Juan Liao

Spectrophotometric-dual-enzyme-simultaneous assay in one reaction solution: chemometrics and experimental models.

Spectrophotometric-dual-enzyme-simultaneous assay in one reaction solution (SDESA) is proposed. SDESA requires the following: (a) Enzyme A acts on Substrate A to release Product A bearing the longest difference absorbance peak (λ(A)) much larger than that of Product B (λ(B)) formed by Enzyme B action on Substrate B; λ(B) is close to the longest isoabsorbance wavelength of Product A and Substrate A (λ(0)); (b) absorbance at λ(A) and λ(0) is quantified via swift alternation of detection wavelengths and corrected on the basis of absorbance additivity; (c) inhibition/activation on either enzyme by any substance is eliminated; (d) Enzyme A is quantified via an integration strategy if levels of Substrate A are lower than the Michaelis constant. Chemometrics of SDESA was tested with γ-glutamyltransferase and lactate-dehydrogenase of complicated kinetics. γ-Glutamyltransferase releases p-nitroaniline from γ-glutamyl-p-nitroaniline with λ(0) at 344 nm and λ(A) close to 405 nm, lactate-dehydrogenase consumes reduced nicotinamide dinucleotide bearing λ(B) at 340 nm. Kinetic analysis of reaction curve yielded lactate-dehydrogenase activity free from inhibition by p-nitroaniline; the linear range of initial rates of γ-glutamyltransferase via the integration strategy, and that of lactate-dehydrogenase after interference elimination, was comparable to those by separate assays, respectively; the quantification limit of either enzyme by SDESA at 25-fold higher activity of the other enzyme remained comparable to that by a separate assay. To test potential application, SDESA of alkaline phosphatase (ALP) and β-D-galactosidase as enzyme-linked-immunoabsorbent assay (ELISA) labels were examined. ALP releases 4-nitro-1-naphthol from 4-nitronaphthyl-1-phosphate with λ(0) at 405 nm and λ(A) at 458 nm, β-D-galactosidase releases 4-nitrophenol from β-D-(4-nitrophenyl)-galactoside with λ(B) at 405 nm. No interference from substrates/products made SDESA of β-galactosidase and ALP simple for ELISA of penicillin G and clenbuterol in one well, and the quantification limit of either hapten was comparable to that via a separate assay. Hence, SDESA is promising.

Comparison of activity indexes for recognizing enzyme mutants of higher activity with uricase as model

BackgroundFor screening a library of enzyme mutants, an efficient and cost-effective method for reliable assay of enzyme activity and a decision method for safe recognition of mutants of higher activity are needed. The comparison of activity concentrations of mutants in lysates of transformed Escherichia coli cells against a threshold is unsafe to recognize mutants of higher activity due to variations of both expression levels of mutant proteins and lysis efficiency of transformed cells. Hence, by a spectrophotometric method after verification to measure uricase activity, specific activity calculated from the level of total proteins in a lysate was tested for recognizing a mutant of higher activity.ResultsDuring uricase reaction, the intermediate 5-hydroxyisourate interferes with the assay of uric acid absorbance, but the measurement of absorbance at 293 nm in alkaline borate buffer was reliable for measuring uricase initial rates within a reasonable range. The level of total proteins in a lysate was determined by the Bradford assay. Polyacrylamide gel electrophoresis analysis supported different relative abundance of uricase mutant proteins in their lysates; activity concentrations of uricase in such lysates positively correlated with levels of total proteins. Receiver-operation-curve analysis of activity concentration or specific activity yielded area-under-the-curve close to 1.00 for recognizing a mutant with > 200% improvement of activity. For a mutant with just about 80% improvement of activity, receiver-operation-curve analysis of specific activity gave area-under-the-curve close to 1.00 while the analysis of activity concentration gave smaller area-under-the-curve. With the mean plus 1.4-fold of the standard deviation of specific activity of a starting material as the threshold, uricase mutants whose activities were improved by more than 80% were recognized with higher sensitivity and specificity.ConclusionSpecific activity calculated from the level of total proteins is a favorable index for recognizing an enzyme mutant with small improvement of activity.

Optimization of pH values to formulate the bireagent kit for serum uric acid assay.

A new formulation of the bireagent kit for serum uric acid assay was developed based on the effects of pH on enzyme stability. At 4 °C, half‐lives of uricases from Bacillus fastidious and Arthrobacter globiforms were longer than 15 months at pH 9.2, but became shorter at pH below 8.0; half‐lives of ascorbate oxidase and peroxidase were comparable at pH 6.5 and 7.0, but became much shorter at pH higher than 7.4. In the new formulation of the bireagent kit, Reagent A contained peroxidase, 4‐aminoantipyrine, and ascorbate oxidase in 50 mM phosphate buffer at pH 6.5; Reagent B contained B. fastidious or A. globiforms uricase in 50 mM sodium borate buffer at pH 9.2; Reagents A and B were mixed at 4:1 to produce a final pH from 7.2 to 7.6 for developing a stable color. The new bireagent kit consumed smaller quantities of three enzymes for the same shelf life. With the new bireagent kit, there were linear responses of absorbance at 546 nm to uric acid up to 34 mM in reaction mixtures and a good correlation of uric acid levels in clinical sera with those by a commercial kit, but stronger resistance to ascorbate. Therefore, the new formulation was advantageous.

Soluble Expression in Escherichia coli of Active Human Cyclic Nucleotide Phosphodiesterase Isoform 4B2 in Fusion with Maltose-Binding Protein

Recombinant expression in Escherichia coli of human cyclic nucleotide phosphodiesterase 4B2 (hPDE4B2) fused to maltose-binding-protein (MBP-hPDE4B2) was investigated. hPDE4B2 DNA amplified via nested RT-PCR with total RNAs from U937 cells was ligated with pMAL-p2x. After induction at 18 °C for 16 h, soluble MBP-hPDE4B2 was produced in E. coli. MBP-hPDE4B2 after amylose-resin chromatography showed 35% homogeneity, and its Michaelis-Menten constant was 10±2 μM (n=3). Rolipram had a dissociation constant of 9±2 nM (n=2), and zinc ion was a potent inhibitor. Hence, MBP-hPDE4B2 was expressed in E. coli as a soluble active protein.

Comparison of modification of a bacterial uricase with N‐hydroxysuccinimide esters of succinate and carbonate of monomethoxyl poly(ethylene glycol)

Uricase after modification with monomethoxy poly(ethylene glycol) (mPEG) is currently the sole agent to treat refractory gout. For formulating Bacillus fastidious uricase, succinimidyl carbonate of mPEG‐5000 (SC‐mPEG5k) and succinimidyl succinate of mPEG‐5000 (SS‐mPEG5k) were compared. SC‐mPEG5k possessed higher purity, comparable reaction rate constant with glycine but lower hydrolysis rate, and stronger effectiveness to modify amino groups. The uricase possessed two types of amino groups bearing a 25‐fold difference in reactivity with SC‐mPEG5k or SS‐mPEG5k at pH 9.2. Oxonate and xanthine concentration‐dependently protected the bacterial uricase from inactivation during PEGylation. With SC‐mPEG5k at a molar ratio of 200 to uricase subunits and oxonate of 50 µM, the PEGylated uricase (1) retained about 73% of the original activity, (2) displayed about 10% reactivity to rabbit anti‐sera recognizing the native uricase, (3) elicited IgG in rats accounting for about 5% of that by the native uricase, (4) exhibited circulation half‐life time of about 25 H in cock plasma in vivo, and (5) concurrently maintained uric acid at lowered levels for over 20 H. Hence, PEGylation with SC‐mPEG under the protection of a competitive inhibitor was a practical approach to formulation of the bacterial uricase; protection of enzymes by competitive inhibitors during PEGylation may have universal significance.

Method to screen aromatic ligands in mixtures for quantitative affinities to target using magnetic separation of bound ligands along with HPLC and UV photometry detection

AbstractA new screening method was tested to estimate the affinity of an aromatic ligand in a mixture through competitive binding against an exogenous reference ligand. The target protein was immobilized on magnetic particles and one or several ligand(s) in each reaction mixture were prepared by parallel combinatorial-synthesis without purification. Specifically, the binding of aromatic biotin derivatives to streptavidin immobilized on magnetic particles was used as the model. An approximation equation that works under the condition of binding ratios below 0.1 for the candidate ligand and the reference ligand was developed. It was found that the relative affinity of each biotinylated aromatic candidate ligand in mixtures was consistent with that by using HPLC-MS analyses or by a homogenous method using its purified counterpart. Hence, this new screening method using HPLC-UV is considered to be highly effective. FigureThe representative procedure to realize the new screening technique for quantitative affinity of an aromatic candidate ligand of unknown quantity in a mixture sample

Estimation of affinities of ligands in mixtures via magnetic recovery of target-ligand complexes and chromatographic analyses: chemometrics and an experimental model

BackgroundThe combinatorial library strategy of using multiple candidate ligands in mixtures as library members is ideal in terms of cost and efficiency, but needs special screening methods to estimate the affinities of candidate ligands in such mixtures. Herein, a new method to screen candidate ligands present in unknown molar quantities in mixtures was investigated.ResultsThe proposed method involves preparing a processed-mixture-for-screening (PMFS) with each mixture sample and an exogenous reference ligand, initiating competitive binding among ligands from the PMFS to a target immobilized on magnetic particles, recovering target-ligand complexes in equilibrium by magnetic force, extracting and concentrating bound ligands, and analyzing ligands in the PMFS and the concentrated extract by chromatography. The relative affinity of each candidate ligand to its reference ligand is estimated via an approximation equation assuming (a) the candidate ligand and its reference ligand bind to the same site(s) on the target, (b) their chromatographic peak areas are over five times their intercepts of linear response but within their linear ranges, (c) their binding ratios are below 10%. These prerequisites are met by optimizing primarily the quantity of the target used and the PMFS composition ratio.The new method was tested using the competitive binding of biotin derivatives from mixtures to streptavidin immobilized on magnetic particles as a model. Each mixture sample containing a limited number of candidate biotin derivatives with moderate differences in their molar quantities were prepared via parallel-combinatorial-synthesis (PCS) without purification, or via the pooling of individual compounds. Some purified biotin derivatives were used as reference ligands. This method showed resistance to variations in chromatographic quantification sensitivity and concentration ratios; optimized conditions to validate the approximation equation could be applied to different mixture samples. Relative affinities of candidate biotin derivatives with unknown molar quantities in each mixture sample were consistent with those estimated by a homogenous method using their purified counterparts as samples.ConclusionsThis new method is robust and effective for each mixture possessing a limited number of candidate ligands whose molar quantities have moderate differences, and its integration with PCS has promise to routinely practice the mixture-based library strategy.

Comparison of Candidate Pairs of Hydrolytic Enzymes for Spectrophotometric-dual-enzyme-simultaneous-assay

Spectrophotometric-dual-enzyme-simultaneous-assay (SDESA) for enzyme-linked-immunosorbent-assay (ELISA) of two components in one well is a patented platform when a special pair of labels is accessible. With microplate readers, alkaline phosphatase on 4-nitro-1-naphthylphosphate (4NNPP) served as label A; Pseudomonas aeruginosa arylsulfatase (PAAS) and acetylcholinesterase (AChE) on their substrates derived from 4-nitrophenol/analogue served as candidate label B, and were compared for SDESA with an engineered alkaline phosphatase of Eschrichia coli (ECAP). For SDESA, the interference from overlapped absorbance was corrected based on linear additivity of absorbance to derive initial rates reflected by absorbance change at 450 nm for ECAP and at 405 nm for PAAS or AChE, after the correction of spontaneous hydrolysis. For SDESA with ECAP, AChE already had sufficient activity in an optimized buffer; PAAS was more favorable for substrate stability and product absorbance except for lower activity. Therefore, PAAS engineered for sufficient activity plus alkaline phosphatase is absorbing for ELISA via SDESA.

An integration strategy to measure enzyme activities for detecting irreversible inhibitors with dimethoate on butyrylcholinesterase as a model

An integration strategy was investigated to measure initial rates of horse butyrylcholinesterase (BChE) at 50.0 µmol L−1 butyrylthiocholine (BTCh) for detecting irreversible inhibitors as pollutants in environment and foods with dimethoate as a model. In this integration strategy: (a) if BTCh consumption within 5.0 min was >60%, BChE initial rates were derived from maximal reaction rates, estimated by an improved integrated method, according to Michaelies-Menten kinetics at 47.0 µmol L−1 BTCh and Michaelis-Menten constant at 94.0 µmol L−1; (b) or else initial rates were determined by the classical initial rate method. Thus, the differences in BChE initial rates without and after dimethoate treatment indexed final dimethoate contents in reaction mixtures to treat BChE. Results supported that this integration strategy determined BChE activities with a linear range about two magnitudes and an upper limit about twice that by the classical initial rate method alone at 2.0 mmol L−1 BTCh. The coefficient of variation with this integration strategy was below 5%. The difference in BChE initial rates before and after dimethoate treatment was proportional to final dimethoate contents in reaction mixtures. By enzymatic analyses, the molar contents of dimethoate extracted from polluted cabbages were consistent with the summed molar contents of dimethoate and dimethoxon by gas-chromatography. Therefore, this integration strategy was effective to detect irreversible inhibitors as pollutants in environment and foods.

Chromogenic substrate from 4-nitro-1-naphthol for hydrolytic enzyme of neutral or slightly acidic optimum pH: 4-Nitro-1-naphthyl-β-d-galactopyranoside as an example

At pH from 5.5 to 7.6, absorptivity of 4-nitro-1-naphthol at 450 nm is over 2.1-fold of that of para-nitrophenol at 405 nm and over 9.6-fold of that of ortho-nitrophenol at 415 nm. On 4-nitro-1-naphthyl-β-D-galactopyranoside at pH 7.4, catalytic efficiency of Escherichia coli β-D-galactosidase is 3-fold of that on para-nitrophenyl-β-D-galactopyranoside and about 40% of that on ortho-nitrophenyl-β-D-galactopyranoside, and produces a lower quantification limit of penicillin G by enzyme-linked-immunoabsorbent-assay. Hence, 4-nitro-1-naphthol is favorable to prepare chromogenic substrates of hydrolytic enzymes of neutral or slightly acidic optimum pH.