Determination of L-phenylalanine based on an NADH-detecting biosensor

Cytotoxic activity of l-lysine alpha-oxidase against leukemia cells

Cancer cells exhibit higher proliferation rates than normal cells, and as a consequence, a higher nutritional demand for metabolites such as amino acids. Such cells demonstrate high expression of amino acid transporters and are significantly dependent on the external uptake of amino acids. Moreover, some types of cancer cells exhibit oncogenic mutations that render them auxotrophic to certain amino acids. This metabolic difference between tumor and normal cells has been explored for developing anticancer drugs. Enzymes capable of depleting certain amino acids in the bloodstream can be employed to inhibit the proliferation of cancer cells and promote cell death. Certain microbial enzymes, such as l-asparaginase and l-amino acid oxidases, have been studied for this purpose. In this paper, we discuss the role of l-asparaginase, the only enzyme currently used as a chemotherapeutic agent. We also review the studies on a new potential antineoplastic agent, l-lysine α-oxidase, an enzyme of l-amino acid oxidase family.

Structural insights into the enhanced thermostability of cysteine substitution mutants of L-histidine decarboxylase from Photobacterium phosphoreum

Enzymatic amino acid assays are important in physiological research and clinical diagnostics because abnormal amino acid concentrations in biofluids are associated with various diseases. L-histidine decarboxylase from Photobacterium phosphoreum (PpHDC) is a pyridoxal 5'-phosphate-dependent enzyme and a candidate for use in an L-histidine quantitation assay. Previous cysteine substitution experiments demonstrated that the PpHDC C57S mutant displayed improved long-term storage stability and thermostability when compared with those of the wild-type enzyme. In this study, combinational mutation experiments of single cysteine substitution mutants of PpHDC were performed, revealing that the PpHDC C57S/C101V/C282V mutant possessed the highest thermostability. The stabilizing mechanism of these mutations were elucidated by solving the structures of PpHDC C57S and C57S/C101V/C282V mutants by X-ray crystallography. In the crystal structures, two symmetry-related PpHDC molecules form a domain-swapped homodimer. The side chain of S57 is solvent exposed in the structure, indicating that the C57S mutation eliminates chemical oxidation or disulfide bond formation with a free thiol group, thereby providing greater stability. Residues 101 and 282 form hydrophobic interactions with neighboring hydrophobic residues. Mutations C101V and C282V enhanced thermostability of PpHDC by filling a cavity present in the hydrophobic core (C101V) and increasing hydrophobic interactions.

Electrochemical biosensors based on multienzyme systems: Main groups, advantages and limitations - A review

In the review, the principles and main purposes of using multienzyme systems in electrochemical biosensors are analyzed. Coupling several enzymes allows an extension of the spectrum of detectable substances, an increase in the biosensor sensitivity (in some cases, by several orders of magnitude), and an improvement of the biosensor selectivity, as showed on the examples of amperometric, potentiometric, and conductometric biosensors. The biosensors based on cascade, cyclic and competitive enzyme systems are described alongside principles of function, advantages, disadvantages and practical use for real sample analyses in various application areas (food production and quality control, clinical diagnostics, environmental monitoring). The complications and restrictions regarding the development of multienzyme biosensors are evaluated. The recommendations on the reasonability of elaboration of novel multienzyme biosensors are given.

Microfluidic screening system based on boron-doped diamond electrodes and dielectrophoretic sorting for directed evolution of NAD(P)-dependent oxidoreductases

We report the development of a micro total analysis system (μTAS) based on electrochemical measurements and dielectrophoretic sorting for screening of NAD(P)-dependent oxidoreductases. In this system, the activity of enzymes immobilized on microbeads, together with their encoding DNA, can be measured with a boron-doped diamond (BDD) electrode in each compartment (∼30 nL) of the microfluidic system. The 30 nL droplets containing microbead-displayed genes of enzymes with higher activity can then be recovered by dielectrophoretic sorting. Previously, we developed the NAD(P)H-measuring device containing the BDD electrode for high-throughput measurement of the activity of NAD(P)-dependent oxidoreductases. In this study, we fabricated an encapsulating device and a droplet-sorting device for nanoliter-size droplets, for the first time, and then combined these three devices to construct a μTAS for directed evolution of NAD(P)-dependent oxidoreductases. We confirmed that this system works by proof-of-principle experiments and successfully applied this system for screening of randomized libraries of NAD-dependent dehydrogenases.

Resolution Mechanism and Characterization of an Ammonium Chloride-Tolerant, High-Thermostable, and Salt-Tolerant Phenylalanine Dehydrogenase from Bacillus halodurans

As phenylalanine dehydrogenase (PheDH) plays an important role in the synthesis of chiral drug intermediates and detection of phenylketonuria, it is significant to obtain a PheDH with specific and high activity. Here, a PheDH gene, pdh, encoding a novel BhPheDH with 61.0% similarity to the known PheDH from Microbacterium sp., was obtained. The BhPheDH showed optimal activity at 60 °C and pH 7.0, and it showed better stability in hot environment (40-70 °C) than the PheDH from Nocardia sp. And its activity and thermostability could be significantly increased by sodium salt. After incubation for 2 h in 3 M NaCl at 60 °C, the residual activity of the BhPheDH was found to be 1.8-fold higher than that of the control group (without NaCl). The BhPheDH could tolerate high concentration of ammonium chloride and its activity could be also enhanced by the high concentration of ammonium salts. These characteristics indicate that the BhPheDH possesses better thermostability, ammonium chloride tolerance, halophilic mechanism, and high salt activation. The mechanism of thermostability and high salt tolerance of the BhPheDH was analyzed by molecular dynamics simulation. These results provide useful information about the enzyme with high-temperature activity, thermostability, halophilic mechanism, tolerance to high concentration of ammonium chloride, higher salt activation and enantio-selectivity, and the application of molecular dynamics simulation in analyzing the mechanism of these distinctive characteristics.

Construction and evaluation of a novel bifunctional phenylalanine–formate dehydrogenase fusion protein for bienzyme system with cofactor regeneration

Phenylalanine dehydrogenase (PheDH) plays an important role in enzymatic synthesis of l-phenylalanine for aspartame (sweetener) and detection of phenylketonuria (PKU), suggesting that it is important to obtain a PheDH with excellent characteristics. Gene fusion of PheDH and formate dehydrogenase (FDH) was constructed to form bifunctional multi-enzymes for bioconversion of l-phenylalanine coupled with coenzyme regeneration. Comparing with the PheDH monomer from Microbacterium sp., the bifunctional PheDH–FDH showed noteworthy stability under weakly acidic and alkaline conditions (pH 6.5–9.0). The bifunctional enzyme can produce 153.9 mM l-phenylalanine with remarkable performance of enantiomers choice by enzymatic conversion with high molecular conversion rate (99.87 %) in catalyzing phenylpyruvic acid to l-phenylalanine being 1.50-fold higher than that of the separate expression system. The results indicated the potential application of the PheDH and PheDH–FDH with coenzyme regeneration for phenylpyruvic acid analysis and l-phenylalanine biosynthesis in medical diagnosis and pharmaceutical field.

Surpassingly competitive electromagnetic field enhancement at the silica/silver interface for selective intracellular surface enhanced Raman scattering detection

A thin plasmonic nanofilm is formed by preformed silver nanoparticles (30 nm) in the matrix of poly(vinyl alcohol) adsorbed on silica microparticles (1.5 μm) (SiO2@Ag-PVA). By applying finite element method (FEM) analysis the surface enhanced Raman spectroscopy (SERS) enhancement factors (EFs) can reach 10(5) with higher values from 10(9) to 10(11) in the silver layer of 5 nm thickness. Nanoparticles in the SiO2@Ag-PVA nanofilm need at least 15 nm radius to exhibit SERS EFs greater than 10(7). High values of this enhancement at the silver/silica interface of spherical geometry can be reached faster by using a 532 nm compared to 785 nm excitation wavelength. By this approach different SERS spectral features can be distinguished between live fibroblasts with spread ("healthy" state) or round ("unhealthy" state) shapes. Characteristic features of secondary protein structures, detection of different acidic conditions and cholesterol with at least a 3-fold higher sensitivity are examined. Moreover, a greater amount of glucose (glucogen) and also tyrosine can be monitored in real time. This is important in identification of higher risk of diabetes as well as in several genetic metabolic disorders (e.g., phenylketonuria, tyrosinaemia type II and tyrosinosis).

Mediatorless N(2) incorporated diamond nanowire electrode for selective detection of NADH at stable low oxidation potential

The electrocatalytic properties of a N2 incorporated diamond nanowire (N-DNW) unmodified electrode towards the oxidation of nicotinamide adenine dinucleotide (NADH) was critically evaluated. The electrochemical behavior of the N-DNW unmodified electrode was examined and compared with that of boron-doped diamond, glassy carbon electrode, and graphite electrodes. The N-DNW electrode had high selectivity and high sensitivity for the differential pulse voltammetric detection of NADH in the presence of ascorbic acid at the lower and stable oxidation potential. Moreover, it exhibited strong stability after prolonged usage. The oxidation peak potential at the N-DNW electrode remained unchanged even after exposure to the solution, followed by washing, drying, and storage in laboratory air for 20 days, with minimization of surface contamination. Therefore, the N-DNW unmodified electrode shows promise for the detection of NADH and is attractive for use in a dehydrogenase based biosensor and other analytical applications.

Sensitive electrochemical detection of NADH and ethanol at low potential based on pyrocatechol violet electrodeposited on single walled carbon nanotubes-modified pencil graphite electrode

In this work, the electrodeposition of pyrocatechol violet (PCV) was initially investigated by the electrochemical surface plasmon resonance (ESPR) technique. Subsequently, PCV was used as redox-mediator and was electrodeposited on the surface of pencil graphite electrode (PGE) modified with single-wall carbon nanotubes (SWCNTs). Owing to the remarkable synergistic effect of SWCNTs and PCV, PGE/SWCNTs/PCV exhibited excellent electrocatalytic activity towards dihydronicotinamide adenine dinucleotide (NADH) oxidation at low potential (0.2V vs. SCE) with fast amperometric response (<10s), broad linear range (1.3-280 μM), good sensitivity (146.2 μA mM(-1)cm(-2)) and low detection limit (1.3 μM) at signal-to-noise ratio of 3. Thus, this PGE/SWCNTs/PCV could be further used to fabricate a sensitive and economic ethanol biosensor using alcohol dehydrogenase (ADH) via a glutaraldehyde/BSA cross-linking procedure.

L-Amino acid oxidases from microbial sources: types, properties, functions, and applications

L-Amino acid oxidases (LAAOs), which catalyze the stereospecific oxidative deamination of L-amino acids to α-keto acids and ammonia, are flavin adenine dinucleotide-containing homodimeric proteins. L-Amino acid oxidases are widely distributed in diverse organisms and have a range of properties. Because expressing LAAOs as recombinant proteins in heterologous hosts is difficult, their biotechnological applications have not been thoroughly advanced. LAAOs are thought to contribute to amino acid catabolism, enhance iron acquisition, display antimicrobial activity, and catalyze keto acid production, among other roles. Here, we review the types, properties, structures, biological functions, heterologous expression, and applications of LAAOs obtained from microbial sources. We expect this review to increase interest in LAAO studies.

Silver nanoparticle-treated filter paper as a highly sensitive surface-enhanced Raman scattering (SERS) substrate for detection of tyrosine in aqueous solution

Highly sensitive SERS substrates based on deposition of silver nanoparticles on commercially available filter paper were prepared in this work, and used to overcome problems found in analyses of aqueous samples. To prepare silver nanoparticle- (AgNP) doped filter substrates, a silver mirror reaction was used. The procedures for substrate preparation were systematically optimized. Pretreatment of filter paper, reaction time, temperature, and concentration of reagents for silver mirror reactions were studied. The morphologies of the resulting substrates were characterized by field-emission scanning electron microscopy (FE-SEM) and correlated with the SERS signals by probing with p-nitrothiophenol (pNTP). Filter papers with different pretreatments were found to have different sizes and distributions of AgNPs. The best performance was found when filter paper was pre-treated with ammonia solution before growth of AgNPs. Based on the SEM images, the resulting AgNPs had roughly spherical shape with a high degree of uniformity. The silver-coated filter paper substrates provide much higher SERS signals compared to glass substrates and the reproducibility was improved significantly. Based on statistical analyses, the relative standard deviations for substrate-to-substrate and spot-to-spot were both were less than 8% and the enhancement factors for the substrates were, in general, higher than 107. The SERS substrates were used to selectively detect tyrosine in aqueous solution. Results indicate that filter-based SERS substrates are highly suited to detection of tyrosine. Compared to glass-based SERS substrates, 50 times more SERS signal was observed in detection of tyrosine. The linear range can be up to 100 μM with a detection limit of 625 nM (SN(-1)=3).

Immobilization of phenylalanine dehydrogenase and its application in flow-injection analysis system for determination of plasma phenylalanine

Phenylalanine dehydrogenase (L-PheDH) from Sporosarcina ureae was immobilized on DEAE-cellulose, modified initially with 2-amino-4,6-dichloro-s-triazine followed by hexamethylenediamine and glutaraldehyde. The highest activity of immobilized PheDH was determined as 95.75 U/g support with 56% retained activity. The optimum pH value of immobilized L-PheDH was shifted from pH 10.4 to 11.0. The immobilized L-PheDH showed activity variations close to the maximum value in a wider temperature range of 45-55 °C, whereas it was 40 °C for the native enzyme. The pH and the thermal stability of the immobilized L-PheDH were also better than the native enzyme. At pH 10.4 and 25 °C, K (m) values of the native and the immobilized L-PheDH were determined as K(m Phe) = 0.118, 0.063 mM and K(m NAD)(+) = 0.234, 0.128 mM, respectively. Formed NADH at the exit of packed bed reactor column was detected by the flow-injection analysis system. The conversion efficiency of the reactor was found to be 100% in the range of 5-600 μM Phe at 9 mM NAD(+) with a total flow rate of 0.1 mL/min. The reactor was used for the analyses of 30 samples each for 3 h per day. The half-life period of the reactor was 15 days.

Expression of esophageal cancer related gene 4 (ECRG4), a novel tumor suppressor gene, in esophageal cancer and its inhibitory effect on the tumor growth in vitro and in vivo

The ECRG4 gene was initially identified and cloned in our laboratory from human normal esophageal epithelium (GenBank accession no. AF325503). We revealed the expression of ECRG4 protein was downregulated in 68.5% (89/130) ESCC samples using tissue microarray. The low ECRG4 protein expression was significantly associated with regional lymph node metastasis, primary tumor size, and tumor stage in ESCC (p < 0.05). ECRG4 mRNA expression was downregulated in ESCC due to the hypermethylation in the gene promoter. The treatment with 5-aza-2'-deoxycytidine, which is a DNA methyltransferase inhibitor restored ECRG4 mRNA expression in ESCC cells. The result indicated that promoter hypermethylation may be 1 main mechanism leading to the silencing of ECRG4. The high expression of ECRG4 in patients with ESCC was associated with longer survival compared with those with low ECRG4 expression by Kaplan-Meier survival analysis (p < 0.05). ECRG4 protein was an independent prognostic factor for ESCC by multivariable Cox proportional hazards regression analysis (p < 0.05). The restoration of ECRG4 expression in ESCC cells inhibited cell proliferation, colony formation, anchorage-independent growth, cell cycle progression and tumor growth in vivo (p < 0.05). The transfection of ECRG4 gene in ESCC cells inhibited the expression of NF-kappaB and nuclear translocation, in addition to the expression of COX-2, a NF-kappaB target gene, was attenuated. Taken together, ECRG4 is a novel candidate tumor suppressor gene in ESCC, and ECRG4 protein is a candidate prognostic marker for ESCC.

Noninvasive measurement of phenylalanine by iontophoretic extraction in patients with phenylketonuria

Phenylketonuria is an autosomal recessive disorder characterized by elevated concentrations of phenylalanine. Elevated phenylalanine concentrations can impair intellectual functions and the disease is treated with a lifelong diet and frequent monitoring of plasma phenylalanine concentrations. Previous in vitro studies have demonstrated the feasibility of iontophoretically enhanced transdermal transport of phenylalanine. Here we evaluate the feasibility of transdermal iontophoretic extraction of phenylalanine in vivo. Phenylalanine was iontophoretically extracted from the skin of healthy volunteers and of patients with phenylketonuria for up to 6 h and concentrations were compared with those measured in plasma. The amount of phenylalanine iontophoretically extracted from the skin declined over time, suggesting contribution of phenylalanine from the skin in the initial extraction. Phenylalanine iontophoretically extracted from skin correlated with plasma phenylalanine levels at plasma levels above 300 micromol/L. This correlation supports the feasibility of iontophoretic phenylalanine extraction for monitoring phenylketonuria.

Purification of recombinant phenylalanine dehydrogenase by partitioning in aqueous two-phase systems

This study presents the partitioning and purification of recombinant Bacillus badius phenylalanine dehydrogenase (PheDH) in aqueous two-phase systems (ATPS) composed of polyethylene glycol 6000 (PEG-6000) and ammonium sulfate. A single-step operation of ATPS was developed for extraction and purification of recombinant PheDH from E. coli BL21 (DE3). The influence of system parameters including; PEG molecular weight and concentration, pH, (NH(4))(2)SO(4) concentration and NaCl salt addition on enzyme partitioning were investigated. The best optimal system for the partitioning and purification of PheDH was 8.5% (w/w) PEG-6000, 17.5% (w/w) (NH(4))(2)SO(4) and 13% (w/w) NaCl at pH 8.0. The partition coefficient, recovery, yield, purification factor and specific activity values were of 92.57, 141%, 95.85%, 474.3 and 10424.97 U/mg, respectively. Also the K(m) values for L-phenylalanine and NAD(+) in oxidative deamination were 0.020 and 0.13 mM, respectively. Our data suggested that this ATPS could be an economical and attractive technology for large-scale purification of recombinant PheDH.

Penicillamine determination using a tyrosinase micro-rotating biosensor

Tyrosinase [EC 1.14.18.1], immobilized on a rotating disk, catalyzed the oxidation of catechols to o-benzoquinone, whose back electrochemical reduction was detected on glassy carbon electrode surface at -150mV versus Ag/AgCl/NaCl 3M. Thus, when penicillamine (PA) was added to the solution, this thiol-containing compound participate in Michael type addition reactions with o-benzoquinone to form the corresponding thioquinone derivatives, decreasing the reduction current obtained proportionally to the increase of its concentration. This method could be used for sensitive determination of PA in drug and human synthetic serum samples. A linear range of 0.02-80 microM (r=0.999) was obtained for amperometric determination of PA in buffered pH 7.0 solutions (0.1 M phosphate buffer). The biosensor has a reasonable reproducibility (R.S.D.<4.0%) and a very stable amperometric response toward this compound (more than 1 month).

Development of infrared optical sensor for selective detection of tyrosine in biological fluids

In this paper, a new and simple evanescent wave type of infrared biosensor is described for the selective detection of tyrosine in biological fluids. This sensor is based on the formation of copper complexes between the sensing phase and tyrosine. To demonstrate that this principle was applicable to the selective detection of tyrosine, a proline-modified sensing phase was synthesized on the surface of the internal reflection elements. This sensing phase was saturated with copper ions to allow it to interact with tyrosine units in aqueous solution through the formation of stable proline-Cu2+-tyrosine complexes. Tyrosine exhibits a unique spectral feature in its absorption band at 1515 cm-1. This band significantly differs from those of other amino acids and provides a further method for the discrimination of tyrosine. By investigating the signals from 12 amino acids, only three amino acids, each containing a phenyl group, could be sensed selectively by this sensing phase. Based on the unique absorption of tyrosine located at 1515 cm-1, tyrosine can be selectively detected. To perform quantitative analyses of tyrosine using this sensing phase, a theoretical working equation was developed and correlated with the experimental data. The analytical results indicated that the developed equations do explain and predict the detection behaviors of the proposed sensing scheme. Using the optimal conditions, the regression coefficients for standard curves of tyrosine recorded in the region of concentrations below 600 microM were higher than 0.996 under either equilibrium or non-equilibrium conditions. Detection limit of tyrosine when using this method was ca. 3 microM.

Fabrication of comb interdigitated electrodes array (IDA) for a microbead-based electrochemical assay system

This research is directed towards developing a more sensitive and rapid electrochemical sensor for enzyme labeled immunoassays by coupling redox cycling at interdigitated electrode arrays (IDA) with the enzyme label beta-galactosidase. Coplanar and comb IDA electrodes with a 2.4 microm gap were fabricated and their redox cycling currents were measured. ANSYS was used to model steady state currents for electrodes with different geometries. Comb IDA electrodes enhanced the signal about three times more than the coplanar IDAs, which agreed with the results of the simulation. Magnetic microbead-based enzyme assay, as a typical example of biochemical detection, was done using the comb and coplanar IDAs. The enzymes could be placed close to the sensing electrodes (approximately 10 microm for the comb IDAs) and detection took less than 1 min with a limit of detection of 70 amol of beta-galactosidase. We conclude that faster and more sensitive assays can be achieved with the comb IDA.

Analysis of thiols with tyrosinase-modified carbon paste electrodes based on blocking of substrate recycling

The enzyme, tyrosinase, was immobilized inside carbon paste electrodes (CPE) for the analysis of thiol-containing compounds such as the reduced form of glutathione (GSH) and L-cysteine. The measuring principle of this sensor is based on the blocking of the substrate recycling process between the enzyme and the electrode. The current response is monitored at -0.050 V versus Ag/AgCl. At this low potential, interferences from easily oxidizable species such as ascorbic acid and uric acid are minimized. The tyrosinase CPE is characterized both in steady state experiments and by flow injection analysis (FIA). GSH is used as the model thiol-containing compound for the study. The highest response for GSH was obtained around pH 6.5. A detection limit of 100 nM and 1 microM is achieved for GSH in steady state and in flow measurements, respectively. The analytical range for GSH is dependent on the concentration of the tyrosinase substrate (catechol). In steady state experiments, and at a lower substrate concentration (10 microM catechol), a linear range of 1-8 microM is found for GSH as compared with 5-30 microM at a higher substrate concentration of 20 microM catechol. Current response of the tyrosinase CPE is not affected by the oxidized form of GSH and L-cysteine (glutathione disulfide, GSSG, and L-cystine, respectively) and sulfur-containing compound such as methionine. The tyrosinase CPE can also detect coenzyme A, which makes it possible to construct biosensors based on enzymes producing or utilizing coenzyme A.

Modified carbon paste electrodes for flow injection amperometric determination of isocitrate dehydrogenase activity in serum

A carbon paste electrode modified with the adsorbed products of the electrochemical oxidation of adenosine triphosphate is described. The electrode was applied to the amperometric electrocatalytic detection of the reduced form of both nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate. The catalytic oxidation current shows a linear dependence on the concentration of the reduced form of nicotinamide adenine dinucleotide up to 1x10(-4)M, with a detection limit of 5x10(-9)M. Modified carbon paste electrodes were coated with an electrogenerated film of nonconducting poly(o-phenylenediamine) to obtain a stable amperometric response for at least 150h. In addition to static measurements, determination of both reduced cofactors was carried out in a flow injection analysis system with a thin-layer amperometric detection cell. The electrocatalytic monitoring of reduced nicotinamide adenine dinucleotide phosphate was applied to flow injection measurement of isocitrate dehydrogenase activity in serum. The results were in good agreement with those for the standard spectrophotometric test kit. The proposed method consumed less time and reagents and provided better precision than the standard method.

Renewable reagentless hydrogen peroxide sensor based on direct electron transfer of horseradish peroxidase immobilized on colloidal gold-modified electrode

A novel renewable reagentless hydrogen peroxide (H(2)O(2)) sensor based on the direct electron transfer of horseradish peroxidase (HRP) is proposed. The direct electrochemistry of HRP immobilized on a colloidal gold-modified carbon paste electrode (Au-CPE) was investigated using electrochemical methods. The immobilized HRP displayed a pair of redox peaks in 0.1M phosphate buffer (PB), pH 7.0, with a formal potential of -0.346 V. The response showed a surface-controlled electrode process with an electron transfer rate constant of 6.04+/-0.18s(-1) determined in the scan rate range from 120 to 500 mV/s. The biosensor displayed an excellent electrocatalytic response to the reduction of H(2)O(2) without the aid of an electron mediator. The sensor surface could be renewed quickly and reproducibly by a simple polish step. The calibration range of H(2)O(2) was 0-0.3mM with linear relation from 0.48 to 50 microM and a detection limit of 0.21 microM at 3 sigma. The response showed Michaelis-Menten behavior at higher H(2)O(2) concentrations. The K(app)(M) value of HRP at HRP-Au-CPE was determined to be 3.69+/-0.71 mM.

Synthesis of a highly substituted N(6)-linked immobilized NAD(+) derivative using a rapid solid-phase modular approach: suitability for use with the kinetic locking-on tactic for bioaffinity purification of NAD(+)-dependent dehydrogenases

This study is concerned with further development of the kinetic locking-on strategy for bioaffinity purification of NAD(+)-dependent dehydrogenases. Specifically, the synthesis of highly substituted N(6)-linked immobilized NAD(+) derivatives is described using a rapid solid-phase modular approach. Other modifications of the N(6)-linked immobilized NAD(+) derivative include substitution of the hydrophobic diaminohexane spacer arm with polar spacer arms (9 and 19.5 A) in an attempt to minimize nonbiospecific interactions. Analysis of the N(6)-linked NAD(+) derivatives confirm (i) retention of cofactor activity upon immobilization (up to 97%); (ii) high total substitution levels and high percentage accessibility levels when compared to S(6)-linked immobilized NAD(+) derivatives (also synthesized with polar spacer arms); (iii) short production times when compared to the preassembly approach to synthesis. Model locking-on bioaffinity chromatographic studies were carried out with bovine heart l-lactate dehydrogenase (l-LDH, EC 1.1.1.27), bakers yeast alcohol dehydrogenase (YADH, EC 1.1.1.1) and Sporosarcinia sp. l-phenylalanine dehydrogenase (l-PheDH, EC 1.4.1.20), using oxalate, hydroxylamine, and d-phenylalanine, respectively, as locking-on ligands. Surprisingly, two of these test NAD(+)-dependent dehydrogenases (lactate and alcohol dehydrogenase) were found to have a greater affinity for the more lowly substituted S(6)-linked immobilized cofactor derivatives than for the new N(6)-linked derivatives. In contrast, the NAD(+)-dependent phenylalanine dehydrogenase showed no affinity for the S(6)-linked immobilized NAD(+) derivative, but was locked-on strongly to the N(6)-linked immobilized derivative. That this locking-on is biospecific is confirmed by the observation that the enzyme failed to lock-on to an analogous N(6)-linked immobilized NADP(+) derivative in the presence of d-phenylalanine. This differential locking-on of NAD(+)-dependent dehydrogenases to N(6)-linked and S(6)-linked immobilized NAD(+) derivatives cannot be explained in terms of final accessible substitutions levels, but suggests fundamental differences in affinity of the three test enzymes for NAD(+) immobilized via N(6)-linkage as compared to thiol-linkage.

Electrocatalytic detection of NADH and glycerol by NAD(+)-modified carbon electrodes

The electrochemical oxidation of the adenine moiety in NAD+ and other adenine nucleotides at carbon paste electrodes gives rise to redox-active products which strongly adsorb on the electrode surface. Carbon paste electrodes modified with the oxidation products of NAD+ show excellent electrocatalytic activity toward NADH oxidation, reducing its overpotential by about 400 mV. The rate constant for the catalytic oxidation of NADH, determined by rotating disk electrode measurements and extrapolation to zero concentration of NADH, was found to be 2.5 x 10(5) M-1 s-1. The catalytic oxidation current allows the amperometric detection of NADH at an applied potential of +50 mV (Ag/AgCl) with a detection limit of 4.0 x 10(-7) M and linear response up to 1.0 x 10(-5) M NADH. These modified electrodes can be used as amperometric transducers in the design of biosensors based on coupled dehydrogenase enzymes and, in fact, we have designed an amperometric biosensor for glycerol based on the glycerol dehydrogenase (GlDH) system. The enzyme GlDH and its cofactor NAD+ were co-immobilized in a carbon paste electrode using an electropolymerized layer of nonconducting poly(o-phenylenediamine) (PPD). After partial oxidation of the immobilized NAD+, the modified electrode allows the amperometric detection of the NADH enzymatically obtained at applied potential above 0 V (Ag/AgCl). The resulting biosensor shows a fast and linear response to glycerol within the concentration range of 1.0 x 10(-6)-1.0 x 10(-4) M with a detection limit of 4.3 x 10(-7) M. The amperometric response remains stable for at least 3 days. The biosensor was applied to the determination of glycerol in a plant-extract syrup, with results in good agreement with those for the standard spectrophotometric method.