Coelenterazine-Type Bioluminescence-Induced Optical Probes for Sensing and Controlling Biological Processes

Bioluminescence-based probes have long been used to quantify and visualize biological processes in vitro and in vivo. Over the past years, we have witnessed the trend of bioluminescence-driven optogenetic systems. Typically, bioluminescence emitted from coelenterazine-type luciferin–luciferase reactions activate light-sensitive proteins, which induce downstream events. The development of coelenterazine-type bioluminescence-induced photosensory domain-based probes has been applied in the imaging, sensing, and control of cellular activities, signaling pathways, and synthetic genetic circuits in vitro and in vivo. This strategy can not only shed light on the mechanisms of diseases, but also promote interrelated therapy development. Here, this review provides an overview of these optical probes for sensing and controlling biological processes, highlights their applications and optimizations, and discusses the possible future directions.

Luciferase-free Luciferin Electrochemiluminescence

Luciferin is one of Nature's most widespread luminophores, and enzymes that catalyze luciferin luminescence are the basis of successful commercial "glow" assays for gene expression and metabolic ATP formation. Herein we report an electrochemical method to promote firefly's luciferin luminescence in the absence of its natural biocatalyst-luciferase. We have gained experimental and computational insights on the mechanism of the enzyme-free luciferin electrochemiluminescence, demonstrated its spectral tuning from green to red by means of electrolyte engineering, proven that the colour change does not require, as still debated, a keto/enol isomerization of the light emitter, and gained evidence of the electrostatic-assisted stabilization of the charge-transfer excited state by double layer electric fields. Luciferin's electrochemiluminescence, as well as the in situ generation of fluorescent oxyluciferin, are applied towards an optical measurement of diffusion coefficients.

A New Method for Albuminuria Measurement Using a Specific Reaction between Albumin and the Luciferin of the Firefly Squid Watasenia scintillans

This study demonstrates that the luciferin of the firefly squid Watasenia scintillans, which generally reacts with Watasenia luciferase, reacted with human albumin to emit light in proportion to the albumin concentration. The luminescence showed a peak wavelength at 540 nm and was eliminated by heat or protease treatment. We used urine samples collected from patients with diabetes to quantify urinary albumin concentration, which is essential for the early diagnosis of diabetic nephropathy. Consequently, we were able to measure urinary albumin concentrations by precipitating urinary proteins with acetone before the reaction with luciferin. A correlation was found with the result of the immunoturbidimetric method; however, the Watasenia luciferin method tended to produce lower albumin concentrations. This may be because the Watasenia luciferin reacts with only intact albumin. Therefore, the quantification method using Watasenia luciferin is a new principle of urinary albumin measurement that differs from already established methods such as immunoturbidimetry and high-performance liquid chromatography.

Orthogonal Bioluminescent Probes from Disubstituted Luciferins

Bioluminescence imaging with luciferase-luciferin pairs is routinely used to monitor cellular functions. Multiple targets can be visualized in tandem using luciferases that process unique substrates, but only a handful of such orthogonal probes are known. Multiplexed studies require additional robust, light-emitting molecules. In this work, we report new luciferins for orthogonal imaging that comprise disubstituted cores. These probes were found to be bright emitters with various engineered luciferases. The unique patterns of light output also provided insight into enzyme-substrate interactions necessary for productive emission. Screening studies identified mutant luciferases that could preferentially process the disubstituted analogues, enabling orthogonal imaging with existing bioluminescent reporters. Further mutational analyses revealed the origins of substrate selectivity. Collectively, this work provides insights into luciferase-luciferin features relevant to bioluminescence and expands the number of probes for multicomponent tracking.

Multi-Modal Multi-Spectral Intravital Macroscopic Imaging of Signaling Dynamics in Real Time during Tumor-Immune Interactions

A major obstacle in studying the interplay between cancer cells and the immune system has been the examination of proposed biological pathways and cell interactions in a dynamic, physiologically relevant system in vivo. Intravital imaging strategies are one of the few molecular imaging techniques that can follow biological processes at cellular resolution over long periods of time in the same individual. Bioluminescence imaging has become a standard preclinical in vivo optical imaging technique with ever-expanding versatility as a result of the development of new emission bioluminescent reporters, advances in genomic techniques, and technical improvements in bioluminescence imaging and processing methods. Herein, we describe an advance of technology with a molecular imaging window chamber platform that combines bioluminescent and fluorescent reporters with intravital macro-imaging techniques and bioluminescence spectral unmixing in real time applied to heterogeneous living systems in vivo for evaluating tumor signaling dynamics and immune cell enzyme activities concurrently.

A bioluminescent probe for longitudinal monitoring of mitochondrial membrane potential

Mitochondrial membrane potential (ΔΨm) is a universal selective indicator of mitochondrial function and is known to play a central role in many human pathologies, such as diabetes mellitus, cancer and Alzheimer's and Parkinson's diseases. Here, we report the design, synthesis and several applications of mitochondria-activatable luciferin (MAL), a bioluminescent probe sensitive to ΔΨm, and partially to plasma membrane potential (ΔΨp), for non-invasive, longitudinal monitoring of ΔΨm in vitro and in vivo. We applied this new technology to evaluate the aging-related change of ΔΨm in mice and showed that nicotinamide riboside (NR) reverts aging-related mitochondrial depolarization, revealing another important aspect of the mechanism of action of this potent biomolecule. In addition, we demonstrated application of the MAL probe for studies of brown adipose tissue (BAT) activation and non-invasive in vivo assessment of ΔΨm in animal cancer models, opening exciting opportunities for understanding the underlying mechanisms and for discovery of effective treatments for many human pathologies.

Coelenterazine-Dependent Luciferases as a Powerful Analytical Tool for Research and Biomedical Applications

: The functioning of bioluminescent systems in most of the known marine organisms is based on the oxidation reaction of the same substrate-coelenterazine (CTZ), catalyzed by luciferase. Despite the diversity in structures and the functioning mechanisms, these enzymes can be united into a common group called CTZ-dependent luciferases. Among these, there are two sharply different types of the system organization-Ca2+-regulated photoproteins and luciferases themselves that function in accordance with the classical enzyme-substrate kinetics. Along with deep and comprehensive fundamental research on these systems, approaches and methods of their practical use as highly sensitive reporters in analytics have been developed. The research aiming at the creation of artificial luciferases and synthetic CTZ analogues with new unique properties has led to the development of new experimental analytical methods based on them. The commercial availability of many ready-to-use assay systems based on CTZ-dependent luciferases is also important when choosing them by first-time-users. The development of analytical methods based on these bioluminescent systems is currently booming. The bioluminescent systems under consideration were successfully applied in various biological research areas, which confirms them to be a powerful analytical tool. In this review, we consider the main directions, results, and achievements in research involving these luciferases.

Genomic and experimental data provide new insights into luciferin biosynthesis and bioluminescence evolution in fireflies

Fireflies are among the most charismatic insects for their spectacular bioluminescence, but the origin and evolution of bioluminescence remain elusive. Especially, the genic basis of luciferin (D-luciferin) biosynthesis and light patterns is largely unknown. Here, we present the high-quality reference genomes of two fireflies Lamprigera yunnana (1053 Mb) and Abscondita terminalis (501 Mb) with great differences in both morphology and luminous behavior. We sequenced the transcriptomes and proteomes of luminous organs of two species. We created the CRISPR/Cas9-induced mutants of Abdominal B gene without luminous organs in the larvae of A. terminalis and sequenced the transcriptomes of mutants and wild-types. Combining gene expression analyses with comparative genomics, we propose a more complete luciferin synthesis pathway, and confirm the convergent evolution of bioluminescence in insects. Using experiments, the function of the firefly acyl-CoA thioesterase (ACOT1) to convert L-luciferin to D-luciferin was validated for the first time. Comparisons of three-dimension reconstruction of luminous organs and their differentially expressed genes among two species suggest that two positive genes in the calcium signaling pathway and structural difference of luminous organs may play an important role in the evolution of flash pattern. Altogether, our results provide important resources for further exploring bioluminescence in insects.

Seeing (and Using) the Light: Recent Developments in Bioluminescence Technology

Bioluminescence has long been used to image biological processes in vivo. This technology features luciferase enzymes and luciferin small molecules that produce visible light. Bioluminescent photons can be detected in tissues and live organisms, enabling sensitive and noninvasive readouts on physiological function. Traditional applications have focused on tracking cells and gene expression patterns, but new probes are pushing the frontiers of what can be visualized. The past few years have also seen the merger of bioluminescence with optogenetic platforms. Luciferase-luciferin reactions can drive light-activatable proteins, ultimately triggering signal transduction and other downstream events. This review highlights these and other recent advances in bioluminescence technology, with an emphasis on tool development. We showcase how new luciferins and engineered luciferases are expanding the scope of optical imaging. We also highlight how bioluminescent systems are being leveraged not just for sensing-but also controlling-biological processes.

Plants with genetically encoded autoluminescence

Autoluminescent plants engineered to express a bacterial bioluminescence gene cluster in plastids have not been widely adopted because of low light output. We engineered tobacco plants with a fungal bioluminescence system that converts caffeic acid (present in all plants) into luciferin and report self-sustained luminescence that is visible to the naked eye. Our findings could underpin development of a suite of imaging tools for plants.

A new brilliantly blue-emitting luciferin-luciferase system from Orfelia fultoni and Keroplatinae (Diptera)

Larvae of O. fultoni (Keroplatidae: Keroplatinae), which occur along river banks in the Appalachian Mountains in Eastern United States, produce the bluest bioluminescence among insects from translucent areas associated to black bodies, which are  located mainly in the anterior and posterior parts of the body. Although closely related to Arachnocampa spp (Keroplatidae: Arachnocampininae), O.fultoni has a morphologically and biochemically distinct bioluminescent system which evolved independently, requiring a luciferase enzyme, a luciferin, a substrate binding fraction (SBF) that releases luciferin in the presence of mild reducing agents, molecular oxygen, and no additional cofactors. Similarly, the closely related Neoceroplatus spp, shares the same kind of luciferin-luciferase system of Orfelia fultoni. However, the molecular properties, identities and functions of luciferases, SBF and luciferin of Orfelia fultoni and other  luminescent members of the Keroplatinae subfamily still remain to be fully elucidated. Using O. fultoni as a source of luciferase, and the recently discovered non-luminescent cave worm Neoditomiya sp as the main source of luciferin and SBF, we isolated and initially characterized these compounds. The luciferase of O. fultoni is a stable enzyme active as an apparent trimer (220 kDa) composed of ~70 kDa monomers, with an optimum pH of 7.8. The SBF, which is found in the black bodies in Orfelia fultoni and in smaller dark granules in Neoditomiya sp, consists of a high molecular weight complex of luciferin and proteins, apparently associated to mitochondria. The luciferin, partially purified from hot extracts by a combination of anion exchange chromatography and TLC, is a very polar and weakly fluorescent compound, whereas its oxidized product displays blue fluorescence with an emission spectrum matching the bioluminescence spectrum (~460 nm), indicating that it is oxyluciferin. The widespread occurrence of luciferin and SBF in both luminescent and non-luminescent Keroplatinae larvae indicate an additional important biological function for the substrate, and therefore the name keroplatin.

A Nanobionic Light-Emitting Plant

The engineering of living plants for visible light emission and sustainable illumination is compelling because plants possess independent energy generation and storage mechanisms and autonomous self-repair. Herein, we demonstrate a plant nanobionic approach that enables exceptional luminosity and lifetime utilizing four chemically interacting nanoparticles, including firefly luciferase conjugated silica (SNP-Luc), d-luciferin releasing poly(lactic-co-glycolic acid) (PLGA-LH₂), coenzyme A functionalized chitosan (CS-CoA) and semiconductor nanocrystal phosphors for longer wavelength modulation. An in vitro kinetic model incorporating the release rates of the nanoparticles is developed to maximize the chemiluminescent lifetimes to exceed 21.5 h. In watercress (Nasturtium officinale) and other species, the nanoparticles circumvent limitations such as luciferin toxicity above 400 μM and colocalization of enzymatic reactions near high adenosine triphosphate (ATP) production. Pressurized bath infusion of nanoparticles (PBIN) is introduced to deliver a mixture of nanoparticles to the entire living plant, well described using a nanofluidic mathematical model. We rationally design nanoparticle size and charge to control localization within distinct tissues compartments with 10 nm nanoparticles localizing within the leaf mesophyll and stomata guard cells, and those larger than 100 nm segregated in the leaf mesophyll. The results are mature watercress plants that emit greater than 1.44 × 10¹² photons/sec or 50% of 1 μW commercial luminescent diodes and modulate "off" and "on" states by chemical addition of dehydroluciferin and coenzyme A, respectively. We show that CdSe nanocrystals can shift the chemiluminescent emission to 760 nm enabling near-infrared (nIR) signaling. These results advance the viability of nanobionic plants as self-powered photonics, direct and indirect light sources.

Orthogonal Luciferase-Luciferin Pairs for Bioluminescence Imaging

Bioluminescence imaging with luciferase-luciferin pairs is widely used in biomedical research. Several luciferases have been identified in nature, and many have been adapted for tracking cells in whole animals. Unfortunately, the optimal luciferases for imaging in vivo utilize the same substrate and therefore cannot easily differentiate multiple cell types in a single subject. To develop a broader set of distinguishable probes, we crafted custom luciferins that can be selectively processed by engineered luciferases. Libraries of mutant enzymes were iteratively screened with sterically modified luciferins, and orthogonal enzyme-substrate "hits" were identified. These tools produced light when complementary enzyme-substrate partners interacted both in vitro and in cultured cell models. Based on their selectivity, these designer pairs will bolster multicomponent imaging and enable the direct interrogation of cell networks not currently possible with existing tools. Our screening platform is also general and will expedite the identification of more unique luciferases and luciferins, further expanding the bioluminescence toolkit.

In Vitro Biotransformation of Two Human CYP3A Probe Substrates and Their Inhibition during Early Zebrafish Development

At present, the zebrafish embryo is increasingly used as an alternative animal model to screen for developmental toxicity after exposure to xenobiotics. Since zebrafish embryos depend on their own drug-metabolizing capacity, knowledge of their intrinsic biotransformation is pivotal in order to correctly interpret the outcome of teratogenicity assays. Therefore, the aim of this in vitro study was to assess the activity of cytochrome P450 (CYP)—a group of drug-metabolizing enzymes—in microsomes from whole zebrafish embryos (ZEM) of 5, 24, 48, 72, 96 and 120 h post-fertilization (hpf) by means of a mammalian CYP substrate, i.e., benzyloxy-methyl-resorufin (BOMR). The same CYP activity assays were performed in adult zebrafish liver microsomes (ZLM) to serve as a reference for the embryos. In addition, activity assays with the human CYP3A4-specific Luciferin isopropyl acetal (Luciferin-IPA) as well as inhibition studies with ketoconazole and CYP3cide were carried out to identify CYP activity in ZLM. In the present study, biotransformation of BOMR was detected at 72 and 96 hpf; however, metabolite formation was low compared with ZLM. Furthermore, Luciferin-IPA was not metabolized by the zebrafish. In conclusion, the capacity of intrinsic biotransformation in zebrafish embryos appears to be lacking during a major part of organogenesis.

Brominated Luciferins Are Versatile Bioluminescent Probes

We report a set of brominated luciferins for bioluminescence imaging. These regioisomeric scaffolds were accessed by using a common synthetic route. All analogues produced light with firefly luciferase, although varying levels of emission were observed. Differences in photon output were analyzed by computation and photophysical measurements. The brightest brominated luciferin was further evaluated in cell and animal models. At low doses, the analogue outperformed the native substrate in cells. The remaining luciferins, although weak emitters with firefly luciferase, were inherently capable of light production and thus potential substrates for orthogonal mutant enzymes.

Rapid and scalable assembly of firefly luciferase substrates

Bioluminescence imaging with luciferase-luciferin pairs is a popular method for visualizing biological processes in vivo. Unfortunately, most luciferins are difficult to access and remain prohibitively expensive for some imaging applications. Here we report cost-effective and efficient syntheses of d-luciferin and 6'-aminoluciferin, two widely used bioluminescent substrates. Our approach employs inexpensive anilines and Appel's salt to generate the luciferin cores in a single pot. Additionally, the syntheses are scalable and can provide multi-gram quantities of both substrates. The streamlined production and improved accessibility of luciferin reagents will bolster in vivo imaging efforts.

Bioluminescent imaging of TRAIL-induced apoptosis through detection of caspase activation following cleavage of DEVD-aminoluciferin

Apoptosis, the most common and well-defined form of programmed cell death (PCD), is often impaired in cancer and neurodegenerative diseases and can limit conventional therapy. Bioluminescent molecular imaging was employed to study apoptosis in human colon cancer cells that have been treated with various doses of the therapeutic agent TRAIL (tumor necrosis factor-related apoptosis inducing ligand). While monitoring therapeutic response through a proluminescent, caspase-activated DEVD-aminoluciferin reagent (Caspase-Glo 3/7) which produced strong, stable signals, alternate preparations of the reagent were explored for non-invasive imaging methods. Dissolving the lyophilized DEVD-aminoluciferin compound in Dulbecco's PBS instead of lysis buffer (along with heat inactivation of an accompanying exogenous luciferase protein by heating at 85 degrees C for 20 minutes) yielded a minimally invasive apoptosis detector, with maximum luminescence intensities 2.5-fold stronger than those produced by D-luciferin at a final concentration of 100 microg/mL. Bioluminescent imaging of cancer therapeutic response through minimally invasive detection of caspase activation may serve as an important tool in monitoring apoptosis in vivo and in vitro.

GroEL Chaperone Binding to Beetle Luciferases and the Implications for Refolding When Co-expressed

The folding of many proteins including luciferase in vivo requires the assistance of molecular chaperone proteins. To understand how a chaperone targets luciferase, we took three luciferases that give different bioluminescence with the same luciferin substrate and with differences in homology. The three luciferase genes, firefly luciferase (FF-Luc) (from Pyrocoelia miyako), and red (RE-Luc) and green (GR-Luc) bioluminescence-emitting luciferases (from Phrixothrix railroad-worms), were expressed in Escherichia coli to produce fusion proteins with predicted molecular masses. Subsequently, we observed that DnaK and GroEL were co-purified along with recombinant luciferase. Although the amount of co-purified DnaK was almost the same compared to FF-Luc, GroEL was 25 and 32 times higher in GR-Luc and RE-Luc respectively. Furthermore, co-expression of GroEL/GroES along with luciferase substantially refolded RE-Luc and GR-Luc compared to FF-Luc.

Neural progenitor cell survival in mouse brain can be improved by co-transplantation of helper cells expressing bFGF under doxycycline control

Cell-based therapy of neurological disorders is hampered by poor survival of grafted neural progenitor cells (NPCs). We hypothesized that it is possible to enhance the survival of human NPCs (ReNcells) by co-transplantation of helper cells expressing basic fibroblast growth factor (bFGF) under control of doxycycline (Dox). 293 cells or C17.2 cells were transduced with a lentiviral vector encoding the fluorescent reporter mCherry and bFGF under tetracycline-regulated transgene expression (Tet-ON). The bFGF secretion level in the engineered helper cells was positively correlated with the dose of Dox (Pearson correlation test; r=0.95 and 0.99 for 293 and C17.2 cells, respectively). Using bioluminescence imaging (BLI) as readout for firefly luciferase-transduced NPC survival, the addition of both 293-bFGF and C17.2-bFGF helper cells was found to significantly improve cell survival up to 6-fold in vitro, while wild-type (WT, non-transduced) helper cells had no effect. Following co-transplantation of 293-bFGF or C17.2-bFGF cells in the striatum of Rag2(-/-) immunodeficient mice, in vivo human NPC survival could be significantly improved as compared to no helper cells or co-transplantation of WT cells for the first two days after co-transplantation. This enhancement of survival in C17.2-bFGF group was not achieved without Dox administration, indicating that the neuroprotective effect was specific for bFGF. The present results warrant further studies on the use of engineered helper cells, including those expressing other growth factors injected as mixed cell populations.

Visualization of ATP with luciferin-luciferase reaction in mouse skeletal muscles using an "in vivo cryotechnique"

Adenosine triphosphate (ATP) is a well-known energy source for muscle contraction. In this study, to visualize localization of ATP, a luciferin-luciferase reaction (LLR) was performed in mouse skeletal muscle with an "in vivo cryotechnique" (IVCT). First, to confirm if ATP molecules could be trapped and detected after glutaraldehyde (GA) treatment, ATP was directly attached to glass slides with GA, and LLR was performed. The LLR was clearly detected as an intentional design of the ATP attachment. The intensity of the light unit by LLR was correlated with the concentration of the GA-treated ATP in vitro. Next, LLR was evaluated in mouse skeletal muscles with IVCT followed by freeze-substitution fixation (FS) in acetone-containing GA. In such tissue sections the histological structure was well maintained, and the intensity of LLR in areas between muscle fibers and connective tissues was different. Moreover, differences in LLR among muscle fibers were also detected. For the IVCT-FS tissue sections, diaminobenzidine (DAB) reactions were clearly detected in type I muscle fibers and erythrocytes in capillaries, which demonstrated flow shape. Thus, it became possible to perform microscopic evaluation of the numbers of ATP molecules in the mouse skeletal muscles with IVCT, which mostly reflect living states.

The location and nature of general anesthetic binding sites on the active conformation of firefly luciferase; a time resolved photolabeling study

Firefly luciferase is one of the few soluble proteins that is acted upon by a wide variety of general anesthetics and alcohols; they inhibit the ATP–driven production of light. We have used time–resolved photolabeling to locate the binding sites of alcohols during the initial light output, some 200 ms after adding ATP. The photolabel 3-azioctanol inhibited the initial light output with an IC50 of 200 µM, close to its general anesthetic potency. Photoincorporation of [³H]3-azioctanol into luciferase was saturable but weak. It was enhanced 200 ms after adding ATP but was negligible minutes later. Sequencing of tryptic digests by HPLC–MSMS revealed a similar conformation–dependence for photoincorporation of 3-azioctanol into Glu-313, a residue that lines the bottom of a deep cleft (vestibule) whose outer end binds luciferin. An aromatic diazirine analog of benzyl alcohol with broader side chain reactivity reported two sites. First, it photolabeled two residues in the vestibule, Ser-286 and Ile-288, both of which are implicated with Glu-313 in the conformation change accompanying activation. Second, it photolabeled two residues that contact luciferin, Ser-316 and Ser-349. Thus, time resolved photolabeling supports two mechanisms of action. First, an allosteric one, in which anesthetics bind in the vestibule displacing water molecules that are thought to be involved in light output. Second, a competitive one, in which anesthetics bind isosterically with luciferin. This work provides structural evidence that supports the competitive and allosteric actions previously characterized by kinetic studies.

Quantitation of cellular and topical uptake of luciferin-oligoarginine conjugates

A major challenge confronting the further advancement of using molecular transporters conjugated to small molecular weight therapeutics in the clinic is the development of linkers that would allow for the controllable release of a free drug/probe only after cell entry. Development of assays that would allow for the rapid real-time quantification of transporter conjugate uptake and cargo release in cells and animals would greatly help in their development. In this chapter, we describe a imaging method that quantitatively measures transporter conjugate uptake and cargo release in real-time in both cell culture and animal models.

Identification of a functional luciferase gene in the non-luminous diurnal firefly, Lucidina biplagiata

We isolated a luciferase gene (LbLuc) from the non-luminous diurnal firefly, Lucidina biplagiata, with high similarity to that from the nocturnal firefly, Photinus pyralis. The recombinant LbLuc showed luminescence activity comparable to that of the luciferases from P. pyralis and Luciola cruciata. To understand the non-luminosity of L. biplagiata, we determined the amount of luciferase in the adult specimen using the luciferin-luciferase reaction and found that the content of luciferase in L. biplagiata was estimated to be only 0.1% of that in L. cruciata. As previously reported, the content of luciferin in L. biplagiata was less than 0.1% of that in L. cruciata. Thus, the non-luminosity of L. biplagiata might be explained by low levels of both luciferase and luciferin.

Profiling of drug action using reporter mice and molecular imaging

Reporter mice associated to molecular imaging represent a major asset for the study of the spatio-temporal effects of drugs in living animals. The field is still relatively young and so far the number of animals genetically modified to express a given reporter gene ubiquitously and under the control of specific drugs is still limited. For a reporter animal the indispensable elements for the application to drug research and development are (i) the short life of the reporter enabling to have a clear view of the onset as well as the termination of drug effects, (ii) the generalized, drug-dependent activation of the reporter, and (iii) imaging modality suitable for high-throughput analysis. Because of its relative cheapness and ease to perform, in addition to all the above considerations, bioluminescence-based imaging is now regarded as the best imaging technology to be applied to the field of drug research. We show here the application of reporter mouse systems for drug screening in living animals in order to compare drug potency on target and specificity of action.

Bioluminescence: from chemical bonds to photons

The biological transformation of chemical to photic energy involves an enzyme-mediated chemiluminescent reaction, in which one of the products exists in an electronically excited state, emitting a photon as it returns to the ground state. The colour of bioluminescence differs in different organisms, ranging from the deep blue (460 nm) of certain crustacea, through the bluish green (490 nm) of some bacteria, the green (530 nm) of mushrooms to the red (about 600 nm) of the railroad worm. In one case, energy transfer has been demonstrated from the enzyme system to material that emits light with a longer wavelength. The energies involved range from about 165 to 250 kJ/einstein (40 to 60 kcal/einstein). Boyle first showed that air was involved in bioluminescence in 1668 in his experiments with an air pump. Over the past 100 years, it has become clear that most if not all bioluminescent systems require molecular oxygen. The recent isolation and characterization of an oxygen-containing (peroxide) enzyme intermediate from the bacterial system is described and a reaction mechanism is postulated. This scheme is compared with other hypothetical mechanisms, in particular those involving a four-membered ring intermediate, a dioxetane, in which the simultaneous cleavage of two bonds leaves one product in an excited state. I shall discuss the special role of luciferases in bioluminescence, especially in flashing mechanisms involving 'precharged' intermediates.

Dye induced quenching of firefly luciferase-luciferin bioluminescence

The quenching of firefly bioluminescence (BL) in presence of xanthene dyes and tetratolylporphyrin was investigated. The BL intensity was quenched with an altered decay pattern in presence of xanthene dyes and tetratolylporphyrin. The electronic absorption spectra indicate that there is no significant interaction occurring between the dyes and the BL components in the ground state. The BL quenching decay rate and fluorescence quenching studies of luciferin by the dyes suggest an energy transfer through an exciplex, involving oxyluciferin, in the excited state and the dyes, in the ground state. The bimolecular quenching rate constant (K(q)) values obtained from fluorescence studies varied between 7.7 x 10(12) and 19.8 x 10(12)M(-1)s(-1). The magnitude of the bimolecular quenching rate constants confirmed the complex formation between dye and excited oxyluciferin. The exciplex subsequently undergoes a non-radiative decay to the ground state via a combination of heavy atom induced and Förster-type energy transfer. The decay rate constants in presence and in absence of dyes vary between 7.47 x 10(-4) and 7.6 x 10(-2)s(-1). In the presence of dyes the effective decay rate constants (k(eff)) increased while the lifetime of light emitting species decreased. The kinetic studies in presence of singlet oxygen scavengers, like beta-carotene and NaN(3), prove that there is no significant quenching of the firefly BL due to the formation of singlet oxygen.

ABCG2/BCRP expression modulates D-Luciferin based bioluminescence imaging

Bioluminescence imaging (BLI) is becoming indispensable to the study of transgene expression during development and, in many in vivo models of disease such as cancer, for high throughput drug screening in vitro. Because reaction of d-luciferin with firefly luciferase (fLuc) produces photons of sufficiently long wavelength to permit imaging in intact animals, use of this substrate and enzyme pair has become the method of choice for performing BLI in vivo. We now show that expression of the ATP-binding cassette (ABC) family transporter ABCG2/BCRP affects BLI signal output from the substrate d-luciferin. In vitro studies show that d-luciferin is a substrate for ABCG2/BCRP but not for the MDR1 P-glycoprotein (ABCB1/Pgp), multidrug resistance protein 1 (MRP1/ABCC1), or multidrug resistance protein 2 (MRP2/ABCC2). d-Luciferin uptake within cells is shown to be modulated by ABC transporter inhibitors, including the potent and selective ABCG2/BCRP inhibitor fumitremorgin C. Images of xenografts engineered to express transgenic ABCG2/BCRP, as well as xenografts derived from the human prostate cancer cell line 22Rv1 that naturally express ABCG2/BCRP, show that ABCG2/BCRP expression and function within regions of interest substantially influence d-luciferin-dependent bioluminescent output in vivo. These findings highlight the need to consider ABCG2/BCRP effects during d-luciferin-based BLI and suggest novel high throughput methods for identifying new ABCG2/BCRP inhibitors.

Visualizing the dynamics of p21(Waf1/Cip1) cyclin-dependent kinase inhibitor expression in living animals

Although the role of p21(Waf1/Cip1) gene expression is well documented in various cell culture studies, its in vivo roles are poorly understood. To gain further insight into the role of p21(Waf1/Cip1) gene expression in vivo, we attempted to visualize the dynamics of p21(Waf1/Cip1) gene expression in living animals. In this study, we established a transgenic mice line (p21-p-luc) expressing the firefly luciferase under the control of the p21(Waf1/Cip1) gene promoter. In conjunction with a noninvasive bioluminescent imaging technique, p21-p-luc mice enabled us to monitor the endogenous p21(Waf1/Cip1) gene expression in vivo. By monitoring and quantifying the p21(Waf1/Cip1) gene expression repeatedly in the same mouse throughout its entire lifespan, we were able to unveil the dynamics of p21(Waf1/Cip1) gene expression in the aging process. We also applied this system to chemically induced skin carcinogenesis and found that the levels of p21(Waf1/Cip1) gene expression rise dramatically in benign skin papillomas, suggesting that p21(Waf1/Cip1) plays a preventative role(s) in skin tumor formation. Surprisingly, moreover, we found that the level of p21(Waf1/Cip1) expression strikingly increased in the hair bulb and oscillated with a 3-week period correlating with hair follicle cycle progression. Notably, this was accompanied by the expression of p63 but not p53. This approach, together with the analysis of p21(Waf1/Cip1) knockout mice, has uncovered a novel role for the p21(Waf1/Cip1) gene in hair development. These data illustrate the unique utility of bioluminescence imaging in advancing our understanding of the timing and, hence, likely roles of specific gene expression in higher eukaryotes.

A Shuffled CYP1A Library Shows Both Structural Integrity and Functional Diversity

The cytochrome P450 enzymes (P450s) that mediate mammalian xenobiotic metabolism are highly versatile monooxygenases, which show wide and overlapping substrate ranges but generally poor catalytic rates. Re-engineering of these P450s may enable the development of useful biocatalysts for industrial applications. In the current study, restriction enzyme-mediated DNA family shuffling was used to create a library from human CYP1A1 and CYP1A2. Among sequenced clones (four randomly selected and eight functional clones), 5.9 +/- 2.3 crossovers and 1.5 +/- 1.5 spontaneous mutations (mean +/- S.D.) were detected per mutant. A high level of structural integrity as well as diverse functionality were found, with 53% of clones expressed at significant levels (>50 nM P450 hemoprotein) and 23% of clones showing activity on one or more of the following compounds: luciferin 6'-chloroethyl ether (luciferin-CEE), luciferin 6'-methyl ether (luciferin-ME), 6'-deoxyluciferin (luciferin-H), the ethylene glycol ester of luciferin 6'-methyl ether, 7-ethoxyresorufin, and p-nitrophenol (PNP). Different activity profiles were seen with higher specific activity on individual compounds (e.g., clone 22; 9 times the CYP1A1 specific activity toward luciferin-CEE), novel activities (e.g., clone 35; activity toward luciferin-H and PNP), and broadening of substrate range observed in particular clones (e.g., clone 9; activity toward both selective substrates luciferin-ME and luciferin-CEE as well as toward luciferin-H and PNP). In summary, forms were found with distinct and novel activity profiles, despite the relatively small number of mutants examined. In addition, the whole-cell metabolic assays described here provide simple, high-throughput methods useful for screening larger libraries.

Fully codon-optimized luciferase uncovers novel temperature characteristics of the Neurospora clock

We report the complete reconstruction of the firefly luciferase gene, fully codon optimized for expression in Neurospora crassa. This reporter enhances light output by approximately 4 log orders over that with previously available versions, now producing light that is visible to the naked eye and sufficient for monitoring the activities of many poorly expressed genes. Time lapse photography of strains growing in race tubes, in which the frq or eas/ccg-2 promoter is used to drive luciferase, shows the highest levels of luciferase activity near the growth front and newly formed conidial bands. Further, we have established a sorbose medium colony assay that will facilitate luciferase-based screens. The signals from sorbose-grown colonies of strains in which the frq promoter drives luciferase exhibit the properties of circadian rhythms and can be tracked for many days to weeks. This reporter now makes it possible to follow the clock in real time, even in strains or under conditions in which the circadian rhythm in conidial banding is not expressed. This property has been used to discover short, ca. 15-h period rhythms at high temperatures, at which banding becomes difficult to observe in race tubes, and to generate a high-resolution temperature phase-response curve.

A GUS/Luciferase Fusion Reporter for Plant Gene Trapping and for Assay of Promoter Activity with Luciferin-Dependent Control of the Reporter Protein Stability

A gene-trapping vector carrying a GUS/Luciferase dual reporter gene was developed to establish an efficient and convenient screening system for T-DNA-based gene trapping in plants. A key feature of this gene trap scheme is to place two different types of reporters, luciferase (Luc) and beta-glucuronidase (GUS), as a fusion protein within a trapped gene to probe the activity of the gene. Luc is then utilized as a non-invasive, vital and highly sensitive screening reporter to identify trapped lines, including direct screening of the trapped lines from the primary T-DNA mutant pools. GUS is utilized as a histochemical assay reporter to analyze detailed cellular expression patterns. Transgenic expression studies in Arabidopsis showed that this fusion reporter protein retains functional enzyme activity for both GUS and Luc. Using this system in Arabidopsis, we were able to identify 3,737 trapped lines from 26,900 individual T-DNA insertion lines. Sequence determination of the T-DNA insertion loci in the genome of 78 trapped lines identified GUS/Luc fusions with 27 annotated Arabidopsis genes which included a subset of transcription factors, protein kinases, regulatory proteins and metabolic enzymes. Of these, particular expression patterns of four tagged genes were further confirmed by analyzing putative promoter regions of the corresponding wild-type genes. Furthermore, the protein stability of the GUS/Luc fusion reporter was controlled by application of luciferase substrate (luciferin), overcoming the excessive stability problem of GUS that causes misrepresentation of the transcriptional activity of a promoter. These results demonstrate the utility of the GUS/Luc dual reporter system as a gene trap reporter for studying plant genome function and also as a convenient dual reporter system for study of gene expression.

Real-time analysis of uptake and bioactivatable cleavage of luciferin-transporter conjugates in transgenic reporter mice

Many therapeutic leads fail to advance clinically because of bioavailability, selectivity, and formulation problems. Molecular transporters can be used to address these problems. Molecular transporter conjugates of otherwise poorly soluble or poorly bioavailable drugs or probes exhibit excellent solubility in water and biological fluids and at the same time an enhanced ability to enter tissues and cells and with modification to do so selectively. For many conjugates, however, it is necessary to release the drug/probe cargo from the transporter after uptake to achieve activity. Here, we describe an imaging method that provides quantification of transporter conjugate uptake and cargo release in real-time in animal models. This method uses transgenic (luciferase) reporter mice and whole-body imaging, allowing noninvasive quantification of transporter conjugate uptake and probe (luciferin) release in real time. This process effectively emulates drug-conjugate delivery, drug release, and drug turnover by an intracellular target, providing a facile method to evaluate comparative uptake of new transporters and efficacy and selectivity of linker release as required for fundamental studies and therapeutic applications.

Enzymatic photochemical sensing using luciferase-immobilized polymer nanoparticles covered with artificial cell membrane

We prepared phospholipid polymer nanoparticles immobilized with luciferase, and the nanoparticles were applied as photochemical sensing nanoparticles. An amphiphilic water-soluble polymer having a phosphorylcholine group was used as an emulsifier and a surface modifier to prepare the nanoparticles. The polymer was composed of three kinds of monomer units, that is, 2-methacryloyloxyethyl phosphorylcholine (MPC) as a hydrophilic and bioinert unit, n-butyl methacrylate as a hydrophobic unit and p-nitrophenyl ester having methacrylate as an enzyme-immobilizing unit. The p-nitrophenyl ester groups to immobilize the proteins were located on the surface of the nanoparticles. Luciferase was immobilized by the reaction between the p-nitrophenyl ester groups and the amino group. The enzymatic reaction on the nanoparticles was followed using a microdialysis system with an optical fiber having a 800 microm diameter in the probe. The nanoparticles conjugated with luciferase reacted with ATP, luciferin and oxygen. It is concluded that the nanoparticles are a promising tool for a photochemical sensing microdiagnostic system.

Firefly luciferase produces hydrogen peroxide as a coproduct in dehydroluciferyl adenylate formation

Firefly luciferase catalyzes the synthesis of H2O2 from the same substrates as the bioluminescence reaction: ATP and luciferin (D-LH2). About 80% of the enzyme-bound intermediate D-luciferyl adenylate (D-LH2-AMP) is oxidized into oxyluciferin, and a photon is emitted during this reaction. The enzyme pathway responsible for the generation of H2O2 is a side reaction in which D-LH2-AMP is oxidized into dehydroluciferyl adenylate (L-AMP). Like the bioluminescence reaction, the luciferase-catalyzed synthesis of H2O2 and L-AMP is a stereospecific process, involving only the natural D enantiomer. However, the intramolecular electron transfer postulated as essential to the light emission process is not involved in this side reaction.

Stereoisomeric bio-inversion key to biosynthesis of fireflyd-luciferin

The chirality of the luciferin substrate is critical to the luciferin-luciferase reaction producing bioluminescence. In firefly, the biosynthetic pathway of D-luciferin is still unclear, although it can be synthesized in vitro from D-cysteine. Here, we show that the firefly produces both D- and L-luciferin, and that the amount of active D-luciferin increases gradually with maturation stage. Studies of firefly body extracts indicate the possible conversion of L-cysteine via L-luciferin into D-luciferin, suggesting that the biosynthesis is enzymatically regulated by stereoisomeric bio-inversion of L-luciferin. We conclude that the selection of chirality in living organisms is not as rigid as previously thought.

Luciferin Derivatives for Enhanced in Vitro and in Vivo Bioluminescence Assays

In vivo bioluminescence imaging has become a cornerstone technology for preclinical molecular imaging. This imaging method is based on light-emitting enzymes, luciferases, which require specific substrates for light production. When linked to a specific biological process in an animal model of human biology or disease, the enzyme-substrate interactions become biological indicators that can be studied noninvasively in living animals. Signal intensity in these animal models depends on the availability of the substrate for the reaction within living cells in intact organs. The biodistribution and clearance rates of the substrates are therefore directly related to optimal imaging times and signal intensities and ultimately determine the sensitivity of detection and predictability of the model. Modifications of d-luciferin, the substrate for the luciferases obtained from beetle, including fireflies, result in novel properties and offer opportunities for improved bioassays. For this purpose, we have synthesized a conjugate, glycine-d-aminoluciferin, and investigated its properties relative to those of d-aminoluciferin and d-luciferin. The three substrates exhibited different kinetic properties and different intracellular accumulation profiles due to differences in their molecular structure, which in turn influenced their biodistribution in animals. Glycine-d-aminoluciferin had a longer in vivo circulation time than the other two substrates. The ability to assay luciferase in vitro and in vivo using these substrates, which exhibit different pharmacokinetic and pharmacodynamic properties, will provide flexibility and improve current imaging capabilities.

Kinetic characterization and in vitro toxicity evaluation of a luciferase less susceptible to HPV chemical inhibition

Enzyme-based in vitro toxicity assays are often susceptible to inhibition by test compounds. A mutant luciferase selected to be less susceptible to inhibition by chloroform (CNBLuc03-06) and other high production volume (HPV) chemicals, consisting of three point mutations was created and characterized. The mutant luciferase was less inhibited by chloroform, other HPV chemicals and common surfactant release reagents (Triton-X and SDS) compared to the wild-type. Inhibition was shown to be competitive. CNBLuc03-06 was a factor of 1.5-3.2 more active than wild type and exhibited a much higher affinity for ATP. CNBLuc03-06 was more thermostable than wild-type and also more active at pH values higher than 10. Both luciferases exhibited a significant tradeoff between activation and susceptibility to chemical inhibition in the presences of the reducing agent DTT. Inhibition to HPV chemicals was eliminated using an "optimum" formulation of DTT and co-solvent ethanol. The performance of CNBLuc03-06 in cell-based in vitro toxicity assays was shown to be superior to the current commercial formulation.

New bioluminogenic substrates for monoamine oxidase assays

Novel bioluminogenic substrates were designed for probing monoamine oxidase (MAO) activity based on a simple and effective beta-elimination strategy. By modifying the amino group and the central core of luciferin derivatives, we have developed a series of substrates useful for assays of MAO A or B, or both. One of these substrates, exhibiting low Km values and high signal-to-background ratios with both isozymes, was shown to accurately measure the Ki values of known MAO inhibitors. This substrate is a key component in the development of a highly sensitive homogeneous MAO assay for high-throughput screening (HTS) of compounds in drug discovery and for monitoring MAO activity in complex biological systems. This design strategy should be applicable to fluorogenic MAO substrates and could broaden the structural requirements of substrates for other enzyme assays.

Interaction of dimethyl-and monomethyloxyluciferin with recombinant wild-type and mutant firefly luciferases

Dissociation constants (Ks) in the pH range 6.5-9.0 for complexes of luciferin, dimethyloxyluciferin (DMOL), and monomethylluciferin (MMOL) with recombinant wild-type and mutant (His433Tyr) luciferases from the Luciola mingrelica firefly were determined by fluorescent titration. The protonated effectors were bound by the wild-type and mutant luciferases better than the nonprotonated ones. The affinity of DMOL for the mutant luciferase was higher than for the wild-type luciferase at alkaline pH, whereas the affinity of MMOL was higher at all pH values studied. The fluorescence emission and excitation spectra of DMOL and MMOL in buffer solution (pH 7.8) were obtained in the absence and presence of luciferase. The fluorescence maxima of DMOL and MMOL complexes with luciferase were 20 and 100 nm, respectively, shifted to shorter wavelengths as compared to the values in buffer solution. This was explained by nonspecific and specific influence of the protein microenvironment on the fluorescence spectra of DMOL and its specific influence on the MMOL fluorescence spectra.

Effect of different salts and detergents on luciferin-luciferase luminescence of the enchytraeid Fridericia heliota

The study addresses the effect produced by different inorganic salts and detergents (SDS, Triton X-100, the Tween series) on the ATP-dependent bioluminescent reaction catalyzed by the luciferase of the new earthworm species Fridericia heliota (Annelida: Clitellata: Oligochaeta: Enchytraeidae). It has been shown that the effect of divalent metal salts on luminescence is determined by the action of cations. Three of them - Mg(2+), Mn(2+) and Ca(2+) - can stimulate luciferase activity at concentrations varying within a wide range, and Mn(2+) can act as a 100%-effective substitute for Mg(2+) in F. heliota luminescence reaction in vitro. The inhibitory effect of monovalent metal salts on luminescence is largely determined by the action of the anion part of the molecule. The effectiveness of the inhibitory effect of anions increases in the following order: Cl(-)<CO(3)(-2) approximately SO(3)(-2) approximately Br(-)<SO(4)(-2) approximately PO(4)(-3)<NO(3)(-)<I(-)<<Cr(2)O(7)(-2)<<Fe(CN)(6)(-3). Of the sodium salts, dodecyl sulfate, which is an anionic detergent, produces the strongest inhibitory effect on luciferase. On the contrary, nonionic detergents produce a stimulatory effect on the F. heliota luciferase. The action of the most effective of them - Triton X-100 - is determined by its ability to reduce the actual concentration of lipid inhibitors in the reaction mixture.

Long-term tracing of adenoviral expression in rat and rabbit using luciferase imaging

BACKGROUND

Luciferase optical imaging provides a novel method to monitor transgene expression in small living animals. As the genetic and immunological heritages of particular animals significantly affect the expression of adenovirus-delivered transgenes, it is essential to know the expression patterns specific to athymic nude and Sprague-Dawley rats, two strains commonly used in rodent models. In this study we set out to determine these patterns. At the same time, we tested luciferase optical imaging in a larger animal, the rabbit.

METHODS

A recombinant luciferase adenoviral vector was injected subcutaneously or intramuscularly into athymic nude rats, Sprague-Dawley rats, and Dutch Belted rabbits. The luciferase expression was assessed using a cooled charge-coupled device.

RESULTS

The luminescent signal was capable of passing through at least 1.3 cm of muscle tissue and proved to be much stronger when luciferin was delivered via a local injection than by an intraperitoneal injection. Although the types of immune cells differed between immunodeficient and immunocompetent rats, similar amounts and patterns of luciferase expression were observed in the musculature in two rat strains during the 1st month after a viral intramuscular injection. The duration of luciferase expression was longer than 15 months in athymic nude rats, 9 months in Sprague-Dawley rats, and 6 months in rabbits following a direct viral injection.

CONCLUSIONS

Luciferase expression after adenoviral gene delivery can persist for longer than 6 months, even in immunocompetent animals. Live imaging of luciferase expression can be performed not only in small animals, but also in larger animals such as rabbits.

Firefly luciferase exhibits bimodal action depending on the luciferin chirality

Firefly luciferase is able to convert L-luciferin into luciferyl-CoA even under ordinary aerobic luciferin-luciferase reaction conditions. The luciferase is able to recognize strictly the chirality of the luciferin structure, serving as the acyl-CoA synthetase for L-luciferin, whereas d-luciferin is used for the bioluminescence reaction. D-Luciferin inhibits the luciferyl-CoA synthetase activity of L-luciferin, whereas L-luciferin retards the bioluminescence reaction of D-luciferin, meaning that both enzyme activities are prevented by the enantiomer of its own substrate.

Identification of the luciferin-luciferase system and quantification of coelenterazine by mass spectrometry in the deep-sea luminous ostracod Conchoecia pseudodiscophora

The bioluminescence system of the ostracod Conchoecia pseudodiscophora, which is abundant in the Sea of Japan, has been characterized. The luminescence (lambda(max)=463 nm) is produced by a luciferin-luciferase reaction, and the luciferin has been identified as coelenterazine. Coelenterazine, coelenteramide, and coelenteramine from C. pseudodiscophora were quantified by LC-ESI-MS/MS analysis. The coelenterazine content was estimated to be approximately 230 pg per animal by using a calibration curve of synthetic coelenterazine. The reaction between homogenates of C. pseudodiscophora and synthetic coelenterazine showed luminescence activity; this suggests that a coelenterazine-type luciferase is present.

Luciferase real-time bioluminescence imaging for the study of viral pathogenesis

Herpes simplex virus 1 (HSV-1) is a common and significant neurotropic human pathogen that infects 80% of all persons by adulthood. During acute HSV-1 infection, virus replicates peripherally in epithelia, enters axonal terminals, and is transported retrogradely to sensory nerve ganglia, where HSV-1 may establish latency or progress to life-threatening infection of the central nervous system. Studies of viral and host factors that influence pathogenesis have largely used experimental mouse models that rely on sacrifice of infected mice to determine distribution and titer of virus. Although this experimental paradigm has provided important data, it precludes real-time investigations of the same animal over the entire course of disease progression. This limits potentially significant insights from animal-to-animal variations in host-pathogen relationships. Unexpected sites of infection also may be missed because appropriate tissues are not analyzed for virus. To improve investigations of viral and host factors that determine HSV-1 pathogenesis, we have validated bioluminescence imaging (BLI) as a technique to monitor infection with a recombinant strain KOS HSV-1 virus that expresses firefly luciferase (FL). This imaging technique allows repetitive, noninvasive monitoring of HSV-1 in living mice. In this chapter, we describe the protocols that we use for in vivo BLI of HSV-1 infection.

Noninvasive in vivo whole body luminescent analysis of luciferase labelled orthotopic prostate tumours

Light photons refracted through living tissues can be used to noninvasively monitor the proliferation of cells expressing bioluminescent markers. We demonstrate the use of a luminometer for noninvasive in vivo whole body luminometric analysis (in vivo WBLA) of luciferase-expressing prostate tumours growing orthotopically in nude mice, and thus hidden from visual inspection. In this procedure, the intraperitoneally (i.p.) inoculated luciferin, the luciferase substrate, reaches the tumours rapidly and the light photons generated by the tumour are recorded by placing the anaesthetised mice in the detection chamber of a luminometer, over the detector slot. We show that the number of recorded light photons is proportional to the tumour mass and to the luciferase activity recorded in vitro. The procedure is applied to demonstrate the use of paclitaxel as an antineoplasic agent with its well characterised antiproliferative activity.

Homogeneous enzyme immunoassay modified for application to luminescence-based biosensors

Application of immunoassay to biosensors for use in the point-of-care setting ideally requires immunoassay without separation steps and with small volumes of both sample and reagents. The suitability of cloned enzyme donor immunoassay (CEDIA), one of a few homogeneous immunoassays available, was investigated for application to biosensors. This method is based on the bacterial enzyme beta-galactosidase, which has been genetically engineered by others into two inactive fragments, enzyme donor (ED) and enzyme acceptor (EA). Association of the ED and EA fragments in the assay results in formation of active enzyme, which acts on substrate to generate a detectable signal. Sensitivity of commercially available CEDIA kits were compared, with respect to the sample and reagent volumes, using three different signal generation processes. The CEDIA kit for valproic acid and three substrates, a colorimetric (chlorophenol red-beta-D-galactopyranoside), a chemiluminescent (Lumi-Gal 530), and a bioluminescent (Beta-Glo Assay System), were employed in the study. Our results indicate that the high sensitivity of the bioluminogenic substrate, D-luciferin-O-beta-galactopyranoside, with short assay time and small volumes of sample and reagents required for the assay, simple handling, and relatively low expense, make this substrate, together with CEDIA, suitable for application to biosensors intended for drug and metabolite monitoring in the point-of-care setting.