Kinetics of inhibition of firefly luciferase by dehydroluciferyl-coenzyme A, dehydroluciferin and L-luciferin

Genome assembly of Genji firefly (Nipponoluciola cruciata) reveals novel luciferase-like luminescent proteins without peroxisome targeting signal

The Genji firefly, Nipponoluciola cruciata, is an aquatic firefly endemic to Japan, inhabiting a wide area of the Japanese archipelago. The luminescence of fireflies is a scientifically interesting phenomenon, and many studies have evaluated this species in Japan. In this study, we sequenced the whole genome of male N. cruciata and constructed a high-quality genome assembly of 662 Mb with a BUSCO completeness of 99.1% in the genome mode. Using the detected set of 15,169 protein-coding genes, the genomic structures and genetic background of luminescence-related genes were also investigated. We found four new firefly luciferase-like genes in the genome. The highest bioluminescent activity was observed for LLa2, which originated from ancestral PDGY, a mitochondrial acyl-CoA synthetase. A thioesterase candidate, NcruACOT1, which is involved in d-luciferin biosynthesis, was expressed in the lantern. Two opsins were also detected and the absorption wavelength of the UV-type opsin candidate shifted from UV to blue. These findings provide an important resource for unravelling the adaptive evolution of fireflies in terms of luminescence and vision.

Synthesis and Evaluation of Glycosyl Luciferins

Measuring glycosidase activity is important to monitor any aberrations in carbohydrate hydrolase activity, but also for the screening of potential glycosidase inhibitors. To this end, synthetic substrates are needed which provide an enzyme-dependent read-out upon hydrolysis by the glycosidase. Herein, we present two new routes for the synthesis of caged luminescent carbohydrates, which can be used for determining glycosidase activity with a luminescent reporter molecule. The substrates were validated with glycosidase and revealed a clear linear range and enzyme-dependent signal upon the in situ generation of the luciferin moiety from the corresponding nitrile precursors. Besides, we showed that these compounds could directly be synthesized from unprotected glycosyl-α-fluorides in a two-step procedure with yields up to 75 %. The intermediate methyl imidate appeared a key intermediate which also reacted with d-cysteine to give the corresponding d-luciferin substrate rendering this a highly attractive method for synthesizing glycosyl luciferins in good yields.

Comparative Investigation of the Chemiluminescent Properties of a Dibrominated Coelenterazine Analog

Chemi- and bioluminescence are remarkable light-emitting phenomena, in which thermal energy is converted into excitation energy due to a (bio)chemical reaction. Among a wide variety of chemi-/bioluminescent systems, one of the most well-known and studied systems is that of marine imidazopyrazinones, such as Coelenterazine and Cypridina luciferin. Due to the increasing usefulness of their chemi-/bioluminescent reactions in terms of imaging and sensing applications, among others, significant effort has been made over the years by researchers to develop new derivatives with enhanced properties. Herein, we report the synthesis and chemiluminescent characterization of a novel dibrominated Coelenterazine analog. This novel compound consistently showed superior luminescence, in terms of total light output and emission lifetime, to natural imidazopyrazinones and commercially available analogs in aprotic media, while being capable of yellow light emission. Finally, this new compound showed enhanced chemiluminescence in an aqueous solution when triggered by superoxide anion, showing potential to be used as a basis for optimized probes for reactive oxygen species. In conclusion, bromination of the imidazopyrazinone scaffold appears to be a suitable strategy for obtaining Coelenterazines with enhanced properties.

Development of a Coelenterazine Derivative with Enhanced Superoxide Anion-Triggered Chemiluminescence in Aqueous Solution

Superoxide anion is a reactive oxygen species (ROS) of biological interest. More specifically, it plays a role in intra- and intercellular signaling, besides being associated with conditions such as inflammation and cancer. Given this, efforts have been made by the research community to devise new sensing strategies for this ROS species. Among them, the chemiluminescent reaction of marine Coelenterazine has been employed as a sensitive and dynamic probing approach. Nevertheless, chemiluminescent reactions are typically associated with lower emissions in aqueous solutions. Herein, here we report the synthesis of a new Coelenterazine derivative with the potential for superoxide anion sensing. Namely, this novel compound is capable of chemiluminescence in a dose-dependent manner when triggered by this ROS species. More importantly, the light-emission intensities provided by this derivative were relevantly enhanced (intensities 2.13 × 101 to 1.11 × 104 times higher) in aqueous solutions at different pH conditions when compared to native Coelenterazine. The half-life of the chemiluminescent signal is also greatly increased for the derivative. Thus, a new chemiluminescence molecule with significant potential for superoxide anion sensing was discovered and reported for the first time.

Biosynthesis-Inspired Deracemizative Production of D-Luciferin In Vitro by Combining Luciferase and Thioesterase

Due to the strict enantioselectivity of firefly luciferase (FLuc), only D-luciferin can be used as a substrate for the bioluminescence (BL) reaction. Unfortunately, luciferin racemizes easily and accumulation of nonluminous L-luciferin has negative influences on the light-emitting reaction. By a detailed analysis of luciferin chirality, however, it becomes clarified that L-luciferin is the biosynthetic precursor of D-luciferin in fireflies and undergoes the enzymatic chiral inversion. By the chiral inversion reaction, the enantiopurity of luciferin can be maintained in the reaction mixture for applications using FLuc. Thus, chirality is crucial for the BL reaction and essential for investigating and applying the biosynthesis of D-luciferin. Here, we describe the methods for the analysis of chiral inversion reaction using high-performance liquid chromatography (HPLC) with a chiral column. We also introduce an example of an in vitro deracemizative BL reaction system using a combination of FLuc and fatty acyl-CoA thioesterase, which is inspired by the chiral inversion mechanism in the biosynthetic pathway of D-luciferin.

A higher spectral range of beetle bioluminescence with infraluciferin

Coleopteran bioluminescence is unique in that beetle luciferases emit colors ranging between green (ca.550 nm) and red (ca.600 nm), including intermediate colors such as yellow and orange, allowing up to 3 simultaneous parameters to be resolved in vitro with natural luciferin (D-LH₂). Here, we report a more than doubling of the maximum bioluminescence wavelength range using a single synthetic substrate, infraluciferin (iLH₂). We report that different luciferases can emit colors ranging from visible green to near-infrared (nIR) with iLH_2, including in human cells. iLH₂ was designed for dual color far-red to nIR bioluminescence imaging (BLI) in small animals and has been utilized in different mouse models of cancer (including a metastatic hepatic model showing detailed hepatic morphology) and for robust dual parameter imaging in vivo (including in systemic hematological models). Here, we report the properties of different enzymes with iLH₂: Lampyrid wild-type (WT) Photinus pyralis (Ppy) firefly luciferase, Ppy-based derivatives previously engineered to be thermostable with D-LH₂, and also color-shifted Elaterid-based enzymes: blue-shifted Pyrearinus termitilluminans derivative Eluc (reported D-LH₂ λmax = 538 nm) and red-shifted Pyrophorus plagiopthalamus derivative click beetle red (CBR) luciferase (D-LH₂ λmax = 618 nm). As purified enzyme, in bacteria or in human cells, Eluc emitted green light (λmax = 536 nm) with DL-iLH₂ whereas Ppy Fluc (λmax = 689 nm), x2 Fluc (λmax = 704 nm), x5 Fluc (λmax = 694 nm), x11 Fluc (λmax = 694 nm) and CBR (λmax = 721 nm) produced far-red to nIR peak wavelengths. Therefore, with iLH_2, enzyme λmaxes can be separated by ca.185nm, giving almost non-overlapping spectra. This is the first report of single-substrate bioluminescence color emission ranging from visible green to nIR in cells and may help shed light on the color tuning mechanism of beetle luciferases. We also report on the reason for the improvement in activity of x11 Fluc with iLH₂ and engineer an improved infraluciferase (iluc) based on this mutant.

Methods for Detecting PER2:LUCIFERASE Bioluminescence Rhythms in Freely Moving Mice

Circadian rhythms are driven by daily oscillations of gene expression. An important tool for studying cellular and tissue circadian rhythms is the use of a gene reporter, such as bioluminescence from the reporter gene luciferase controlled by a rhythmically expressed gene of interest. Here we describe methods that allow measurement of circadian bioluminescence from a freely moving mouse housed in a standard cage. Using a LumiCycle In Vivo (Actimetrics), we determined conditions that allow detection of circadian rhythms of bioluminescence from the PER2 reporter, PER2::LUC, in freely behaving mice. The LumiCycle In Vivo applies a background subtraction that corrects for effects of room temperature on photomultiplier tube (PMT) output. We tested delivery of d-luciferin via a subcutaneous minipump and in the drinking water. We demonstrate spikes in bioluminescence associated with drinking bouts. Further, we demonstrate that a synthetic luciferase substrate, CycLuc1, can support circadian rhythms of bioluminescence, even when delivered at a lower concentration than d-luciferin, and can support longer-term studies. A small difference in phase of the PER2::LUC bioluminescence rhythms, with females phase leading males, can be detected with this technique. We share our analysis scripts and suggestions for further improvements in this method. This approach will be straightforward to apply to mice with tissue-specific reporters, allowing insights into responses of specific peripheral clocks to perturbations such as environmental or pharmacological manipulations.

Synthesis of bioluminescent gold nanoparticle-luciferase hybrid systems for technological applications

Bioluminescent gold nanoparticles (AuNPs) were synthesized in situ using dithiol-terminated polyethylene glycol (PEG(SH)2) as reducer and stabilizing agents. Hybrid Au/F3O4 nanoparticles were also produced in a variation of synthesis, and both types of nanostructures had the polymer capping replaced by L-cysteine (Cys). The four types of nanoparticles, PEG(SH)2AuNPs, PEG(SH)2Au/F3O4NPs, CysAuNPs, and CysAu/F3O4NPs were associated with purified recombinant Pyrearinus termitilluminans green emitting click beetle luciferase (PyLuc) and Phrixotrix hirtus (RELuc) red-emitting railroad worm luciferase. Enzyme association with PEG(SH)2 was also investigated as a control. Luciferases were chosen because they catalyze bioluminescent reactions used in a wide range of bioanalytical applications, including ATP assays, gene reporting, high-throughput screening, bioluminescence imaging, biosensors and other bioluminescence-based assays. The immobilization of PyLuc and RELuc promoted partial suppression of the enzyme luminescence activity in a functionalization-dependent way. Association of PyLuc and RELuc with AuNPs increased the enzyme operational stability in relation to the free enzyme, as evidenced by the luminescence intensity from 0 to 7 h after substrate addition. The stability of the immobilized enzymes was also functionalization-dependent and the association with CysAuNPs was the condition that combined more sustained luminescent activity with a low degree of luminescence quenching. The higher enzymatic stability and sustained luminescence of luciferases associated with nanoparticles may improve the applicability of bioluminescence for bioimaging and biosensing purposes.

Optimized bioluminescence analysis of adenosine triphosphate (ATP) released by platelets and its application in the high throughput screening of platelet inhibitors

Activated platelets release adenosine trisphosphate (ATP) and bioluminescence analysis of ATP release is usually used to monitor activation of platelets induced by various stimulants. However, bioluminescence analysis of ATP possesses poor linearity, the signal is quickly attenuated, and the accuracy of ATP release from platelets is hard to determine accurately enough to be used in a high throughput screening of platelet inhibitors. The present study was designed to optimize bioluminescence analysis of ATP released by platelets and expand its application in high throughput screening of platelet inhibitors. The results showed that accuracy of ATP analysis was significantly improved by adding coenzyme A (CoA) and signal attenuation of ATP analysis was greatly postponed by adding bovine serum albumin (BSA) both in Hank’s balanced salt solution (HBSS) and Tyrode’s buffer. Furthermore, ATP release of activated platelets and inhibitory effects of Ly294002 and Staurosporine on platelet activation were accurately determined by our optimized bioluminescence analysis of ATP. Thus, we have successfully constructed an optimized bioluminescence analysis of ATP which can be used in high throughput screening of platelet inhibitors.

Comparative study of the chemiluminescence of coelenterazine, coelenterazine-e and Cypridina luciferin with an experimental and theoretical approach

Imidazopyrazinone is a typical scaffold present in marine bioluminescence, in which thermal energy is converted into excitation energy in an enzyme-catalyzed reaction. In fact, the imidazopyrazinone scaffold is a common link among organisms of eight phyla. The characterization of the light emission mechanism is essential for the development of future applications in bioimaging, bioanalysis and biomedicine. Herein, we have studied the chemiluminescent reaction of three commercially-available imidazopyrazinones (Cypridina luciferin, Coelenterazine and Coelenterazine-e) in several aprotic solvents at different pH. We have found that at acidic pH only DMF and DMSO consistently present high light emission, while chemiluminescence in other solvents is negligible. We have attributed this to the inability of most solvents to allow for the deprotonation of the imidazopyrazinone core, thereby preventing the oxygenation step. We have also observed that increasing the pH of the solution leads to the inhibition of chemiluminescence, which we attributed to the deprotonation of the dioxetanone intermediate, as the neutral species is the one associated with efficient chemiexcitation. We have also observed that the pK_a of dioxetanone increases with the dielectric constant of the medium. Finally, our work indicated that the chemiexcitation yield increases with increasing polarity of the medium, due to a reduced transition dipole moment associated with S₀ → S₁ transition.

β-Deuterium Isotope Effects on Firefly Luciferase Bioluminescence

A 5,5-d2 -luciferin was prepared to measure isotope effects on reactions of two intermediates in firefly bioluminescence: emission by oxyluciferin and elimination of a putative luciferyl adenylate hydroperoxide to dehydroluciferin. A negligible isotope effect on bioluminescence provides further support for the belief that the emitting species is the keto-phenolate of oxyluciferin and rules out its excited-state tautomerization, one potential contribution to a bioluminescence quantum yield less than unity. A small isotope effect on dehydroluciferin formation supports a single-electron-transfer mechanism for reaction of the luciferyl adenylate enolate with oxygen to form the hydroperoxide or dehydroluciferin. Partitioning between the dioxetanone intermediate (en route to oxyluciferin) and dehydroluciferin is determined, not by the fate of the hydroperoxide, but by that of the radical formed from luciferyl adenylate, and the kinetic isotope effect (KIE) reflects H-atom abstraction by superoxide.

In-depth Characterization of Firefly Luciferase as a Reporter of Circadian Gene Expression in Mammalian Cells

Firefly luciferase (Fluc) is frequently used to report circadian gene expression rhythms in mammalian cells and tissues. During longitudinal assays it is generally assumed that enzymatic substrates are in saturating excess, such that total bioluminescence is directly proportional to Fluc protein level. To test this assumption, we compared the enzyme kinetics of purified luciferase with its activity in mammalian cells. We found that Fluc activity in solution has a lower Michaelis constant (K_m) for luciferin, lower temperature dependence, and lower catalytic half-life than Fluc in cells. In consequence, extracellular luciferin concentration significantly affects the apparent circadian amplitude and phase of the widely used PER2::LUC reporter in cultured fibroblasts, but not in SCN, and we suggest that this arises from differences in plasma membrane luciferin transporter activity. We found that at very high concentrations (>1 mM), luciferin lengthens circadian period, in both fibroblasts and organotypic SCN slices. We conclude that the amplitude and phase of circadian gene expression inferred from bioluminescence recordings should be treated with some caution, and we suggest that optimal luciferin concentration should be determined empirically for each luciferase reporter and cell type.

Supramolecular Control over Split-Luciferase Complementation

Supramolecular split-enzyme complementation restores enzymatic activity and allows for on-off switching. Split-luciferase fragment pairs were provided with an N-terminal FGG sequence and screened for complementation through host-guest binding to cucurbit[8]uril (Q8). Split-luciferase heterocomplex formation was induced in a Q8 concentration dependent manner, resulting in a 20-fold upregulation of luciferase activity. Supramolecular split-luciferase complementation was fully reversible, as revealed by using two types of Q8 inhibitors. Competition studies with the weak-binding FGG peptide revealed a 300-fold enhanced stability for the formation of the ternary heterocomplex compared to binding of two of the same fragments to Q8. Stochiometric binding by the potent inhibitor memantine could be used for repeated cycling of luciferase activation and deactivation in conjunction with Q8, providing a versatile module for in vitro supramolecular signaling networks.

Synthesis of Firefly Luciferin Analogues and Evaluation of the Luminescent Properties

Five new firefly luciferin (1) analogues were synthesized and their light emission properties were examined. Modifications of the thiazoline moiety in 1 were employed to produce analogues containing acyclic amino acid side chains (2-4) and heterocyclic rings derived from amino acids (5 and 6) linked to the benzothiazole moiety. Although methyl esters of all of the synthetic derivatives exhibited chemiluminescence activity, only carboluciferin (6), possessing a pyrroline-substituted benzothiazole structure, had bioluminescence (BL) activity (λmax =547 nm). Results of bioluminescence studies with AMP-carboluciferin (AMP=adenosine monophosphate) and AMP-firefly luciferin showed that the nature of the thiazoline mimicking moiety affected the adenylation step of the luciferin-luciferase reaction required for production of potent BL. In addition, BL of 6 in living mice differed from that of 1 in that its luminescence decay rate was slower.

Simple and ultrastable all-inclusive pullulan tablets for challenging bioassays

Many biodetection systems employ labile enzymes and substrates that need special care, making it hard to routinely use them for point-of-care or field applications. In this work we provide a simple solution to this challenging problem through the creation of all-inclusive pullulan assay tablets. The proposed tablet system not only enhances the long-term stability of both enzymes and organic substrates, but also simplifies the assay procedure. The enhanced stability is attributed to two factors: the restriction of the molecular motion of proteins and impermeability to molecular oxygen afforded by the tables. These tablets dissolve rapidly upon addition to testing samples, making the test very easy to perform. Using the ATP-detecting luciferase-luciferin system as an example, we show that the tablet-based assay can achieve highly sensitive detection of ATP in biological samples and that the activity of the assay tablets remains unchanged for over a month at room temperature.

Mathematical Model of the Firefly Luciferase Complementation Assay Reveals a Non-Linear Relationship between the Detected Luminescence and the Affinity of the Protein Pair Being Analyzed

The firefly luciferase complementation assay is widely used as a bioluminescent reporter technology to detect protein-protein interactions in vitro, in cellulo, and in vivo. Upon the interaction of a protein pair, complemented firefly luciferase emits light through the adenylation and oxidation of its substrate, luciferin. Although it has been suggested that kinetics of light production in the firefly luciferase complementation assay is different from that in full length luciferase, the mechanism behind this is still not understood. To quantitatively understand the different kinetics and how changes in affinity of a protein pair affect the light emission in the assay, a mathematical model of the in vitro firefly luciferase complementation assay was constructed. Analysis of the model finds that the change in kinetics is caused by rapid dissociation of the protein pair, low adenylation rate of luciferin, and increased affinity of adenylated luciferin to the enzyme. The model suggests that the affinity of the protein pair has an exponential relationship with the light detected in the assay. This relationship causes the change of affinity in a protein pair to be underestimated. This study underlines the importance of understanding the molecular mechanism of the firefly luciferase complementation assay in order to analyze protein pair affinities quantitatively.

Quenching the firefly bioluminescence by various ions

Some specific ions could selectively inhibit firefly luciferase while having a negligible effect on renilla luciferase, which may be used in the improved dual luciferase reporter gene assay.

Blocking the entrance of AMP pocket results in hormetic stimulation of imidazolium-based ionic liquids to firefly luciferase

The hormesis characterized by low-concentration stimulation and high-concentration inhibition has gained significant interest over the past decades. Some organic solvents and ionic liquids (ILs) have hormetic concentration responses (HCR) to bioluminescence such as firefly luciferase and Vibrio qinghaiensis sp.-Q67. In this study, we determine the effects of 1-alkyl-3-methylimidazolium chlorine ILs ([Cnmim]Cl, n=2, 4, 6, 8, 10 and 12) to firefly luciferase in order to verify the mechanism of hormesis. The luminescence inhibition toxicity tests show that the stimulation effects of [C8mim]Cl and [C10mim]Cl are obvious, [C6mim]Cl and [C12mim]Cl are minor, and [C2mim]Cl and [C4mim]Cl are rare. The enzyme kinetics show that [C8mim]Cl and [C10mim]Cl are the competitive inhibitors with ATP while [C2mim]Cl and [C4mim]Cl are the noncompetitive ones. Molecular dynamics simulation results reveal that imidazolium rings of [C8mim] and [C10mim] locate at the entrance of luciferin pocket which is adjacent to AMP pocket, while alkyl-chains insert into the bottom of the luciferin pocket. Combining the results from inhibition test, kinetics assay and molecular simulation, we can deduce that occupying AMP pocket by imidazolium ring is responsible for hormetic stimulation.

Development of a cell-based bioassay for phospholipase A2-triggered liposomal drug release

The feasibility of exploiting secretory phospholipase A₂ (sPLA₂) enzymes, which are overexpressed in tumors, to activate drug release from liposomes precisely at the tumor site has been demonstrated before. Although the efficacy of the developed formulations was evaluated using in vitro and in vivo models, the pattern of sPLA₂-assisted drug release is unknown due to the lack of a suitable bio-relevant model. We report here on the development of a novel bioluminescence living-cell-based luciferase assay for the monitoring of sPLA₂-triggered release of luciferin from liposomes. To this end, we engineered breast cancer cells to produce both luciferase and sPLA₂ enzymes, where the latter is secreted to the extracellular medium. We report on setting up a robust and reproducible bioassay for testing sPLA₂-sensitive, luciferin remote-loaded liposomal formulations, using 1,2-distearoyl-sn-glycero-3-phosphatidylcholine/1,2-distearoyl-sn-glycero-3-phosphatidylglycerol (DSPC/DSPG) 7:3 and DSPC/DSPG/cholesterol 4:3:3 as initial test systems. Upon their addition to the cells, the liposomes were degraded almost instantaneously by sPLA₂ releasing the encapsulated luciferin, which provided readout from the luciferase-expressing cells. Cholesterol enhanced the integrity of the formulation without affecting its susceptibility to sPLA₂. PEGylation of the liposomes only moderately broadened the release profile of luciferin. The provided bioassay represents a useful tool for monitoring active drug release in situ in real time as well as for testing and optimizing of sPLA₂-sensitive lipid formulations. In addition, the bioassay will pave the way for future in-depth in vitro and in vivo studies.

Discovery of a series of 2-phenylnaphthalenes as firefly luciferase inhibitors

A series of 2-phenylnaphthalenes as firefly luciferase inhibitors are reported. The most potent compound 5 showed good systemic inhibition in transgenic mice. Kinetic assay indicated 5 is competitive for aminoluciferin and noncompetitive for ATP.

Biosynthesis of firefly luciferin in adult lantern: decarboxylation of L-cysteine is a key step for benzothiazole ring formation in firefly luciferin synthesis

Background

Bioluminescence in fireflies and click beetles is produced by a luciferase-luciferin reaction. The luminescence property and protein structure of firefly luciferase have been investigated, and its cDNA has been used for various assay systems. The chemical structure of firefly luciferin was identified as the ᴅ-form in 1963 and studies on the biosynthesis of firefly luciferin began early in the 1970’s. Incorporation experiments using ¹⁴C-labeled compounds were performed, and cysteine and benzoquinone/hydroquinone were proposed to be biosynthetic component for firefly luciferin. However, there have been no clear conclusions regarding the biosynthetic components of firefly luciferin over 30 years.

Methodology/Principal Findings

Incorporation studies were performed by injecting stable isotope-labeled compounds, including ʟ-[U-¹³C₃]-cysteine, ʟ-[1-¹³C]-cysteine, ʟ-[3-¹³C]-cysteine, 1,4-[D₆]-hydroquinone, and p-[2,3,5,6-D]-benzoquinone, into the adult lantern of the living Japanese firefly Luciola lateralis. After extracting firefly luciferin from the lantern, the incorporation of stable isotope-labeled compounds into firefly luciferin was identified by LC/ESI-TOF-MS. The positions of the stable isotope atoms in firefly luciferin were determined by the mass fragmentation of firefly luciferin.

Conclusions

We demonstrated for the first time that ᴅ- and ʟ-firefly luciferins are biosynthesized in the lantern of the adult firefly from two ʟ-cysteine molecules with p-benzoquinone/1,4-hydroquinone, accompanied by the decarboxylation of ʟ-cysteine.

Firefly luciferase inhibitor-conjugated peptide quenches bioluminescence: a versatile tool for real time monitoring cellular uptake of biomolecules

In this paper, novel firefly luciferase-specific inhibitor compounds (FLICs) are evaluated as potential tools for cellular trafficking of transporter conjugates. As a proof-of-concept, we designed FLICs that were suitable for solid phase peptide synthesis and could be covalently conjugated to peptides via an amide bond. The spacer between inhibitor and peptide was optimized to gain efficient inhibition of recombinant firefly luciferase (FLuc) without compromising the activity of the model peptides. The hypothesis of using FLICs as tools for cellular trafficking studies was ensured with U87Fluc glioblastoma cells expressing firefly luciferase. Results show that cell penetrating peptide (penetratin) FLIC conjugate 9 inhibited FLuc penetrated cells efficiently (IC50 = 1.6 μM) and inhibited bioluminescence, without affecting the viability of the cells. Based on these results, peptide-FLIC conjugates can be used for the analysis of cellular uptake of biomolecules in a new way that can at the same time overcome some downsides seen with other methods. Thus, FLICs can be considered as versatile tools that broaden the plethora of methods that take advantage of the bioluminescence phenomena.

Oxyluciferin photoacidity: the missing element for solving the keto-enol mystery?

The oxyluciferin family of fluorophores has been receiving much attention from the research community and several systematic studies have been performed in order to gain more insight regarding their photophysical properties and photoprotolytic cycles. In this minireview, we summarize the knowledge obtained so far and define several possible lines for future research. More importantly, we analyze the impact of the discoveries on the firefly bioluminescence phenomenon made so far and explain how they re-open again the discussion regarding the identity (keto or enol species) of the bioluminophore.

A biocompatible in vivo ligation reaction and its application for noninvasive bioluminescent imaging of protease activity in living mice

The discovery of biocompatible reactions had a tremendous impact on chemical biology, allowing the study of numerous biological processes directly in complex systems. However, despite the fact that multiple biocompatible reactions have been developed in the past decade, very few work well in living mice. Here we report that D-cysteine and 2-cyanobenzothiazoles can selectively react with each other in vivo to generate a luciferin substrate for firefly luciferase. The success of this "split luciferin" ligation reaction has important implications for both in vivo imaging and biocompatible labeling strategies. First, the production of a luciferin substrate can be visualized in a live mouse by bioluminescence imaging (BLI) and furthermore allows interrogation of targeted tissues using a "caged" luciferin approach. We therefore applied this reaction to the real-time noninvasive imaging of apoptosis associated with caspase 3/7. Caspase-dependent release of free D-cysteine from the caspase 3/7 peptide substrate Asp-Glu-Val-Asp-D-Cys (DEVD-(D-Cys)) allowed selective reaction with 6-amino-2-cyanobenzothiazole (NH(2)-CBT) in vivo to form 6-amino-D-luciferin with subsequent light emission from luciferase. Importantly, this strategy was found to be superior to the commercially available DEVD-aminoluciferin substrate for imaging of caspase 3/7 activity. Moreover, the split luciferin approach enables the modular construction of bioluminogenic sensors, where either or both reaction partners could be caged to report on multiple biological events. Lastly, the luciferin ligation reaction is 3 orders of magnitude faster than Staudinger ligation, suggesting further applications for both bioluminescence and specific molecular targeting in vivo.

Identification and synthesis of substituted pyrrolo[2,3-d]pyrimidines as novel firefly luciferase inhibitors

A novel firefly luciferase inhibitor (3a) with a pyrrolo[2,3-d]pyrimidine core was identified in a cell-based NF-κB luciferase reporter gene assay. It potently inhibited the firefly luciferase derived from Photinus pyralis with an IC(50) value of 0.36 ± 0.05 μM. Kinetic analysis of 3a inhibition showed that it is predominantly competitive with respect to D-luciferin and uncompetitive with respect to ATP. Therefore, several pyrrolo[2,3-d]pyrimidine analogues were prepared to further investigate the structure-activity relationship (SAR) for luciferase inhibition. The most potent inhibitor of this series was 4c, which showed an IC(50) value of 0.06 ± 0.01 μM. In addition, molecular docking studies suggested that both 3a and 4c could be accommodated in the D-luciferin binding pocket, which is expected for a predominantly competitive inhibitor with respect to D-luciferin.

Expedient synthesis of electronically modified luciferins for bioluminescence imaging

Bioluminescence imaging with luciferase enzymes requires access to light-emitting, small-molecule luciferins. Here, we describe a rapid method to synthesize d-luciferin, the substrate for firefly luciferase (Fluc), along with a novel set of electronically modified analogues. Our procedure utilizes a relatively rare, but synthetically useful dithiazolium reagent to generate heteroaromatic scaffolds in a divergent fashion. Two of the luciferin analogues produced with this approach emit light with Fluc in vitro and in live cells. Collectively, our work increases the number of substrates that can be used for bioluminescence imaging and provides a general strategy for synthesizing new collections of luciferins.