Electrochemiluminescent arrays for cytochrome P450-activated genotoxicity screening. DNA damage from benzo[a]pyrene metabolites

Microfluidic electrochemical array for detection of reactive metabolites formed by cytochrome P450 enzymes

A novel, simple, rapid microfluidic array using bioelectronically driven cytochrome P450 enzyme catalysis for reactive metabolite screening is reported for the first time. The device incorporates an eight-electrode screen-printed carbon array coated with thin films of DNA, [Ru(bpy)(2)(PVP)(10)](ClO(4)) {RuPVP}, and rat liver microsomes (RLM) as enzyme sources. Catalysis features electron donation to cyt P450 reductase in the RLMs and subsequent cyt P450 reduction while flowing an oxygenated substrate solution past sensor electrodes. Metabolites react with DNA in the film if they are able, and damaged DNA is detected by catalytic square wave voltammetry (SWV) utilizing the RuPVP polymer. The microfluidic device was tested for a set of common pollutants known to form DNA-reactive metabolites. Logarithmic turnover rates based on SWV responses gave excellent correlation with the rodent liver TD(50) toxicity metric, supporting the utility of the device for toxicity screening. The microfluidic array gave much better S/N and reproducibility than single-electrode sensors based on similar principles.

Carbon nanotube microwell array for sensitive electrochemiluminescent detection of cancer biomarker proteins

This paper describes fabrication of a novel electrochemiluminescence (ECL) immunosensor array featuring capture-antibody-decorated single-wall carbon nanotube (SWCNT) forests residing in the bottoms of 10-μL wells with hydrophobic polymer walls. Silica nanoparticles containing [Ru(bpy)(3)](2+) and secondary antibodies (RuBPY-silica-Ab(2)) are employed in this system for highly sensitive two-analyte detection. Antibodies to prostate specific antigen (PSA) and interleukin-6 (IL-6) were attached to the same RuBPY-silica-Ab(2) particle. The array was fabricated by forming the wells on a conductive pyrolytic graphite chip (1 in. × 1 in.) with a single connection to a potentiostat to achieve ECL. The sandwich immunoassay protocol employs antibodies attached to SWCNTs in the wells to capture analyte proteins. Then RuBPY-silica-Ab(2) is added to bind to the captured proteins. ECL is initiated in the microwells by electrochemical oxidation of tripropyl amine (TprA), which generates excited state [Ru(bpy)(3)](2+) in the 100-nm particles, and is measured with a charge-coupled device (CCD) camera. Separation of the analytical spots by the hydrophobic wall barriers enabled simultaneous immunoassays for two proteins in a single sample without cross-contamination. The detection limit (DL) for PSA was 1 pg mL(-1) and for IL-6 was 0.25 pg mL(-1) (IL-6) in serum. Array determinations of PSA and IL-6 in patient serum were well-correlated with single-protein ELISAs. These microwell SWCNT immunoarrays provide a simple, sensitive approach to the detection of two or more proteins.

Bioelectronic delivery of electrons to cytochrome P450 enzymes

Cytochrome P450s (cyt P450s) are the major oxidative enzymes in human oxidative metabolism of drugs and xenobiotic chemicals. In nature, the iron heme cyt P450s utilize oxygen and electrons delivered from NADPH by a reductase enzyme to oxidize substrates stereo- and regioselectively. Significant research has been directed toward achieving these events electrochemically. This Feature Article discusses the direct electrochemistry of cyt P450s in thin films and the utilization of such films for electrochemically driven biocatalysis. Maintaining and confirming structural integrity and catalytic activity of cyt P450s in films is an essential feature of these efforts. We highlight here our efforts to elucidate the influence of iron heme spin state and secondary structure of human cyt P450s on voltammetric and biocatalytic properties, using methodologies to quantitatively describe the dynamics of these processes in thin films. We also describe the first cyt P450/reductase films that accurately mimic the natural biocatalytic pathway and show how they can be used with voltammetry to elucidate key mechanistic features. Such bioelectronic cyt P450 systems have high value for future drug development, toxicity screening, fundamental investigations, and chemical synthesis systems.

Electrochemical detection of anti-benzo[a]pyrene diol epoxide DNA damage on TP53 codon 273 oligomers

DNA damage from (+/-)-anti-benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide (BPDE) at a hotspot TP53 gene sequence was electrochemically detected. BPDE was exposed to gold electrode immobilized double-stranded DNA oligomers followed by voltammetric measurements in the presence of redox-active C(12)H(25)V(2+)C(6)H(12)V(2+)C(12)H(25) (V(2+) = 4,4'-bipyridyl or viologen, C12-viologen). Square wave voltammograms from BPDE-exposed DNA-modified electrodes showed the emergence of a C12-viologen-DNA complex at -0.37 V versus Ag/AgCl. The peak current intensity of this redox wave was dependent on both BPDE concentration and exposure time. Controls with alternate xenobiotics and DNA sequences showed this redox wave to be primarily due to BPDE damage at the wild-type DNA sequence. The detection limit was determined to be approximately 170 nM BPDE. Mass spectrometry and UV thermal melting experiments provided insight into the BPDE reaction and mirrored the sensor results. This report demonstrates that an electrochemical hybridization sensor can be used to detect sequence-related xenobiotic DNA damage.

Evaluation of electrochemiluminescent metabolic toxicity screening arrays using a multiple compound set

Arrays for screening metabolite-generated toxicity utilizing spots containing DNA, enzyme, and electroluminescent (ECL) polymer ([Ru(bpy)(2)PVP(10)](2+)) were extended to include a fully representative set of metabolic enzymes from human and rat liver microsomes, human and rat liver cytosol, and mouse liver S9 fractions. Array use involves two steps: (1) enzyme activation of the test chemical and metabolite reaction with DNA, and then, (2) capture of ECL resulting from DNA damage using a charge coupled device (CCD) camera. Plots of ECL increase vs enzyme reaction time monitor relative rates of DNA damage and were converted into turnover rates for enzymic production of DNA-reactive metabolites. ECL turnover rates were defined by R, the initial slope of ECL increase versus enzyme reaction time normalized for amounts of enzyme and test chemical. R-values were used to establish correlations for 11 toxic compounds with the standard toxicity metrics rodent liver TD(50) and lethal dose (LD(50)), Ames tests, and Comet assays for in vitro DNA damage. Results support the value of the ECL genotoxicity arrays together with toxicity bioassays for early screening of new chemicals and drug candidates.

Rapid LC-MS drug metabolite profiling using microsomal enzyme bioreactors in a parallel processing format

Silica nanoparticle bioreactors featuring thin films of enzymes and polyions were utilized in a novel high-throughput 96-well plate format for drug metabolism profiling. The utility of the approach was illustrated by investigating the metabolism of the drugs diclofenac (DCF), troglitazone (TGZ), and raloxifene, for which we observed known metabolic oxidation and bioconjugation pathways and turnover rates. A broad range of enzymes was included by utilizing human liver (HLM), rat liver (RLM) and bicistronic human-cyt P450 3A4 (bicis.-3A4) microsomes as enzyme sources. This parallel approach significantly shortens sample preparation steps compared to an earlier manual processing with nanoparticle bioreactors, allowing a range of significant enzyme reactions to be processed simultaneously. Enzyme turnover rates using the microsomal bioreactors were 2-3 fold larger compared to using conventional microsomal dispersions, most likely because of better accessibility of the enzymes. Ketoconazole (KET) and quinidine (QIN), substrates specific to cyt P450 3A enzymes, were used to demonstrate applicability to establish potentially toxic drug-drug interactions involving enzyme inhibition and acceleration.

[Ru(bpy)2dppz]2+ electrochemiluminescence switch and its applications for DNA interaction study and label-free ATP aptasensor

[Ru(bpy)2dppz]2+ electrochemiluminescence (ECL) was studied, and it was used to investigate DNA interaction and develop a label-free ATP aptasensor for the first time. ECL of [Ru(bpy)2dppz]2+ is negligible in aqueous solution, and increases approximately 1000 times when [Ru(bpy)2dppz]2+ intercalates into the nucleic acid structure. The ECL switch behavior of [Ru(bpy)2dppz]2+ is ascribed to the intercalation that shields the phenazine nitrogens from the solvent and results in a luminescent excited state. The ECL switch by DNA was applied to investigate the interaction of [Ru(bpy)2dppz]2+ with herring sperm DNA. The calculated equilibrium constant (K) is 1.35 x 10(6) M(-1), and the calculated binding-site size (s) is 0.88 base pair, which is consistent with the reported values. Moreover, ATP can dramatically affect ECL of the [Ru(bpy)2dppz]2+/ATP aptamer complex. As a result, a label-free, sensitive, and selective [Ru(bpy)2dppz]2+ ECL method for ATP detection was developed. The detection limit is 100 nM for ATP (with a signal-to-noise ratio, S/N, of 3) with a linear range of 0-1 microM. The result demonstrates that [Ru(bpy)2dppz]2+ ECL holds great promise in aptasensors.

Control of electrochemical and ferryloxy formation kinetics of cyt P450s in polyion films by heme iron spin state and secondary structure

Voltammetry of cytochrome P450 (cyt P450) enzymes in ultrathin films with polyions was related for the first time to electronic and secondary structure. Heterogeneous electron transfer (hET) rate constants for reduction of the cyt P450s depended on heme iron spin state, with low spin cyt P450cam giving a value 40-fold larger than high spin human cyt P450 1A2, with mixed spin human P450 cyt 2E1 at an intermediate value. Asymmetric reduction-oxidation peak separations with increasing scan rates were explained by simulations featuring faster oxidation than reduction. Results are consistent with a square scheme in which oxidized and reduced forms of cyt P450s each participate in rapid conformational equilibria. Rate constants for oxidation of ferric cyt P450s in films by t-butyl hydroperoxide to active ferryloxy cyt P450s from rotating disk voltammetry suggested a weaker dependence on spin state, but in the reverse order of the observed hET reduction rates. Oxidation and reduction rates of cyt P450s in the films are also likely to depend on protein secondary structure around the heme iron.

Screening for reactive metabolites using electro-optical arrays featuring human liver cytosol and microsomal enzyme sources and DNA

We demonstrate for the first time the combination of human liver cytosol and microsomal enzyme sources into an electro-optical array to screen for reactive metabolites produced in multi-enzyme metabolic processes.

Electrochemiluminescent immunosensor for detection of protein cancer biomarkers using carbon nanotube forests and [Ru-(bpy)(3)](2+)-doped silica nanoparticles

We report the first electrochemiluminescent immunosensor combining single-wall carbon nanotube forests with RuBPY-silica-secondary antibody nanoparticles for sensitive detection of cancer biomarker prostate specific antigen.

Multiplexed sandwich immunoassays using electrochemiluminescence imaging resolved at the single bead level

A new class of bead-based microarray that uses electrogenerated chemiluminescence (ECL) as a readout mechanism to detect multiple antigens simultaneously is presented. This platform demonstrates the possibility of performing highly multiplexed assays using ECL because all the individual sensing beads in the array are simultaneously imaged and individually resolved by ECL. Duplex and triplex assay results are demonstrated as well as a cross reactivity study.

Differences in metabolite-mediated toxicity of tamoxifen in rodents versus humans elucidated with DNA/microsome electro-optical arrays and nanoreactors

Tamoxifen, a therapeutic and chemopreventive breast cancer drug, was chosen as a model compound because of acknowledged species specific toxicity differences. Emerging approaches utilizing electro-optical arrays and nanoreactors based on DNA/microsome films were used to compare metabolite-mediated toxicity differences of tamoxifen in rodents versus humans. Hits triggered by liver enzyme metabolism were first provided by arrays utilizing a DNA damage end point. The arrays feature thin-film spots containing an electrochemiluminescent (ECL) ruthenium polymer ([Ru(bpy)(2)PVP(10)](2+); PVP, polyvinylpyridine), DNA, and liver microsomes. When DNA damage resulted from reactions with tamoxifen metabolites, it was detected by an increase in light from the oxidation of the damaged DNA by the ECL metallopolymer. The slope of ECL generation versus enzyme reaction time correlated with the rate of DNA damage. An approximate 2-fold greater ECL turnover rate was observed for spots with rat liver microsomes compared to that with human liver microsomes. These results were supported by liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of reaction products using nanoreactors featuring analogous films on silica nanoparticles, allowing the direct measurement of the relative formation rate for alpha-(N(2)-deoxyguanosinyl)tamoxifen. We observed 2-5-fold more rapid formation rates for three major metabolites, i.e., alpha-hydroxytamoxifen, 4-hydroxytamoxifen, and tamoxifen N-oxide, catalyzed by rat liver microsomes compared to human liver microsomes. Comparable formation rates were observed for N-desmethyl tamoxifen with rat and human liver microsomes. A better detoxifying capacity for human liver microsomes than rat liver microsomes was confirmed utilizing glucuronyltransferase in microsomes together with UDP-glucuronic acid. Taken together, lower genotoxicity and higher detoxication rates presented by human liver microsomes correlate with the lower risk of tamoxifen in causing liver carcinoma in humans, provided the glucuronidation pathway is active.

Comparison of DNA-Reactive Metabolites from Nitrosamine and Styrene Using Voltammetric DNA/Microsomes Sensors

Voltammetric sensors made with films of polyions, double-stranded DNA and liver microsomes adsorbed layer-by-layer onto pyrolytic graphite electrodes were evaluated for reactive metabolite screening. This approach features simple, inexpensive screening without enzyme purification for applications in drug or environmental chemical development. Cytochrome P450 enzymes (CYPs) in the liver microsomes were activated by an NADPH regenerating system or by electrolysis to metabolize model carcinogenic compounds nitrosamine and styrene. Reactive metabolites formed in the films were trapped as adducts with nucleobases on DNA. The DNA damage was detected by square-wave voltammetry (SWV) using [Formula: see text] as a DNA-oxidation catalyst. These sensors showed a larger rate of increase in signal vs. reaction time for a highly toxic nitrosamine than for the moderately toxic styrene due to more rapid reactive metabolite-DNA adduct formation. Results were consistent with reported in vivo TD(50) data for the formation of liver tumors in rats. Analogous polyion/ liver microsome films prepared on 500 nm silica nanoparticles (nanoreactors) and reacted with nitrosamine or styrene, provided LC-MS or GC analyses of metabolite formation rates that correlated well with sensor response.

Characterizing metabolic inhibition using electrochemical enzyme/DNA biosensors

Studies of metabolic enzyme inhibition are necessary in drug development and toxicity investigations as potential tools to limit or prevent appearance of deleterious metabolites formed, for example, by cytochrome (cyt) P450 enzymes. In this paper, we evaluate the use of enzyme/DNA toxicity biosensors as tools to investigate enzyme inhibition. We have examined DNA damage due to cyt P450cam metabolism of styrene using DNA/enzyme films on pyrolytic graphite (PG) electrodes monitored via Ru(bpy)(3)(2+)-mediated DNA oxidation. Styrene metabolism initiated by hydrogen peroxide was evaluated with and without the inhibitors, imidazole, imidazole-4-acetic acid, and sulconazole (in micromolar range) to monitor DNA damage inhibition. The initial rates of DNA damage decreased with increased inhibitor concentrations. Linear and nonlinear fits of Michaelis-Menten inhibition models were used to determine apparent inhibition constants (K(I)*) for the inhibitors. Elucidation of the best fitting inhibition model was achieved by comparing correlation coefficients and the sum of the square of the errors (SSE) from each inhibition model. Results confirmed the utility of the enzyme/DNA biosensor for metabolic inhibition studies. A simple competitive inhibition model best approximated the data for imidazole, imidazole-4-acetic acid and sulconazole with K(I)* of 268.2, 142.3, and 204.2 microM, respectively.

Human cyt P450 mediated metabolic toxicity of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) evaluated using electrochemiluminescent arrays

Electrochemiluminescent (ECL) arrays containing polymer ([Ru(bpy)(2)(PVP)(10)](2+), PVP = polyvinylpyridine), DNA, and selected enzymes were employed to elucidate cytochrome (cyt) P450 dependent metabolism of the tobacco specific carcinogen, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). Bioactivated NNK metabolites formed upon H(2)O(2)-enzymatic activation were captured as DNA adducts and detected simultaneously from 36 spot arrays by capturing and quantifying emitted ECL with an overhead CCD camera. Increased ECL emission was dependent on NNK exposure time. Of the enzymes tested, the activity toward NNK bioactivation was cyt P450 1A2 > 2E1 > 1B1 approximately chloroperoxidase (CPO) > myoglobin (Mb) in accordance with reported in vivo studies. Cyt P450/polyion films were also immobilized on 500 nm diameter silica nanospheres for product analysis by LC-MS. Analysis of the nanosphere film reaction media provided ECL array validation and quantitation of the bioactivated NNK hydrolysis product 4-hydroxy-1-(3-pyridyl)-1-butanone (HPB) confirming production of reactive metabolites in the films. Chemical screening in this fashion allows rapid clarification of enzymes responsible for genotoxic activation as well as offering insight into cyt P450-related toxicity and mechanisms.

Microsome biocolloids for rapid drug metabolism and inhibition assessment by LC-MS

Rat liver microsomes attached to nanoparticles were used for LC-MS studies of CYP3A and 2E1 enzymes in metabolism of N-nitroso compounds. Using these biocolloids, turnover rates were measured within 2 min. Inhibitor IC(50) values for ketoconazole (KET) and 4-methylpyrazole (4-MEP) were estimated.

Electrochemical biosensor featuring a two-enzyme pathway and DNA for screening toxic reactive metabolites of arylamines

We demonstrate for the first time a biosensor featuring a sequential two-enzyme pathway suitable to screen potentially toxic reactive metabolites generated during metabolism.

Synergistic metabolic toxicity screening using microsome/DNA electrochemiluminescent arrays and nanoreactors

Platforms based on thin enzyme/DNA films were used in two-tier screening of chemicals for reactive metabolites capable of producing toxicity. Microsomes were used for the first time as sources of cytochrome (cyt) P450 enzymes in these devices. Initial rapid screening involved electrochemiluminescent (ECL) arrays featuring spots containing ruthenium poly(vinylpyridine), DNA, and rat liver microsomes or bicistronically expressed human cyt P450 2E1 (h2E1). Cyt P450 enzymes were activated via the NADPH/reductase cycle. When bioactivation of substrates in the films gives reactive metabolites, they are trapped by covalent attachment to DNA bases. The rate of increase in ECL with enzyme reaction time reflects relative DNA damage rates. "Toxic hits" uncovered by the array were studied in structural detail by using enzyme/DNA films on silica nanospheres as "nanoreactors" to provide nucleobase adducts from reactive metabolites. The utility of this synergistic approach was demonstrated by estimating relative DNA damage rates of three mutagenic N-nitroso compounds and styrene. Relative enzyme turnover rates for these compounds using ECL arrays and LC-UV-MS correlated well with TD 50 values for liver tumor formation in rats. Combining ECL array and nanoreactor/LC-MS technologies has the potential for rapid, high-throughput, cost-effective screening for reactive metabolites and provides chemical structure information that is complementary to conventional toxicity bioassays.

Accurate DNA fragment sizing by capillary electrophoresis with laser-induced fluorescence array for detection of sequence specificity of DNA damage

Cancer has been linked to mutations within specific codons in genes that code for critical biomolecules such as tumor suppressor proteins (e.g., p53). Activated metabolites like benzo[a]pyrenediol epoxide act on preferred nucleotide sequences of DNA, and such mutations have been identified in cancers. DNA reaction site identification depends on accurate analysis of oligonucleotide fragment sizes produced by strand breakage at the damaged sites. Herein, we report a new method for DNA fragment sizing using capillary electrophoresis with laser-induced fluorescence detection (CE-LIF). Absolute sizing accuracy and speed are achieved by utilizing a CE-LIF array with two-color fluorescence detection. Accuracy depends on correcting results with commercial standards by referring them to primary standards with the same sequences and identical labels as sample fragments. The method is demonstrated by detection of a [...GGCGCGCAG...] G reaction site for styrene oxide on an oligonucleotide representing the CYP1B1 gene. This approach avoids the need for radioactive isotopes and is less labor intensive and faster than the alternative PAGE with (32)P end labeling.

Biochemical applications of ultrathin films of enzymes, polyions and DNA

This feature article summarizes recent applications of ultrathin films of enzymes and DNA assembled layer-by-layer (LbL). Using examples mainly from our own research, we focus on systems developed for biocatalysis and biosensors for toxicity screening. Enzyme-poly(L-lysine) (PLL) films, especially when stabilized by crosslinking, can be used for biocatalysis at unprecedented high temperatures or in acidic or basic solutions on electrodes or sub-micron sized beads. Such films have bright prospects for chiral synthesis and biofuel cells. Excellent bioactivity and retention of enzyme structure in these films facilitates their use in detailed kinetic studies. Biosensors and arrays employing DNA-enzyme films show great promise in predicting genotoxicity of new drug and chemical product candidates. These devices combine metabolic biocatalysis, reactive metabolite-DNA reactions, and DNA damage detection. Catalytic voltammetry or electrochemiluminescence (ECL) can be used for high throughput arrays utilizing multiple LbL "spots" of DNA, enzyme and metallopolymer. DNA-enzyme films can also be used to produce nucleobase adduct toxicity biomarkers for detection by LC-MS. These approaches provide valuable high throughput tools for drug and chemical product development and toxicity prediction.

Electrochemiluminescent/voltammetric toxicity screening sensor using enzyme-generated DNA damage

Simultaneous optical and voltammetric detection of bioactivated genotoxicity is reported for the first time employing ultrathin films of DNA, model metabolic enzymes, and electrochemiluminescence (ECL) generating metallopolymer [Ru(bpy)2PVP10]2+ on pyrolytic graphite (PG) electrodes. Cytochrome P450cam and myoglobin were used as model monoxygenase enzymes to mimic in vivo processes. Sensor film growth and component amounts were monitored using a quartz crystal microbalance (QCM). Subsequent to the enzyme reaction, DNA damage in the sensor films was measured simultaneously using a simple apparatus combining a standard voltammetry cell coupled with an optical fiber and photomultiplier tube. The model enzyme reaction converted styrene to styrene oxide, which reacts with DNA nucleobases. ECL and SWV signals increased with enzyme reaction time on the scale of several min, and provided relative enzyme turnover rates for DNA damage suitable for toxicity screening applications. Within 1 min, the sensor detects approximately 3 damaged bases per 10,000 DNA bases using this simultaneous detection.

Thin film voltammetry of metabolic enzymes in rat liver microsomes

We report herein thin film voltammetry and kinetics of electron transfer for redox proteins in rat liver microsomes for the first time. Films were made layer-by-layer from liver microsomes and polycations on pyrolytic graphite electrodes. Cyclic voltammograms were chemically reversible with a midpoint potential of -0.48 V vs SCE at 0.1 V s(-1) in pH 7.0 phosphate buffer. Reduction peak potentials shifted negative at higher scan rates, and oxidation-reduction peak current ratios were approximately 1 consistent with non-ideal quasireversible thin film voltammetry. Analysis of oxidation-reduction peak separations gave an average apparent surface electron transfer rate constant of 30 s(-1). Absence of significant electrocatalytic reduction of O(2) or H(2)O(2) and lack of shift in midpoint potential when CO is added that indicates lack of an iron heme cofactor suggest that peaks can be attributed to oxidoreductases present in the microsomes rather than cytochrome P450 enzymes.

Genotoxicity screening for N-nitroso compounds. Electrochemical and electrochemiluminescent detection of human enzyme-generated DNA damage from N-nitrosopyrrolidine

We report for the first time voltammetric/electrochemiluminescent sensors applied to predict genotoxicity of N-nitroso compounds bioactivated by human cytochrome P450 enzymes.