The role of protein globule in firefly luciferase catalysis

The important role of the dynamic structure of firefly luciferase in enzyme functioning is a subject of this literature review. Due to the domain alternation, the optimal configuration of the active site is created for each stage of the luciferin oxidation. The diversity of bioluminescence spectra is explained by the combined emission of several coexisting forms of electronically excited oxyluciferin. The superposition of two or three emitter forms recorded in the bioluminescence spectra indicates that different luciferase conformers coexist in the reaction medium in dynamic equilibrium. The relationship between the thermal stability of the protein globule and the bioluminescence spectra is also discussed.

Luciola mingrelica firefly luciferase as a marker in bioluminescent immunoassays

Chemical modification of the enzymes with biospecific macromolecules is used in various fields of biotechnology to impart new functions or improve their properties and is a fast and convenient way to get the final products. The preparation of highly active, stable, and functionally active conjugates of the thermostable luciferase through the NH2-groups or free SH-groups of the enzyme with target molecules of different molecular weight (albumin, avidin from chicken eggs, antibodies, and progesterone) is described. The obtained conjugates were successfully tested as a reporter in bioluminescent immunoassay for the detection of the molecules and pathogens. Thus, the luc-albumin (Luc-Alb) and luc-insulin (Luc-Ins) conjugates were used in competitive ELISA for the detection of an analyte (albumin or insulin) in the samples. Luc-progesterone (Luc-Pg) was used in the rapid homogeneous immunoassay of progesterone by the BRET technique with the detection limit of 0.5 ng/ml. Luciferase conjugates with avidin (Luc-Avi) and secondary and primary antibodies (Luc-RAM and Luc-Sal) were used for enzyme immunoassay detection of Salmonella paratyphi A cells with the cell detection limit of 5 × 104 CFU/ml. To reduce the detection limit of Salmonella cells, we developed a pseudo-homogeneous bioluminescent enzyme immunoassay of cells using a new matrix for the analyte capture-polystyrene microparticles coated with Pluronic F108, covalently labeled with Sal antibodies. This allowed to achieve efficient trapping of cells from solution, significantly reduced nonspecific sorption and decreased the cell detection limit to 2.7 × 103 CFU/ml without prior concentration of the sample. The methodology that was developed in this study can be applied for the development of novel bioanalytical systems based on firefly luciferases.

Application of Bioluminescent Methods to Study the Effect of the Membrane-active Antibiotic Colistin on Bacterial Cells

For the first time, recombinant Escherichia coli cells expressing thermostable Luciola mingrelica firefly luciferase were used to study the effect of the membrane-active antibiotic colistin on live cells. Simple, fast and highly sensitive bioluminescent methods were developed for measurement of luciferase activity and ATP concentration inside and outside E. coli cells incubated in a nutrient medium, or in saline. Luciferase proved to be an informative protein marker for detecting the irreversible changes in cell membrane permeability. The study of kinetics of intra- and extracellular ATP concentration at different concentrations of colistin showed that the rate of decrease in intracellular ATP concentration significantly exceeded the rate of accumulation of extracellular ATP concentration. This fact could not be explained only by the release of ATP from the cell with an increase in the permeability of the outer cell membrane under the action of colistin. The loss of a significant part of intracellular ATP in presence of the colistin is probably due to a decrease in the activity of the respiratory chain enzymes and ATP synthase which operate in the cytoplasmic cell membrane, which leads to a decrease in the rate of ATP synthesis or even to its halt.

Mechanisms of increased mitochondria-dependent necrosis in Wiskott-Aldrich syndrome platelets

Wiskott-Aldrich syndrome (WAS) is associated with thrombocytopenia of unclear origin. We investigated real-time cytosolic calcium dynamics, mitochondrial membrane potential and phoszphatidylserine (PS) exposure in single fibrinogen-bound platelets using confocal microscopy. The WAS platelets had higher resting calcium levels, more frequent spikes, and their mitochondria more frequently lost membrane potential followed by PS exposure (in 22.9% of platelets vs. 3.9% in controls; P<0.001) after the collapse of the last mitochondria. This phenomenon was inhibited by the mitochondrial permeability transition pore inhibitor cyclosporine A, as well by xestospongin C and lack of extracellular calcium. Thapsigargin by itself caused accelerated cell death in the WAS platelets. The number of mitochondria was predictive of PS exposure: 33% of platelets from WAS patients with fewer than five mitochondria exposed PS, while only 12% did among those that had five or more mitochondria. Interestingly, healthy donor platelets with fewer mitochondria also more readily became procoagulant upon PAR1/PAR4 stimulation. Collapse of single mitochondria led to greater cytosolic calcium increase in WAS platelets if they had one to three mitochondria compared with platelets containing higher numbers. A computer systems biology model of platelet calcium homeostasis showed that smaller platelets with fewer mitochondria could have impaired calcium homeostasis because of higher surface-to-volume ratio and greater metabolic load, respectively. There was a correlation (C=0.81, P<0.02) between the mean platelet size and platelet count in the WAS patients. We conclude that WAS platelets readily expose PS via a mitochondria-dependent necrotic mechanism caused by their smaller size, which could contribute to the development of thrombocytopenia.

A Novel Streptavidin-luciferase Fusion Protein: Preparation, Properties and Application in Hybridization Analysis of DNA

A streptavidin-luciferase fusion protein comprising the thermostable mutant form of firefly luciferase Luciola mingrelica and minimal core streptavidin was constructed. The streptavidin-luciferase fusion was mainly produced in a tetrameric form with high luciferase and biotin-binding activities. It was shown that fusion has the same K_m values for ATP and luciferin and the bioluminescence spectra as initial luciferase. The linear dependence of the bioluminescence signal on the content of the fusion was observed within the range of 10^-18 -10^-13 mol per well. Successful application of obtained fusion in a biospecific bioluminescence assay based on biotin-streptavidin interactions was demonstrated by the example of a specific DNA hybridization analysis. A DNA hybridization analysis for Escherichia coli cells identification was developed using unique for these cells gadB fragment encoding glutamate decarboxylase. The amplified biotinylated GadB fragments were hybridized with the immobilized oligonucleotide probes; then, the biotin in the DNA duplexes was detected using the streptavidin-luciferase fusion protein. To reach the high sensitivity of the assay, we optimized the conditions of the assay. It was shown that the use of Pluronic for plate modification resulted in a significant reduction in the DNA detection limit which finally was 0.4 ng per well.

Firefly Luciferase-based Fusion Proteins and their Applications in Bioanalysis

Firefly luciferase is widely used in molecular biology and bioanalytical systems as a reporter molecule due to the high quantum yield of the bioluminescence, availability of stable mutant forms of the enzyme with prescribed spectral characteristics and abundance of bacterial expression systems suitable for production of recombinant proteins in limitless quantities. In this review, we described fusion proteins of luciferase with biotin-binding domain and streptavidin, with proteins A and G, antibodies, with DNA- and RNA-binding proteins, as well as fusion proteins designed for BRET systems. The firefly luciferase-based fusion proteins are represented as an effective tool for the development of different bioanalytical systems such as (1) systems in which luciferase is attached to the surface of the target and the bioluminescence signal is detected from the specific complexes formed; (2) BRET-based systems, in which the specific interaction induces changes in the bioluminescence spectrum; and (3) systems that use modified or split luciferases, in which the luciferase activity changes under the action of the analyte. All these systems have wide application in biochemical analysis of physiologically important compounds, for the detection of pathogenic bacteria and viruses, for evaluation of protein-protein interactions, assaying of metabolites involved in cell communication and cell signaling.

A simplified ATP method for the rapid control of cell viability in a freeze-dried BCG vaccine

We propose a simple and cost-effective ATP method for controlling the specific activity of a freeze-dried BCG vaccine. A freeze-dried BCG vaccine is reconstituted with 1ml saline and incubated for 15min at room temperature and then for 1h at 37°C. The vaccine is then treated with apyrase to remove extracellular ATP. After that, the cells are lysed with DMSO and the ATP content in the lysate is measured by the bioluminescence method. To implement the method, we developed a kit that requires no time-consuming preparation before the analysis. We demonstrated the linear relationship between the experimental values of the specific activity (10⁶CFU/mg) and intracellular ATP content (ATP, pmol/mg) for different batches of the studied BCG vaccines; the proportionality coefficient was К=0.36±0.02. We proposed a formula for calculating the specific activity from the measured content of intracellular ATP (ATP, pmol/mg). The comparison of the measured and calculated values of the specific activity (10⁶CFU/mg) shows that these values are similar; their differences fall within the allowable range of deviations for the specific activity values of the BCG vaccine.

Bioluminescence assay for cell viability

Theoretical aspects of the adenosine triphosphate bioluminescence assay based on the use of the firefly luciferin-luciferase system are considered, as well as its application for assessing cell viability in microbiology, sanitation, medicine, and ecology. Various approaches for the analysis of individual or mixed cultures of microorganisms are presented, and capabilities of the method for investigation of biological processes in live cells including necrosis, apoptosis, as well as for investigation of the dynamics of metabolism are described.

Strategy of mutual compensation of green and red mutants of firefly luciferase identifies a mutation of the highly conservative residue E457 with a strong red shift of bioluminescence

Bioluminescence spectra of firefly luciferases demonstrate highly pH-sensitive spectra changing the color from green to red light when pH is lowered from alkaline to acidic. This reflects a change of ratio of the green and red emitters in the bimodal spectra of bioluminescence. We show that the mutations strongly stabilizing green (Y35N) or red (H433Y) emission compensate each other leading to the WT color of firefly luciferase. We further used this compensating ability of Y35N to search for strong red-shifting mutations in the C-domain of firefly luciferase by random mutagenesis. The discovered mutation E457K substantially increased the contribution of the red emitter and caused a 12 nm red shift of the green emitter as well. E457 is highly conservative not only in beetle luciferases but also in a whole ANL superfamily of adenylating enzymes and forms a conservative structural hydrogen bond with V471. Our results suggest that the removal of this hydrogen bond only mildly affects luciferase properties and that most of the effect of E457K is caused by the introduction of positive charge. E457 forms a salt bridge with R534 in most ANL enzymes including pH-insensitive luciferases which is absent in pH-sensitive firefly luciferases. The mutant A534R shows that this salt bridge is not important for pH-sensitivity but considerably improves in vivo thermostability. Although E457 is located far from the oxyluciferin-binding site, the properties of the mutant E457K suggest that it affects color by influencing the AMP binding.

Point mutations in firefly luciferase C-domain demonstrate its significance in green color of bioluminescence

Firefly luciferase is a two-domain enzyme that catalyzes the bioluminescent reaction of firefly luciferin oxidation. Color of the emitted light depends on the structure of the enzyme, yet the exact color-tuning mechanism remains unknown by now, and the role of the C-domain in it is rarely discussed, because a very few color-shifting mutations in the C-domain were described. Recently we reported a strong red-shifting mutation E457K in the C-domain; the bioluminescence spectra of this enzyme were independent of temperature or pH. In the present study we investigated the role of the residue E457 in the enzyme using the Luciola mingrelica luciferase with a thermostabilized N-domain as a parent enzyme for site-directed mutagenesis. We obtained a set of mutants and studied their catalytic properties, thermal stability and bioluminescence spectra. Experimental spectra were represented as a sum of two components (bioluminescence spectra of putative "red" and "green" emitters); λmax of these components were constant for all the mutants, but the ratio of these emitters was defined by temperature and mutations in the C-domain. We suggest that each emitter is stabilized by a specific conformation of the active site; thus, enzymes with two forms of the active site coexist in the reactive media. The rigid structure of the C-domain is crucial for maintaining the conformation corresponding to the "green" emitter. We presume that the emitters are the keto- and enol forms of oxyluciferin.

Approaches to engineer stability of beetle luciferases

Luciferase enzymes from fireflies and other beetles have many important applications in molecular biology, biotechnology, analytical chemistry and several other areas. Many novel beetle luciferases with promising properties have been reported in the recent years. However, actual and potential applications of wild-type beetle luciferases are often limited by insufficient stability or decrease in activity of the enzyme at the conditions of a particular assay. Various examples of genetic engineering of the enhanced beetle luciferases have been reported that successfully solve or alleviate many of these limitations. This mini-review summarizes the recent advances in development of mutant luciferases with improved stability and activity characteristics. It discusses the common limitations of wild-type luciferases in different applications and presents the efficient approaches that can be used to address these problems.

Thermostabilization of firefly luciferase by in vivo directed evolution

Firefly luciferase is widely used in a number of areas of biotechnology and molecular biology. However, rapid inactivation of wild-type (WT) luciferases at elevated temperatures often hampers their application. A simple non-lethal in vivo screening scheme was used to identify thermostable mutants of luciferase in Escherichia coli colonies. This scheme allowed carrying out each cycle of mutagenesis in a rapid and efficient manner. Four rounds of directed evolution were conducted on a part of the gene coding for amino acid residues 130-390 of Luciola mingrelica luciferase. The resultant mutant designated 4TS had a half-life of 10 h at 42°C, which is 65-fold higher compared with the WT luciferase. Moreover, the mutant 4TS showed a 1.9-fold increase in specific activity, 5.7-fold reduction of K(m) for ATP and a higher-temperature optimum compared with the WT enzyme. 4TS contains eight mutations, four of which are suggested to be mainly responsible for the enhancement of thermostability: R211L, A217V, E356K and S364C. Thus, directed evolution with non-lethal colony screening for in vivo bioluminescence activity proved to be an effective and efficient approach for increasing thermal stability of luciferase while retaining high catalytic activity.

[Rapid bioluminescent antibiotic susceptibility assay]

Rapid testing of pathogen susceptibility to antibiotics is of great practical value for rational chemotherapy of pyoinflammatory deseases and postoperative complications of microbial etiology. The standard microbiological methods, i.e., the disk diffusion method and the method of serial dilutions are labour- and time-consuming (not less than 18-36 hours). The method of the authors is based on measuring bioluminescence resulting from interaction of adenosine-5'-triphosphate (ATP) and ATP reagent, a standard reaction mixture of firefly luciferase (an enzyme) and luciferin. The bioluminescence intensity is proportional to the ATP concentration in the reaction mixture and the ATP concentration is proportional to the number of the pathogen viable cells in the sample. The bioluminescence intensity value in the pathogen suspension aliquots with and without (control) the antibiotic were compared after the incubation for 5 hours and the coefficient of the microbial cell growth inhibition was calculated. Satisfactory correlation (R2 > 88%) of the results of the bioluminescent assay and the assay with the disk diffusion method and the method of serial dilutions was observed.

Quenching of the fluorescence of Tyr and Trp residues of firefly luciferase from Luciola mingrelica by the substrates

Luciferase of the firefly Luciola mingrelica is characterized by fluorescence of not only the unique Trp residue (lambda(em) = 340 nm), but also that of Tyr residues (lambda(em) = 308 nm). Quenching of the intrinsic fluorescence of the luciferase by its substrates luciferin and ATP (AMP) has been studied. Luciferin (LH2) quenches Trp fluorescence more efficiently than the fluorescence of Tyr residues. Two centers of quenching of Tyr fluorescence by ATP have been found corresponding apparently to the allosteric and active sites of the luciferase with K(s(ATP)) = 20 and 110 microM, respectively. The influence of one substrate on the affinity of luciferase to the second was investigated using fluorescence. ATP (AMP) binding to the allosteric sites of the luciferase significantly affects the affinity of luciferase to LH2. Formation of the complex between the luciferase and LH2 affects the affinity of both allosteric and active sites of the luciferase to ATP (AMP). The observed effects are probably connected with conformational changes in the luciferase molecule upon its interaction with the substrates.

Bioluminescence spectra of native and mutant firefly luciferases as a function of pH

Bioluminescence spectra of the wild-type recombinant Luciola mingrelica firefly luciferase and its mutant form with the His433Tyr point mutation were obtained within the pH 5.6-10.2 interval. The spectra are shown to be a superposition of the spectra of the three forms of the electronically excited reaction product oxyluciferin: ketone (lambdamax = 618 nm), enol (lambdamax = 587 nm), and enolate-ion (lambdamax = 556 nm). The shift in lambdamax by 40 nm to the red region in the mutant luciferase bioluminescence at the pH optimum of enzyme activity (pH 7.8) is explained by the change in the relative content of different oxyluciferin forms due to the shift in the ketone <--> enol <--> enolate equilibria. A computer model of the luciferase-oxyluciferin-AMP complex was constructed and the structure of amino acid residues participating in the equilibrium is proposed. Computer models of the protein region near the His433 residue for the wild type and mutant luciferases are also proposed. Comparison of the models shows that the His433Tyr mutation increases flexibility of the polypeptide loop that binds the N and C domains of luciferase. As a result, the flexibility of the C domain amino acid residues in the emitter microenvironment increases, and this increase may be the reason for the observed differences in the bioluminescence spectra of the native and mutant luciferases.

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.

Dimethyl-and monomethyloxyluciferins as analogs of the product of the bioluminescence reaction catalyzed by firefly luciferase

The absorption and fluorescence spectra of dimethyloxyluciferin (DMOL) and monomethyloxyluciferin (MMOL) were studied at pH 3.0-12.0. In the range of pH 3.0-8.0, the fluorescence spectrum of DMOL exhibits a maximum at lambda(em) = 639 nm. At higher pH values an additional emission maximum appears at lambda(em) = 500 nm (wavelength of excitation maximum lambda(ex) = 350 nm), which intensity increases with time. It is shown that this peak corresponds to the product of DMOL decomposition at pH > 8.0. The absorption spectra of MMOL were studied in the range of pH 6.0-9.0. At pH 8.0-9.0, the absorption spectrum of MMOL exhibits one peak at lambda(abs) = 440 nm. At pH 7.3-7.7, an additional band appears with maximum at lambda(abs) = 390 nm. At pH 6.0-7.0 two maxima are observed, at lambda(abs) = 375 and 440 nm. The fluorescence spectra of MMOL (pH 6.0-9.7, lambda(ex) = 440 or 375 nm) exhibit one maximum. It is shown that decomposition of DMOL and MMOL in aqueous solutions results in products of similar structure. DMOL and MMOL are rather stable at the pH optimum of luciferase. It is suggested that they can be used as fluorescent markers for investigation of the active site of the enzyme.

Assessment of photodynamic destruction of Escherichia coli O157:H7 and Listeria monocytogenes by using ATP bioluminescence

Antimicrobial photodynamic therapy was shown to be effective against a wide range of bacterial cells, as well as for fungi, yeasts, and viruses. It was shown previously that photodestruction of yeast cells treated with photosensitizers resulted in cell destruction and leakage of ATP. Three photosensitizers were used in this study: tetra(N-methyl-4-pyridyl)porphine tetratosylate salt (TMPyP), toluidine blue O (TBO), and methylene blue trihydrate (MB). A microdilution method was used to determine MICs of the photosensitizers against both Escherichia coli O157:H7 and Listeria monocytogenes. To evaluate the effects of photodestruction on E. coli and L. monocytogenes cells, a bioluminescence method for detection of ATP leakage and a colony-forming assay were used. All tested photosensitizers were effective for photodynamic destruction of both bacteria. The effectiveness of photosensitizers (in microgram-per-milliliter equivalents) decreased in the order TBO > MB > TMPyP for both organisms. The MICs were two- to fourfold higher for E. coli O157:H7 than for L. monocytogenes. The primary effects of all of the photosensitizers tested on live bacterial cells were a decrease in intracellular ATP and an increase in extracellular ATP, accompanied by elimination of viable cells from the sample. The time courses of photodestruction and intracellular ATP leakage were different for E. coli and L. monocytogenes. These results show that bioluminescent ATP-metry can be used for investigation of the first stages of bacterial photodestruction.

Protein structure and bioluminescent spectra for firefly bioluminescence

Modern theory on general and specific effects of microenvironment on emission spectra was used for explanation of spectral differences for both natural and mutant forms of beetle luciferases, as well as for bioluminescence emitter oxyluciferin in model systems. For the analysis, both authors' and other published data were used. It was shown that active site mutations that resulted in spectral shifts of bioluminescence as a rule caused substantial decrease in the catalytic activity of the enzyme. At the same time, mutations in the conservative regions of the protein amino acid sequence that were in the periphery of the protein globe resulted in red shift of the bioluminescence spectra without affecting catalytic activity. Correlation was observed between the value of spectral shift and polarizability of the introduced amino acid residue: the higher the polarizability, the larger was the red shift of bioluminescence.

Quenching of tryptophan fluorescence of firefly luciferase by substrates

The interaction of firefly luciferase with substrates (luciferin and MgATP) by steady-state and time-resolved fluorescence is studied. The efficient quenching of tryptophan fluorescence of the active enzyme takes place upon its binding with substrates. In the presence of ATP the quenching is of dynamic type and is caused by structural changes in the protein molecule upon ATP binding. A model is proposed in which the complex has smaller fluorescence quantum yield than the free enzyme because of partial quenching of tryptophan fluorescence by the new microenvironment. Quenching of tryptophan fluorescence by luciferin due to the efficient energy transfer from tryptophan to luciferin is discussed. The calculated distance between Trp-419 and luciferin for the L. mingrelica luciferase in the enzyme-substrate complex is less than 12 A.

Fluorescent properties of firefly luciferases and their complexes with luciferin

Fluorescence of luciferases from Luciola mingrelica (single tryptophan residue, Trp-419) and Photinus pyralis (two tryptophan residues, Trp-417, Trp-426) was studied. Analysis of quenching of tryptophan fluorescence showed that the tryptophan residue conserved in all luciferases is not accessible for charged quenchers, which is explained by the presence of positively and negatively charged amino acid residues in the close vicinity to it. An effective energy transfer from tryptophan to luciferin was observed during quenching of tryptophan fluorescence of both luciferases with luciferin. From the data on the energy transfer, the distance between the luciferin molecule and Trp-417 (419) in the luciferin luciferase complex was calculated: 11-15 A for P. pyralis and 12-17 A for L. mingrelica luciferases. The role of the conserved Trp residue in the catalysis is discussed.

[Inhibition of horseradish peroxidase by N-ethylamide of o-sulfobenzoylacetic acid]

The carboxylic groups of horseradish peroxidase were modified by 1-cyclohexyl-3-(2-morpholinoethyl)carbodiimide metho-p-toluenesulfonate by the Koshland method. The catalytic properties of the native and modified peroxidase were studied in the presence of N-ethylamide of o-sulfobenzoylacetic acid (EASBA) at pH 5.0-7.5. In the oxidation of o-dianisidine, EASBA is a competitive inhibitor of the carbidiimide-modified peroxidase, and it increases both K(m) and Vm in the case of the native enzyme. These data show that at least one of the carboxylic groups modified with carbodiimide is located at the area of the peroxidase active site.

Transient increase of tryptophan fluorescence of enzyme caused by photoexcitation of ligand in luciferase-luciferin complex

An experiment was proposed and accomplished that was based on the hypothesis of the dissociation of the luciferase-luciferin complex in photoexcitation. A pump-probe experiment was performed with the use of picosecond laser pulses and was based on the effect of quenching of enzyme tryptophan fluorescence caused by luciferin binding. A photoinduced increase of the tryptophan fluorescence intensity was detected. Experimental results were interpreted on the basis of the assumptions on photoinduced dissociation of the luciferin-luciferase complex and Forster energy transfer from tryptophan to luciferin. Under the assumption on the photoinduced dissociation and stationary quenching of tryptophan fluorescence the rate of propagation of the conformational changes in the protein caused by the complex dissociation was estimated to be >20 m/s.

Quantitative assessment of bacterial contamination of raw milk using bioluminescence

Enumeration of bacteria in raw milk is of public health and economic importance. Among the proposed rapid methods for assessment of bacterial contamination in raw milk, ATP bioluminescence has proved to be one of the most promising (Griffiths, 1991). Several companies produce ATP bioluminescence reagent kits and equipment for analysing raw milk samples for total bacterial count (Sutherland et al. 1994; Reybroeck & Schram, 1995). The principle of ATP bioluminescent bacterial assay is based on the following assumptions (Olsen, 1991). All living organisms contain ATP, ATP is neither associated with dead cells nor absorbed on to surfaces, colloids and so on, and there is a fairly constant ratio of ATP to biomass/number of cells for all microbial taxa independent of metabolic activity or environmental conditions. Of these assumptions, only the first seems to be indisputable. It is not the number of bacterial cells, but rather the colony forming unit (cfu) that is the denomination used when assessing the microbial quality of milk. For Gram-negative rods of the genus Enterobacteriaceae, a cfu is usually derived from a single cell. However, Gram-positive cocci (staphylococci and streptococci) grow in bunches and chains respectively (Gregg, 1991), and estimation of cell numbers may not give good agreement with the colony counts.

Several approaches have been investigated to increase the sensitivity of the bioluminescent method (Pahuski et al. 1991; Sutherland et al. 1994; Reybroeck & Schram, 1995; Froundjian et al. 1999). Although the detection limit achieved by these modifications (104 cfu/ml) may be sufficient for practical use (Bautista et al. 1992; Reybroeck & Schram 1995), the accuracy of the analysis was not significantly improved. The reported values for accuracy of the estimate for cfu/ml in raw milk (Syx) by the bioluminescent method were in the range 0·27–0·87 log units (Bautista et al. 1992; Reybroeck & Schram, 1995). The purpose of the present study was to determine the reasons for the lack of accuracy of the bacterial ATP assay in raw milk.

Fluorescence of tryptophan residues in firefly luciferases and enzyme--substrate complexes

Quenching of tryptophan fluorescence of Luciola mingrelica (single tryptophan residue, Trp-419) and Photinus pyralis (two tryptophan residues, Trp-417 and Trp-426) luciferases with different quenchers (I-, Cs+, acrylamide) was studied. The conserved Trp-417(419) residue was shown to be not accessible to charged particles, and positively and negatively charged amino acid residues are located in close vicinity to it. We found previously unreported effective energy transfer from this tryptophan to luciferin during the quenching of the tryptophan fluorescence. The distance between the luciferin molecule and Trp-417(419) was calculated: 11-15 and 12-17 A for P. pyralis and L. mingrelica luciferases, respectively. The role of the conserved Trp residue in the catalysis is discussed. ATP and AMP are also quenchers of the tryptophan fluorescence of the luciferases. In this case, an allosteric mechanism of the interaction of Trp-417(419) with an excess of ATP (AMP) is proposed.

Model of the active site of firefly luciferase

A model for the spatial structure of firefly luciferase--ATP--luciferin complex is suggested using the coordinates of unliganded luciferase and the enzyme--substrate complex of the adenylating subunit of gramicidin S synthetase known from the literature. Conformational changes in luciferase can occur during substrate binding resulting in a relative orientation of two luciferase domains similar to that in case of the AMP--phenylalanine--synthetase complex. The model is consistent with data on the physicochemical properties of firefly luciferase and its complexes with the substrates.

[Bioluminescent assay of total bacterial contamination of raw milk]

A rapid (30-35 min) bioluminescence assay of total bacterial contamination (TBC) of raw milk was optimized. This method includes incubation of milk samples in the presence of Neonol-10 and medical purity grade pancreatin with further removal of nonbacterial ATP by filtration through a membrane filter, cell disruption by treatment with dimethyl sulfoxide, and measurement of ATP concentration in a reaction with the bioluminescent reagent Immolum. The TBC detection threshold is 0.5 x 10(5) colony-forming units (CFU) per ml milk. Coefficients of correlation between the standard plate count method and bioluminescence assay (R) and residual standard deviations (Sxy) in raw milk samples (n = 140) were 0.83 and 0.54, respectively. In sterilized milk samples artificially contaminated with pure cultures of the main representatives of milk microflora (coli-forms, Staphylococcus aureus, Streptococcus thermophilus, and Streptococcus group D), these values were 0.89-0.99 and 0.09-0.29, respectively. The specific content of ATP was found to be (0.8 +/- 0.1) x 10(-18) mol/CFU in coli-forms; (12.0 +/- 8.1) x 10(-18) mol/CFU in S. aureus; (35.2 +/- 16.9) x 10(-18) mol/CFU in S. thermophilus; and (42.5 +/- 1.3) x 10(-18) in Streptococcus group D.

Immobilization of recombinant firefly luciferase. Physicochemical properties and application

Immobilization of the recombinant Luciola mingrelica and Photinus pyralis firefly luciferases on BrCN-activated Sepharose was investigated. The catalytic properties and analytical characteristics of the immobilized recombinant and native luciferases were comparatively studied. The catalytic properties of the immobilized recombinant L. mingrelica luciferase are close to those of the native luciferase, but the former enzyme appeared to be significantly more stable. The immobilized recombinant luciferases can be used for ATP assay within the 0.01-10000 nM range.

Bioluminescence of free and poly(vinyl alcohol) cryogel-entrapped recombinant Escherichia coli cells expressing firefly luciferase

Bioluminescence of free and poly(vinyl alcohol) cryogel-entrapped recombinant E. coli cells expressing firefly luciferase was investigated. It was shown that bioluminescence intensity and time-course of the bioluminescent signal changed upon immobilization and depended on intracellular ATP concentration and permeability of the cell membrane.

[A bioluminescent method of determining antibiotic sensitivity of microbial cells in septic blood]

A bioluminescence assay for rapid (5-7 h) determination of susceptibility to antibiotics was applied to samples of septic blood and optimized. The method comprises hemolysis of blood, reduction of osmolarity by adding concentrated nutritive media, and further incubation of the samples in the presence or absence of therapeutic doses of the antibiotic examined. Growth of bacteria is estimated by the level of bacterial ATP in the sample, which is determined by a bioluminescence assay. Hemolysis and further incubation of samples in nutrition media reduced the concentration of nonbacterial ATP to a level that did not interfere with the determination of bacterial ATP. There was a positive correlation between the levels of resistance to antibiotics determined by the bioluminescence assay and standard plate counts.

[Comparative evaluation of methods of intracellular ATP extraction from different types of microorganisms for bioluminescent determinationof microbial cells]

The efficiency of dimethyl sulfoxide (DMSO), trichloroacetic acid (TCA), and cetyltrimethylammonium bromide (CTAB) as extractants of intracellular ATP from various microorganisms was compared in bioluminescent measurements of microbial cell concentrations. Extraction with CTAB was found to provide an approximately ten times higher sensitivity of the bioluninescent assay of microbial cells than extraction with DMSO or TCA. In Gram-positive bacteria and yeasts, the ATP concentration in the extract was a linear function of the microbial suspension density only within a cell concentration range of D600 0.02-3.5 in the three types of tested extracts. In Gram-negative bacteria, a significant deviation from the linear dependence between ATP concentration and microbial suspension density was observed in CTAB extracts at D600 > 1.

[The ATP content of the neutrophils and whole blood in the inhabitants of regions in the Altai Territory subjected to nuclear testing exposure]

The bioluminescent method was used in the studies of the influence of ionizing irradiation and/or xenobiotics on the content of ATP in RBC and neutrophils of rats, and in whole blood and neutrophils of 80 examined women of Altai Region exposed to ionizing radiation during a series of nuclear tests in Semipalatinsk in 1949-1965. Deviations from the normal ATP content were measured with due to account of the natural variability of a given metabolite. For rats, deviations from the content of ATP in erythrocytes were short-term, those in neutrophils were long-term. For people, a statistically significant increase in the content of ATP in neutrophils as compared to the control was observed. A non-linear correlation between the content of ATP in neutrophils and the calculated dose of radiation was observed. An increase in the ATP content in whole blood, with regard to the control, was not statistically significant for all groups of examined persons.

[Conservative motifs in enzyme superfamily catalyzing formation of acyladenylates and compounds with carboxy group]

The search of conserved motifs was performed in enzymes catalyzing acyladenylate formation using ATP as AMP-donor. Besides a known motif, we have found a second conserved motif. Screening the SWISS-PROT database for occurrence of the motifs have showed that both motifs are highly characteristic and occur in all proteins of this superfamily. The motifs are separated by 200-250 residues in all sequences. It may suggest that the both motifs belong to the structural unit involved in acyl adenylate formation.

[Measurement of ATP in intact Escherichia coli cells, containing recombinant firefly luciferase]

Recombinant firefly luciferase was expressed in E. coli and its properties inside the intact cells were studied. At low concentrations, antibiotic polymyxin B increases permeability of E. coli cell membrane and concentrations of luciferase substrates inside the cells can thus be varied. Effect of intracellular ATP concentrations on intensity of bioluminescence was studied. Recombinant cells expressing the firefly luciferase gene can be used for investigation of substances which influence synthesis and hydrolysis of ATP inside the cells and cell membrane transport.

[Participation of DnaK chaperone and ATP in in vivo folding of firefly luciferase, synthesized by E. coli cells]

The time-courses of protein accumulation and luciferase activity of Luciola mingrelica firefly luciferase synthesized on plasmid pJG lambda in E. coli strains omega 238 and B178groE7 (DnaK- and GroEL-deficient, correspondingly) and omega 237 and W3110 that are control to them, have been studied. It was shown that the amounts of the luciferase protein synthesized by all strains was approximately equal to 3.0-3.9% of the total cell protein. Luciferase was synthesized in a catalytically inactive form and transformed into an active enzyme during incubation at 21 degrees C. In the DnaK-deficient E. coli strain omega 238, the enzyme transformation into a catalytically active form did not occur which provides evidence for participation of the DnaK chaperone in transformation of newly synthesized luciferase into catalytically active form. Luciferase folding was accelerated with an increase in ATP concentration inside the cell. It is possible to increase the ATP concentration inside the cell by treatment with polymyxin and addition of ATP to the culture medium, which significantly accelerates the luciferase folding.

[Physicochemical properties of recombinant luciferase from the firefly Luciola mingrelica and its mutant forms]

Physico-chemical properties of the recombinant L. mingrelica luciferase synthesized by E. coli cells have been studied. The catalytic and spectral properties of recombinant luciferase were similar to those of the native enzyme but the former was less stable in the presence of the additional Cys residue. The mutant forms of L. mingrelica firefly luciferase with point mutations Cys-82-->Ala, Cys-260-->Ala, Cys-393-->Ala and Thr-204-->Asp, have been constructed using the method of site-specific mutagenesis. Mutations Cys-82,260,393-->Ala changed slightly the Km values for ATP and luciferin but did not influence kcat. The Cys-393-->Ala mutant appeared to be more stable in comparison with the native enzyme. Mutation Thr-204-->Asp resulted in a 8-fold increase in the ATP binding constant and in a 2-fold increase in the kcat, thus indicating that Thr-204 may be located in the ATP-binding region of luciferase. Dithiothreitol, ethylene glycol, bovine serum albumin and trehalose had a stabilizing effect on the native, recombinant and mutant luciferases.

Bioluminescent assay for human lymphocyte blast transformation

One of the basic tests of in vitro evaluation of immune cell functional activity is a proliferative response of lymphocytes on the action of external stimuli such as mitogenic lectines, antigens, etc. We compared two methods used to assess the lymphocyte functional status. (1) [3H]thymidine incorporation and (2) bioluminescence for determination of intracellular ATP in blast cells. Comparison has been done for healthy donors and patients with proven low immunological status. The proposed bioluminescent method for evaluation of the proliferative response was shown to be sensitive enough for diagnostic purposes. This method allows one to process a large number of samples at the same time and correlates highly with the radionuclide test use hazardous radioactive materials.

Bioluminescent assay of bacterial intracellular AMP, ADP, and ATP with the use of a coimmobilized three-enzyme reagent (adenylate kinase, pyruvate kinase, and firefly luciferase)

A three-enzyme coimmobilized system (firefly luciferase, pyruvate kinase, and adenylate kinase) was constructed for the bioluminescent assay of ATP, ADP, and AMP in bacterial cell extracts. Data for the reproducibility and sensitivity of the proposed method are presented. Detection limits were 1.5 pmol of ADP and 15 pmol of AMP in the sample. With this system, changes in adenine nucleotide concentrations in bacterial cells were measured during the actions exerted by external chemical and physical sources, such as additives to nutrient media and low-power He-Ne laser irradiation.

[Kinetics of luciferase gene expression from fireflies Photinus pyralis and Luciola mingrelica in Escherichia coli cells. I. Kinetics of the transformation of recombinant luciferase into enzymatically-active conformers]

The Photinus pyralis and Luciola mingrelica luciferases genes expression has been studied on the pME61 or pJG lambda plasmids with a thermoinducible lambda Pr promoter in the E. coli strain CA using three independent methods: SDS gel electrophoresis to quantify the synthesized luciferase protein, EIA to quantify the enzyme native conformer and activity measurements. The cultures were incubated by the temperature schemes 28 degrees-42 degrees-21 degrees C and 28 degrees-21 degrees C. The maximal amount of the Ph. pyralis protein reached 4.5%, while that of L. mingrelica luciferase was 4.1% of the total amount of the cellular proteins after 10-hr incubation. The amount of the native conformer and the luciferase activity began to grow after a long induction period, reaching the maximal level by hour 50-60 after the thermoinduction. The increase in the enzyme activity correlated well with the increase in the ATP content in the cell. The observed "low-temperature induction" of the enzyme activity is interpreted as a protein transition to an active conformation. This transformation is considerably behind the protein biosynthesis process. Intracellular metabolic reactions have been shown to play an active part in these conformational changes.

[Kinetics of luciferase gene expression from fireflies Photinus pyralis and Luciola mingrelica in Escherichia coli cells. II. The role of pH in the biosynthesis of proteins and their transformation into active conformers]

The Photinus pyralis and Luciola mingrelica luciferases genes expression kinetics has been studied, repectively, on the pME61 and pJG lambda plasmids with a thermoinducible lambda PR promoter in the E. coli strain CA under conditions of a pH shift. An increase in the enzyme activity after a pH shift has been revealed. The maximal amount of the firefly Ph. pyralis luciferase reaches 5.3%, while the maximal amount of the L. mingrelica enzyme reaches 4.9% of the total amount of cellular proteins in case of pHout shift to 9.0 according to the SDS gel electrophoresis data; that in case of a pHout shift to 8.5 is 4.9% and 4.5%, respectively. The enzyme activation correlates well with the dynamics of pHout and pHin.

[Bioluminescence and bioluminescent analysis: development of certain aspects of the problem over the last decade]

Recent data from gene engineering, kinetic and fluorescent studies concerning the structure and functions of firefly luciferase are reviewed. Some new trends in the development of bioluminescent methods of control over bacterial contaminations, dynamics of intracellular processes and methods of bioluminescent detection in immuno- and DNA-assays are described. Possible applications of luciferase genes as markers are considered.

Luciferase from the east European firefly Luciola mingrelica: cloning and nucleotide sequence of the cDNA, overexpression in Escherichia coli and purification of the enzyme

We have cloned cDNA encoding luciferase in Luciola mingrelica, fireflies living near the Black Sea in southern Russia, and obtained high level expression of the cloned sequences in Escherichia coli. The nucleotide sequences of two isolated clones were determined; five single base differences were observed, but none resulted in a change in the encoded amino acid residue. The cDNA encoded a protein of 548 amino acid residues. The overall amino acid sequence identity with the luciferase from Photinus pyralis, the North American firefly, was 67%, while comparison of the L. mingrelica luciferase with L. cruciata and L. lateralis, both indigenous to Japan, showed about 80% of the residues were strictly conserved. A novel overexpression system which employs the regulatory genes of the luminous bacterium Vibrio fischeri allowed growth of cultures to high cell density and high luciferase content, facilitating purification of the enzyme. Luciferase was purified to homogeneity in good yield from lysates of recombinant E. coli by ammonium sulfate fractionation and chromatography on columns of DEAE Sephadex and Blue Sepharose. The physicochemical properties of the luciferases from the available recombinant sources are significantly different and should allow detailed investigations into the mechanism of the bioluminescence reaction and the physical basis of the differences in the color of light emitted from the various enzymes.

[Change in adenine nucleotide pool in E. coli 1257 bacterial cells under the action of a low intensity He-Ne laser]

Evidence is presented of co-immobilized bioluminescent reagents which include firefly luciferase, pyruvate kinase and adenylate kinase used for an adenylate assay in bacterial cells. The changes in the ATP, ADP and AMP content induced by irradiation with a low-power He-Ne-laser depend on the laser power and irradiation dose. The data obtained suggest that laser irradiation causes the activation of adenine nucleotide synthesis de novo.

[The bioanalytical uses of the luciferase from fireflies (a review)]

Main principles of the bioluminescent microassay with the use of firefly luciferase are considered. Literature data and own experimental results of the author on application of luciferase in microbiology, clinical biochemistry, and express assays for antibiotic sensitivity and resistance to biocorrosion are generalized. New approaches to the use of luciferase for the detection of enzyme labels in EIA and for the nonradioactive detection of DNA probes are discussed.