imidazo[1,2-alpha]pyrazin-3(7H)-one with fluorescein were synthesized and characterized. Using the hypoxanthine-xanthine oxidase system for the generation of the superoxide anions, these novel chemiluminescent probes showed higher superoxide-induced chemiluminescence intensity than that of 6-[4-[2-[N(')-(5-fluoresceinyl)thioureido]-ethoxy]phenyl]-2-methylimidazo[1,2-alpha]pyrazin-3(7H)-one (FCLA). When tested at a probe concentration of 1.0 microM, compound 6, in which 6-(4-methoxyphenyl)imidazo[1,2-alpha]pyrazin-3(7H)-one and fluorescein are covalently attached on the secondary and primary hydroxyl faces of gamma-cyclodextrin, respectively, showed green luminescence intensity that was 26 times that of FCLA, which was also the highest luminescence intensity in this present study. At probe concentrations of less than 1.0 microM, the ratio of the superoxide-dependent chemiluminescence intensity to the background chemiluminescence intensity for compound 6 was higher than that of FCLA. This high superoxide-induced chemiluminescence intensity and superoxide specificity in low probe concentrations indicates that 6 can be more effective than FCLA toward the measurement of superoxide anions.

Complex III releases superoxide to both sides of the inner mitochondrial membrane

Mechanisms of mitochondrial superoxide formation remain poorly understood despite considerable medical interest in oxidative stress. Superoxide is produced from both Complexes I and III of the electron transport chain, and once in its anionic form it is too strongly charged to readily cross the inner mitochondrial membrane. Thus, superoxide production exhibits a distinct membrane sidedness or "topology." In the present work, using measurements of hydrogen peroxide (Amplex red) as well as superoxide (modified Cypridina luciferin analog and aconitase), we demonstrate that Complex I-dependent superoxide is exclusively released into the matrix and that no detectable levels escape from intact mitochondria. This finding fits well with the proposed site of electron leak at Complex I, namely the iron-sulfur clusters of the (matrix-protruding) hydrophilic arm. Our data on Complex III show direct extramitochondrial release of superoxide, but measurements of hydrogen peroxide production revealed that this could only account for approximately 50% of the total electron leak even in mitochondria lacking CuZn-superoxide dismutase. We posit that the remaining approximately 50% of the electron leak must be due to superoxide released to the matrix. Measurements of (mitochondrial matrix) aconitase inhibition, performed in the presence of exogenous superoxide dismutase and catalase, confirmed this hypothesis. Our data indicate that Complex III can release superoxide to both sides of the inner mitochondrial membrane. The locus of superoxide production in Complex III, the ubiquinol oxidation site, is situated immediately next to the intermembrane space. This explains extramitochondrial release of superoxide but raises the question of how superoxide could reach the matrix. We discuss two models explaining this result.

Methods for detection of reactive metabolites of oxygen and nitrogen: in vitro and in vivo considerations

Facile detection of reactive oxygen and nitrogen species in biologic systems is often problematic. This is a result of the numerous cellular mechanisms, both enzymatic and nonenzymatic involved in their catabolism/decomposition, the complex and overlapping nature of their reactivities, as well as the often limited intracellular access of detector systems. This review describes approaches to the direct and indirect measurement of different reactive metabolites of oxygen and nitrogen. Particular attention to a method's applicability for in vivo determinations will be addressed.

Coelenterazine: a two-stage antioxidant in lipid micelles

Coelenterazine is a luciferin found in many marine bioluminescent organisms. This luciferin also possesses high antioxidant properties and an exceptional ability to protect cells exposed to oxidative stress. It has been suggested that coelenterazine's antioxidative mechanisms include the formation of an oxidation product, coelenteramine, also endowed with chain-breaking properties. In this work, coelenterazine analogs were shown to delay the onset of lipid peroxidation in a linoleate micellar solution exposed to free radical initiators. Their consumption was accompanied by the concomitant formation of coelenteramine. This was followed by a reduction in the peroxidation rate coinciding with the consumption of coelenterazine's oxidation product coelenteramine. The addition of coelenteramine to micelles reduced the propagation rate of the oxidative process. When coelenterazine analogs oxidizing into an inactive analog of coelenteramine were applied, the delaying effect but not the reduced peroxidation rate nor the consumption of the aminopyrazine was observed. These results demonstrate the role of the oxidation product coelenteramine in the chain-breaking properties of coelenterazine and analogs.

Lucigenin and coelenterazine as superoxide probes in mitochondrial and bacterial membranes

The chemiluminescent superoxide indicators lucigenin and coelenterazine were compared in rat liver submitochondrial particles and cytoplasmic membranes from Paracoccus denitrificans. Qualitative monitoring is possible with both probes, but quantitative work with lucigenin is hampered by its dependence on one-electron reduction before the photon-emitting reaction. Therefore, calibration of measurements on complex I, capable of efficient lucigenin prereduction with reduced nicotinamide adenine dinucleotide, against xanthine oxidase, which in the presence of hypoxanthine is not able to reduce the probe to a significant rate compared to complex I, may give results in error by one order of magnitude. Coelenterazine, although susceptible of storage-dependent high background chemiluminescence, does not require prereduction and is thus a more reliable probe.

Enhanced chemiluminescence of 6-(4-methoxyphenyl)imidazo[1,2-a]pyrazin-3(7H)-one by attachment of a cyclomaltooligosaccharide (cyclodextrin). Attachment of cyclomaltononaose (delta-cyclodextrin)

2-Methyl-6-(4-methoxyphenyl)imidazo[1,2-a]pyrazin-3(7H)-one (MCLA) is an oxygen-induced chemiluminescent compound. It has been shown that the chemiluminescence can be enhanced by forming a cyclomaltooligosaccharide (cyclodextrin)-bound MCLA, and therefore, in continuation of the survey of the types of cyclodextrins, in this study, MCLA was attached to the secondary hydroxyl face of delta-cyclodextrin, which consists of nine D-glucose units. Although the oxygen-induced chemiluminescence efficiency of delta-cyclodextrin-bound MCLA in a pH 8.0 aqueous phosphate buffer was 12 times greater than that of MCLA, the efficiency was markedly lower than that of gamma-cyclodextrin-bound MCLA, which exhibited the highest chemiluminescence efficiency in the previous investigation. Although fluorescence efficiency and light-emitter formation efficiency for delta-cyclodextrin-bound MCLA were similar to those for gamma-cyclodextrin-bound MCLA, singlet-excited state formation efficiency for delta-cyclodextrin-bound MCLA was lower than that for gamma-cyclodextrin-bound MCLA. This study distinctly indicated the optimum cyclodextrin for construction of greatly luminescent cyclodextrin-bound MCLA is gamma-cyclodextrin.

Localization of tumor by chemiluminescence probe during photosensitization action

As a chemiluminescence (CL) probe, 3,7-dihydro-6-[4-[2-(N'-(5-fluoresceinyl) thioureido)ethoxy]phenyl]-2-methylimidazo[1,2-a]pyrazin-3-one dosium salt (FCLA) can sensitively and specifically react with singlet oxygen ((1)O(2)) and superoxide (O(2)(-)). Based on the fact that photosensitization action involves (1)O(2) generation, this report presents a novel method for localization of tumor mediated by FCLA. In experiments, FCLA was used as an optical reporter of (1)O(2) produced from photosensitization reaction of hematoporphyrin derivative. The effects of azide inhibition and deuterium enhancement on photosensitized CL indicated that (1)O(2) molecules generated during photosensitization action could be measured and imaged. In vivo experiments in vivo with tumor-bearing nude mice showed a clear CL image of tumor. The study suggests that CL imaging method may be applicable to the detection of (1)O(2) and the diagnosis of superficial tumors.

Recombinant Gaussia luciferase. Overexpression, purification, and analytical application of a bioluminescent reporter for DNA hybridization

The cDNA for Gaussia luciferase (GLuc), the enzyme responsible for the bioluminescent reaction of the marine copepod Gaussia princeps, has been cloned recently. GLuc (MW = 19 900) catalyzes the oxidative decarboxylation of coelenterazine to produce coelenteramide and light. We report the first quantitative anaytical study of GLuc and examine its potential as a new reporter for DNA hybridization. A plasmid encoding both a biotin acceptor peptide-GLuc fusion protein as well as the enzyme biotin protein ligase (BPL) is engineered by using GLuc cDNA as a starting template. BPL catalyzes the covalent attachment of a single biotin to the fusion protein in vivo. Purification of GLuc is then accomplished by affinity chromatography using immobilized monomeric avidin. Moreover, the in vivo biotinylation enables subsequent complexation of GLuc with streptavidin (SA), thereby avoiding chemical conjugation reactions that are known to inactivate luciferases. Purified GLuc can be detected down to 1 amol with a signal-to-background ratio of 2 and a linear range extending over 5 orders of magnitude. The background luminescence of coelenterazine is the main limiting factor for even higher detectability of GLuc. Furthermore, the GLuc-SA complex is used as a detection reagent in a microtiter well-based DNA hybridization assay. The analytical range extends from 1.6 to 800 pmol/L of target DNA. Biotinylated GLuc produced from 1 L of bacterial culture is sufficient for 150,000 hybridization assays.

Experimental study on photodynamic diagnosis of cancer mediated by chemiluminescence probe

A novel method of photodynamic diagnosis of cancer mediated by chemiluminescence probe is presented. The mechanism for photodynamic therapy involves singlet oxygen ((1)O(2)) generated by energy transfer from photosensitizers. (1)O(2) can react with 3,7-dihydro-6-[4-[2-(N'-(5-fluoresceinyl)thioureido)ethoxy]phenyl]-2-methylimidazo[1,2-a]pyrazin-3-one sodium salt (FCLA), which is a Cypridina luciferin analog and a specific chemiluminescence probe for detecting (1)O(2) and superoxide (O(2)(-)). The reaction of FCLA and (1)O(2) can give emission with peak wavelength at about 532 nm. In the present study, FCLA was chosen as an optical reporter of (1)O(2) produced from the photosensitization reaction of hematoporphyrin derivative in model solution and in nude mice with transplanted mammary cancer. Photosensitized chemiluminescence from the reaction of FCLA with (1)O(2) was detected by a highly sensitive Intensified Charge-Coupled Device detector. The chemiluminescence was markedly inhibited by the addition of 10 mmol/l sodium azide (NaN(3)) to the model solution and minor effects were observed at the addition of 10 micromol/l superoxide dismutase, 20 mmol/l mannitol and 100 microg/ml catalase, respectively, thus indicating that (1)O(2) generation from photosensitization reaction mainly results in light emission. Experiments in vivo with tumor-bearing mice showed a clear chemiluminescence image of tumor. The study suggests that this novel method may be applicable to the diagnosis of superficial tumors.

Imidazolopyrazinones as potential antioxidants

A series of imidazolopyrazinones 3, substituted at C-2, and C-2/C-6, has been prepared. The compounds behaved as quenchers of superoxide anion. The more active compounds are structurally related to coelenterazine, a natural substrate of marine bioluminescence. Theoretical parameters based on Hartree-Fock instabilities have been examined.

Methods of detection of vascular reactive species: nitric oxide, superoxide, hydrogen peroxide, and peroxynitrite

The evanescent nature of reactive oxygen and nitrogen species, the multiple cellular mechanisms evolved to maintain these substances at low (submicromolar) concentrations within the vascular system, and the often multifaceted nature of their reactivities have made measurement of these compounds within the vasculature problematic. This review attempts to provide a critical description of some of the most common approaches to quantification of nitric oxide, superoxide, hydrogen peroxide, and peroxynitrite, with attention to key issues that may influence the utility of a particular assay when adapted for use in vascular cells and tissues.

Chemiluminescent detection of oxidants in vascular tissue. Lucigenin but not coelenterazine enhances superoxide formation

Lucigenin-amplified chemiluminescence has frequently been used to assess the formation of superoxide in vascular tissues. However, the ability of lucigenin to undergo redox cycling in purified enzyme-substrate mixtures has raised questions concerning the use of lucigenin as an appropriate probe for the measurement of superoxide production. Addition of lucigenin to reaction mixtures of xanthine oxidase plus NADH resulted in increased oxygen consumption, as well as superoxide dismutase-inhibitable reduction of cytochrome c, indicative of enhanced rates of superoxide formation. Additionally, it was revealed that lucigenin stimulated oxidant formation by both cultured bovine aortic endothelial cells and isolated rings from rat aorta. Lucigenin treatment resulted in enhanced hydrogen peroxide release from endothelial cells, whereas exposure to lucigenin resulted in inhibition of endothelium-dependent relaxation in isolated aortic rings that was superoxide dismutase inhibitable. In contrast, the chemiluminescent probe coelenterazine had no significant effect on xanthine oxidase-dependent oxygen consumption, endothelial cell hydrogen peroxide release, or endothelium-dependent relaxation. Study of enzyme and vascular systems indicated that coelenterazine chemiluminescence is a sensitive marker for detecting both superoxide and peroxynitrite.

Evaluation of five imidazopyrazinone-type chemiluminescent superoxide probes and their application to the measurement of superoxide anion generated by Listeria monocytogenes

Superoxide-triggered chemiluminescence of five new imidazopyrazinone derivatives was investigated using the hypoxanthine-xanthine oxidase system as the source of superoxide anion. The results showed that they are highly sensitive and have favorable properties in measuring superoxide anion. With those new probes, the generation of superoxide anion from the bacteria Listeria monocytogenes was examined. The results confirmed the previous report that L. monocytogenes is an unusual organism that extracellularly and continuously generates a high level of superoxide anion in the presence of acetaldehyde. The data indicated that two of the probes, 3,7-dihydro-2-methyl-6-phenylethynylimidazo[1,2-a]pyrazin-3- one (4) and its methoxy derivative (5), are highly sensitive and useful in the measurements of superoxide anion and are clearly superior to 3,7-dihydro-2-methyl-6-(4-methoxyphenyl)imidazo[1,2-a]pyrazin-3-on e (MCLA), which-has been generally considered the most sensitive superoxide probe in the past. When tested at a probe concentration of 3.3 microM, the luminescence response and the signal-background ratio of compound 4 were 1.5 and 2.5 times those of MCLA, respectively, and the signal-background ratio of compound 5 was almost 15 times that of MCLA, though the luminescence response of this compound was slightly lower than that of MCLA. The low probe concentration used enhances the usefulness of probes in the measurements of superoxide in functioning biological systems.