Photonic crystal sensor for organophosphate nerve agents utilizing the organophosphorus hydrolase enzyme

We developed an intelligent polymerized crystalline colloidal array (IPCCA) photonic crystal sensing material which reversibly senses the organophosphate compound methyl paraoxon at micromolar concentrations in aqueous solutions. A periodic array of colloidal particles is embedded in a poly-2-hydroxyethylacrylate hydrogel. The particle lattice spacing is such that the array Bragg-diffracts visible light. We utilize a bimodular sensing approach in which the enzyme organophosphorus hydrolase (OPH) catalyzes the hydrolysis of methyl paraoxon at basic pH, producing p-nitrophenolate, dimethylphosphate, and two protons. The protons lower the pH and create a steady-state pH gradient. Protonation of the phenolates attached to the hydrogel makes the free energy of mixing of the hydrogel less favorable, which causes the hydrogel to shrink. The IPCCA's lattice constant decreases, which blueshifts the diffracted light. The magnitude of the steady-state diffraction blueshift is proportional to the concentration of methyl paraoxon. The current detection limit is 0.2 micromol methyl paraoxon per liter.

Fluorescence-based sensing of p-nitrophenol and p-nitrophenyl substituent organophosphates

A novel detection method for organophosphate neurotoxins has been described, based on the fluorescence quenching of a Coumarin derivative. These dyes are similar in structure to some organophosphates (OPs), and they fluoresce in the blue-green region of the spectra. This methodology has been utilized for the detection of organophosphates whose hydrolysis product is p-nitrophenol by using an enzyme, organophosphorus hydrolase (OPH). Coumarin1 in the presence of p-nitrophenol results in a quenching of fluorescence, providing a direct measure of the concentration of p-nitrophenol present in the sample. The decrease in fluorescence intensity is proportional to the paraoxon concentration in the range of 7.0x10(-7)-1.7x10(-4) M. The specificity of this sensing application for p-nitrophenyl substituent OPs has also been demonstrated. OPs are a class of synthetic organic pesticides which generally have a short residual life and can cause numerous acute and chronic health effects. They have been an integral part of the agricultural industry for the past several decades due to their target specificities and selectable toxicities. The toxic nature of these compounds can be attributed to the species-specific inhibition of acetylcholinesterase (AChE), an important enzyme responsible for the regeneration of neural synaptic function. In addition to their wide agricultural and urban usage, they have also been exploited for the development of neurological chemical warfare agents. Currently available technologies for OP detection include sol-gel thin films, screen printed electrodes, acoustic patterning, gas chromatography-mass spectrometry, and various other intricate techniques that have limited field applicabilities. This optically-based approach promises much simpler and more direct detection capabilities.

Calmodulin-mediated reversible immobilization of enzymes

This work demonstrates the use of the protein calmodulin, CaM, as an affinity tag for the reversible immobilization of enzymes on surfaces. Our strategy takes advantage of the of the reversible, calcium-mediated binding of CaM to its ligand phenothiazine and of the ability to produce fusion proteins between CaM and a variety of enzymes to reversibly immobilize enzymes in an oriented fashion to different surfaces. Specifically, we employed two different enzymes, organophosphorus hydrolase (OPH) and beta-lactamase and two different solid supports, a silica surface and cellulose membrane modified by covalently attaching a phenothiazine ligand, to demonstrate the versatility of our immobilization method. Fusion proteins between CaM-OPH and CaM-beta-lactamase were prepared by using genetic engineering strategies to introduce the calmodulin tail at the N-terminus of each of the two enzymes. In the presence of Ca(2+), CaM adopts a conformation that favors interaction between hydrophobic pockets in CaM and phenothiazine, while in the presence of a Ca(2+)-chelating agent such as EGTA, the interaction between CaM and phenothiazine is disrupted, thus allowing for removal of the CaM-fusion protein from the surface under mild conditions. CaM also acts as a spacer molecule, orienting the enzyme away from the surface and toward the solution, which minimizes enzyme interactions with the immobilization surface. Since the method is based on the highly selective binding of CaM to its phenothiazine ligand, and this is covalently immobilized on the surface, the method does not suffer from ligand leaching nor from interference from other proteins present in the cell extract. An additional advantage lies in that the support can be regenerated by passing through EGTA, and then reused for the immobilization of the same or, if desired, a different enzyme. Using a fusion protein approach for immobilization purposes avoids the use of harsh conditions in the immobilization and/or regeneration steps, which could cause inactivation of the immobilized enzyme. Moreover, we have demonstrated that the CaM affinity tag allows immobilization of enzymes on a variety of surfaces without compromising their enzymatic activity substantially; for example, the immobilized OPH retained more than 80% of the activity of the free enzyme. Our results with beta-lactamase showed the feasibility of using a phenothiazine surface in several consecutive loading and regeneration cycles. This can be advantageous when expensive and/or difficult to obtain immobilization surfaces have to be employed; the immobilization surface could be reused to immobilize the same or a different enzyme using the CaM affinity tail. We also determined that the phenothiazine-modified silica particles are stable for long periods of time, i.e., up to 2 years when stored at 4 degrees C. It is envisioned that this type of reversible immobilization may find applications in the development of reversible, reusable biosensors and bioreactors endowed with the additional advantage that the biological element at the surface of the sensor or bioreactor could be replaced under mild conditions when needed to sense or process a different target molecule.

Stability of highly purified human paraoxonase (PON1): Association with human phosphate binding protein (HPBP) is essential for preserving its active conformation(s)

The biological role of human paraoxonase (PON1) remains unclear, whilst there is a consensus that the enzyme has a protective influence. A toxicological role, protecting from environmental poisoning by organophosphate derivatives drove earlier works, and more recently, clinical interest has focused on a protective role in vascular disease. PON1 resides essentially on HDL particles, a complex and dynamic molecular environment. Our recent discovery of the human phosphate binding protein (HPBP), displaying a firm propensity to associate with PON1, has steered new directions for characterizing PON1 functional state. Here, we report investigations on the effect of HPBP on oligomerization, storage and thermal stability of PON1. We found that purified PON1 is as a mixture of at least two states, and that the absence of HPBP favors homo-oligomerization of PON1 into state(s) of higher molecular size. We showed that HPBP allows stabilizing active conformation(s) of PON1 disencumbered of its natural environment. We also showed that PON1 exhibits intrinsically a remarkable thermal stability, and that the association of HPBP strongly contributes to slow the denaturation rate. A hybrid recombinant PON1 was shown more thermostable than the human enzyme, and its stability was unaffected by the presence of HPBP. Altogether, the results strongly encourage further study of the human enzyme.

Organophosphorus hydrolase multilayer modified microcantilevers for organophosphorus detection

We report a biosensor based on organophosphorus hydrolase (OPH) multilayer modified microcantilever (MCL) for detection of organophosphorus compounds (OPs). The assay is based on substrate-dependent bending of the OPH functionalized MCLs. The cantilever bending amplitude at equilibrium was a function of the concentration of paraoxon with the dynamic range extending from 10(-7) to 10(-3)M. The lower detection limit of approximately 10(-7)M for paraoxon was an order of magnitude better than the OPH-based potentiometric and optical biosensors based on pH modulation. There was a good intra-sensor and an acceptable inter-sensor reproducibility as evidenced by the standard errors of 5% and 15%, respectively. OPs measured using this technique included parathion and diisopropyl fluorophosphate (DFP) in the order of paraoxon>DFP>parathion. The conformational change of the OPH was most likely the main origin of MCL bending.

Estrogen esters as substrates for human paraoxonases

Mammalian paraoxonases (PONs 1, 2 and 3) are a highly conserved family of esterases, with uncertain physiological functions and natural substrates. Here we characterize the ability of purified recombinant human PONs to hydrolyze estrogen esters, a class of compounds previously not known to be PON substrates. PONs hydrolyzed estrogen mono- and diesters at position 3 of the steroid A-ring. Diesters were better substrates for the PONs and were very efficiently hydrolyzed, particularly by PON3. Esters at position 17 were not cleaved by the PONs unless an adjacent double bound was present. Purified human serum butyryl cholinesterase also hydrolyzed estrogen esters, however it preferably hydrolyzed the mono-esters. The ability of the PONs' to effectively hydrolyze a variety of estrogen esters provides further insight into the structure of their active sites and suggests that natural compounds with aromatic ester groups might be relevant substrates for the PONs.

Human paraoxonase: A promising approach for pre-treatment and therapy of organophosphorus poisoning

The limited efficiency of medical countermeasures against poisoning by nerve agent justifies efforts to find new prophylactic means and new antidotes. The concept of bioscavengers has emerged as an alternative approach to pharmacological pre- and post-exposure treatments. Catalytic scavengers are enzymes displaying a turnover with OPs as substrates, allowing rapid and efficient protection using administration of small doses. Several reasons have endorsed human paraoxonase (PON1) to be a pertinent candidate as catalytic bioscavenger. The physiological function of PON1 has not yet been unambiguously identified. Considered as a promiscuous enzyme, PON1 appears to be primarily a lactonase and also displays an anti-atherogenic activity closely linked to its localization on HDL particles. A HDL-associated phosphate transporter termed human phosphate binding protein (HPBP) was found to be a partner of natural human PON. In the absence of its natural environment (or mimicry by detergents), human PON1 is unstable and tends to aggregate. Converging data indicate that both the activity and the stability of PON1 are dramatically dependent on the HDL component molecular environment, including HPBP. Therefore, biochemical and physiological characterization of PON1-HPBP complexes, the environment allowing retaining functional enzyme state(s), and the thermal and storage stability of PON1 are mandatory. Synergistic efforts on characterization of recombinant hybrid PON1 expressed in E. coli and natural human PON1 provide information for the future rational design of stable mutants of PON1-based catalytic scavengers to be used as safe and effective countermeasures to OP intoxication.

Découverte et structure cristallographique d’une apolipoprotéine humaine

We report the serendipitous discovery of a human plasma phosphate binding protein (HPBP). This 38 kDa protein is co-purified with paraoxonase (PON1). The association between HPON1 and HPBP is modulated by phosphate and calcium concentrations. The HPBP X-ray structure solved at 1.9 A resolution is similar to the prokaryotic phosphate solute-binding proteins (SBPs) associated with ATP binding cassette transmembrane transporters, though phosphate-SBPs have never been characterized or predicted from nucleic acid databases in eukaryotes. However, HPBP belongs to the family of ubiquitous eukaryotic proteins named DING, meaning that phosphate-SBPs are also widespread in eukaryotes. The absence of complete genes for eukaryotic phosphate-SBP from databases is intriguing, but the astonishing 90% sequence conservation of genes between evolutionary distant species suggests that the corresponding proteins play an important function. HPBP is the first identified transporter capable of binding phosphate ions in human plasma. Thus it is thought to become a new predictor and a potential therapeutic agent for phosphate-related diseases such as atherosclerosis.

Oxidative status and serum PON1 activity in beta-thalassemia minor

OBJECTIVES

Paraoxonase-1 (PON1) deficiency is related to increased susceptibility to low density lipoprotein oxidation and development of atherosclerosis. The aim of this study was to investigate paraoxonase and arylesterase activities along with oxidative status parameters, and to find out if there is any increased susceptibility to atherogenesis, which might be reflected with increased oxidative stress and decreased serum PON1 activity in beta-thalassemia minor (BTM) subjects.

DESIGN AND METHODS

Thirty-two subjects with BTM and 28 healthy subjects as control were enrolled in the study. Serum paraoxonase and arylesterase activities, lipid hydroperoxide (LOOH) levels, total antioxidant capacity (TAC), total oxidant status (TOS) and oxidative stress index (OSI) were determined.

RESULTS

Serum TAC, paraoxonase and arylesterase activities were significantly lower in BTM subjects than controls (for all p<0.001), while TOS, LOOH levels and OSI were significantly higher (p<0.001, p<0.05 and p<0.001; respectively). In BTM subjects, OSI, TOS, LOOH levels and TAC were significantly correlated with serum paraoxonase (r=-0.245, p<0.05; r=-0.231, p<0.05; r=-0.264, p<0.05 and, r=0.342, p<0.05, respectively) and arylesterase activities (r=-0.332, p<0.05, r=-0.308, p<0.05; r=-0.320, p<0.05 and r=0.443, p<0.05). Additionally, hemoglobin level was also correlated with serum paraoxonase (r=0.501, p<0.001) and arylesterase activities (r=0.501, p<0.001), TAC (r=0.402, p<0.05), TOS (r=-0.274, p<0.05) and OSI (r=-0.352, p<0.05).

CONCLUSIONS

Oxidative stress is increased, while serum PON1 activity is decreased in BTM subjects. Decrease in PON1 activity seems to be associated with both the degree of oxidative stress and anemia. BTM subjects may be more prone to development of atherogenesis due to low serum PON1 activity.

Direct detection of stereospecific soman hydrolysis by wild-type human serum paraoxonase

Human serum paraoxonase 1 (HuPON1; EC 3.1.8.1) is a calcium-dependent six-fold beta-propeller enzyme that has been shown to hydrolyze an array of substrates, including organophosphorus (OP) chemical warfare nerve agents. Although recent efforts utilizing site-directed mutagenesis have demonstrated specific residues (such as Phe222 and His115) to be important in determining the specificity of OP substrate binding and hydrolysis, little effort has focused on the substrate stereospecificity of the enzyme; different stereoisomers of OPs can differ in their toxicity by several orders of magnitude. For example, the C+/-P- isomers of the chemical warfare agent soman (GD) are known to be more toxic by three orders of magnitude. In this study, the catalytic activity of HuPON1 towards each of the four chiral isomers of GD was measured simultaneously via chiral GC/MS. The catalytic efficiency (k(cat)/K(m)) of the wild-type enzyme for the various stereoisomers was determined by a simultaneous solution of hydrolysis kinetics for each isomer. Derived k(cat)/K(m) values ranged from 625 to 4130 mm(-1).min(-1), with isomers being hydrolyzed in the order of preference C+P+ > C-P+ > C+P- > C-P-. The results indicate that HuPON1 hydrolysis of GD is stereoselective; substrate stereospecificity should be considered in future efforts to enhance the OPase activity of this and other candidate bioscavenger enzymes.

Accuracy and Biological Variation of Human Serum Paraoxonase 1 Activity and Polymorphism (Q192R) by Kinetic Enzyme Assay

Background: Paraoxonase 1 (PON1) phenotype is a better predictor of atherosclerosis risk than are PON1 genetic polymorphisms alone. Larger studies are required to determine the role of PON1 and there is a need for standardized PON1 assays between laboratories.

Methods: We have adapted 5 enzyme kinetic assays for high-throughput automated analysis of PON1 activity. Using different substrates and reaction conditions, we measured PON1 activity and used activity ratios to identify the PON1 Q192R genetic polymorphisms and assessed the accuracy of the genotype assignments in 79 adult study participants by comparing them with genotypes determined by AlwI restriction enzyme digestion of a 176-bp PCR amplification product from genomic DNA. Imprecision was determined using pooled serum and purified enzyme preparations. Biological variability was estimated by analysis of serial samples from 17 individuals. Variability parameters were compared with total cholesterol as a point of reference to a recognized biomarker of coronary heart disease risk.

Results: Salt stimulation and inhibition ratios were 97.4% and 94.7% correct in assigning Q192R genotype, respectively. Analytical imprecision (CV) was 1.0%–3.0% for phenylacetate and paraoxon substrate assays and 3.0%–8.0% for the para-nitrophenylacetate substrate assays. Combination of the 2 ratios into a double ratio resulted in 100% correct genotype classification.

Conclusion: The described methods for measurement of PON1 activity and accurate genotype assignment are rapid and have potential to facilitate the efficient investigation of PON1 status in clinical and epidemiological studies.

A wireless magnetoelastic biosensor for the direct detection of organophosphorus pesticides

An organophosphorus (OP) pesticide sensor was fabricated by applying a pH-sensitive polymer coating and organophosphorus hydrolase (OPH) enzyme onto the surface of a magnetoelastic sensor, the magnetic analogue of the better-known surface acoustic wave sensor. Organophosphorus hydrolase catalyses the hydrolysis of a wide range of organophosphorus compounds, which changes the pH in the hydrogel. This article describes the application of the magnetoelastic sensor for the detection of OP pesticides by measuring the changes in viscoelasticity caused by the swelling/shrinking of the pH-responsive polymer when exposed to the pesticides. The sensor was successfully used to detect paraoxon and parathion down to a concentration of 1 x 10(-7) and 8.5 x 10(-7) M respectively.

Biosensing paraoxon in simulated environmental samples by immobilized organophosphorus hydrolase in functionalized mesoporous silica

There is a critical need for highly sensitive, cost-effective sensors to conduct ecological analyses for environmental and homeland security-related applications. Enzyme biosensors, which are currently gaining acceptance for environmental monitoring applications, need improvements to deliver faster measurements with stabilized sensing elements, e.g., enzymes. We report here on a method which significantly overcomes this difficulty, and demonstrate its application in a biosensor for aquatic environmental applications. A fast-responding and stable biosensor was developed via immobilization of organophosphorus hydrolase (OPH) in functionalized mesoporous silica (FMS) with pore sizes in tens of nanometers. The OPH-FMS composite was held on glassy carbon electrode by a dried Nafion gel and FMS protected OPH from Nafion-resulted activity loss. The resulting enzyme biosensor, when integrated with an electrochemical instrument, responded rapidly to low paraoxon concentration and achieved steady-state current in less than 10 s, with a detection limit of 4.0x10(-7) M paraoxon. The biosensor was tested for detection of paraoxon in simulated environmental samples, under wide-ranging physicochemical conditions. Results clearly indicate high recovery efficiencies in aqueous solutions (96 to 101%) at different pH, total organic carbon, total dissolved solids, and total suspended solids, and demonstrate the ability of the biosensor unit to continuously monitor paraoxon in aqueous conditions similar to those found in river and lake systems.

Asymmetric fluorogenic organophosphates for the development of active organophosphate hydrolases with reversed stereoselectivity

In order to enhance the enzymatic detoxification rate of organophosphorus (OP) nerve agents we have searched for more active variants of recombinant mammalian paraoxonase (PON1). We have previously identified three key positions in PON1 that affect OP hydrolysis: Leu69, Val346 and His115, that significantly enhance the hydrolysis of cyclosarin (GF), soman, chlorpyrifos-oxon (ChPo), O-isopropyl-O-(p-nitrophenyl)methylphosphonate (IMP-pNP) and diisopropyl fluorophosphate (DFP). GC/FPD analysis compared to residual AChE inhibition assay displayed stereoselective hydrolysis of GF, soman and IMP-pNP, indicating that wild type PON1 and its variant V346A are more active toward the less toxic P(+) optical isomer. In order to obtain new PON1 variants with reversed stereoselectivity, displaying augmented activity toward the more toxic isomer P(-) of nerve agents, we synthesized new asymmetric fluorogenic OPs (Flu-OPs). Six Flu-OPs were prepared containing either ethyl (E), cyclohexyl (C) or pinacolyl (P) alkyl radicals attached to methyl-phosphonyl (MP) moiety analogous to the structure of VX, GF and soman, respectively. The fluorescent moieties are either 3-cyano-4-methyl-7-hydroxy coumarin (MeCyC) or 1,3-dichloro-7-hydroxy-9,9-dimethyl-9H-acridin-2-one (DDAO). The kinetics of AChE and BChE inhibition by these new Flu-OPs display k(i) values 8.5x10(4) to 8.5x10(7) and 5x10(4) to 2x10(6)M(-1)min(-1), respectively. EMP-MeCyC and EMP-DDAO are the most active inhibitors of AChE whereas CMP-MeCyC and CMP-DDAO are better inhibitors of BChE than AChE, indicating accommodation of bulky cyclohexyl group inside the active site of BChE. PMP-MeCyC and PMP-DDAO are the least active inhibitors of both AChE and BChE. CMP-MeCyC and CMP-DDAO were significantly detoxified only by the five-site mutations PON1 variant L69V/S138L/S193P/N287D/V346A. Degradation kinetics of Flu-OPs measured by increase in absorbance of the released fluorogenic group was fit by a two exponential function, indicating faster hydrolysis of the less toxic optical isomer. Interestingly, wt PON1 caused only 50% degradation of racemic EMP-MeCyC, CMP-MeCyC and CMP-DDAO indicating complete hydrolysis of P(+) isomer. This remarkable stereoselectivity was used for the enzymatic separation of the P(-) isomer of CMP-MeCyC. The bimolecular rate constant k(i) for human AChE inhibition by the isolated P(-) isomer of CMP-MeCyC is five-fold larger than that of its P(+) isomer. The marked preference of wt PON1 toward P(+) stereo-isomer of CMP-MeCyC and CMP-DDAO renders their P(-) stereo-isomers suitable for the selection of new OP hydrolase variants with reversed stereoselectivity.

Organophosphorus Hydrolase at the Air−Water Interface: Secondary Structure and Interaction with Paraoxon

The secondary structure of organophosphorus hydrolase (OPH) at the air-water interface was studied using polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The shape and position of the amide I and amide II bands were used to estimate the surface conformation and orientation of OPH. The PM-IRRAS results indicated that the enzyme did not unfold for the range of surface pressure used (0-30 mN/m). At low surface pressures, the signal of amide I was very weak and the intensity was almost the same as amide II. Upon further compression, the PM-IRRAS signal and the ratio of the intensity of amide I and amide II both increase, implying an increased interfacial concentration of the enzyme. From the amide I/amide II ratio and the band position, it was deduced that the enzyme adopts a conformation which gives a higher occupied surface at low surface pressure and rotates to a more vertical orientation at high surface pressures. The compression and decompression of the OPH monolayer indicated that the fingerprint of the secondary structure at the air-water interface was reversible. PM-IRRAS was also used to investigate the pH effect of the subphase on the secondary structure of OPH. The secondary structure of OPH at the air-water interface was well defined when the pH of the subphase was near its isoelectric point (IP, pH 7.6). However, it adopted a different orientation when the subphase pH values were higher or lower than the IP with formation of random coil structure. The hydrolysis of organophosphorus compound paraoxon by OPH was also studied at the air-water interface by PM-IRRAS. The pH effect and the interaction with paraoxon both seem to orientate the enzyme more in the plane of the interface and to produce random coil structure.

Detection of Organophosphorus Compounds by Covalently Immobilized Organophosphorus Hydrolase

As a consequence of organophosphorus (OP) toxins posing a threat to human life globally, organophosphorus hydrolase (OPH) has become the enzyme of choice to detoxify such compounds. Organophosphorus hydrolase was covalently immobilized onto a quartz substrate for utilization in paraoxon detection. The substrate was cleaned and modified prior to chemical attachment. Each modification step was monitored by imaging ellipsometry as the thickness increased with each modification step. The chemically attached OPH was labeled with a fluorescent dye (7-isothiocyanato-4-methylcoumarin) for the detection of paraoxon in aqueous solution, ranging from 10(-9) to 10(-5) M. UV-visible spectra were also acquired for the determination of the hydrolysis product of para-oxon, namely p-nitrophenol.

Serendipitous discovery and X-ray structure of a human phosphate binding apolipoprotein

We report the serendipitous discovery of a human plasma phosphate binding protein (HPBP). This 38 kDa protein is copurified with the enzyme paraoxonase. Its X-ray structure is similar to the prokaryotic phosphate solute binding proteins (SBPs) associated with ATP binding cassette transmembrane transporters, though phosphate-SBPs have never been characterized or predicted from nucleic acid databases in eukaryotes. However, HPBP belongs to the family of ubiquitous eukaryotic proteins named DING, meaning that phosphate-SBPs are also widespread in eukaryotes. The systematic absence of complete genes for eukaryotic phosphate-SBP from databases is intriguing, but the astonishing 90% sequence conservation between genes belonging to evolutionary distant species suggests that the corresponding proteins play an important function. HPBP is the only known transporter capable of binding phosphate ions in human plasma and may become a new predictor of or a potential therapeutic agent for phosphate-related diseases such as atherosclerosis.

Differential effect of lysophospholipids on activities of human plasma paraoxonase1, either soluble or lipid-bound

Interaction of paraoxonase1 (PON1) with lysophospholipids was examined with respect to activity regulation and binding property. Paraoxonase activity of purified PON1 was partially inhibited by palmitoyl-lysophosphatidyl-glycerol (palmitoyl-lysoPG) and lysophosphatidylinositol (lysoPI), which had a stimulatory effect on arylesterase and diazoxonase activities. The selective inhibition of paraoxonase activity by palmitoyl-lysoPG, characterized by noncompetitiveness and charge interaction, was also observed with HDL- or dimyristoylphosphatidylcholine (DMPC)-bound PON1. Meanwhile, lysophosphatidylcholine (lysoPC) stimulated all three activities of purified PON1, although it stimulated DMPC-bound or HDL-bound PON1 to a lesser extent. The stimulatory action of lysophospholipids was observed around their CMC, suggesting that micelle formation of lysophospholpids might be involved in the stimulation of PON1 activity. Presumably in support of this, the tryptophan fluorescence intensity of PON1 was increased by lysophospholipids at concentrations required for the stimulation of PON1 activity. Separately, lysoPC stimulation was less remarkable for DMPC-bound PON1 than for either dimyristoylphosphatidylserine (DMPS)- or dimyristoylphosphatidylglycerol-bound PON1, suggesting a tight association between PON1 and DMPC. In support of this, the stimulatory role of apolipoprotein A-I was less prominent for DMPC-bound PON1 than for DMPS-bound PON1. Taken together, these data suggest that the inhibition of paraoxonase activity by lysoPG or lysoPI may be due to binding to a site distinct from the active center, whereas the stimulation by lysophospholipid may be ascribed to the micelle formation around the lipid-associable region of PON1.

Vitamin C preserves the cardio-protective paraoxonase activity of high-density lipoprotein during oxidant stress

HDL-associated paraoxonase (PON) antioxidant enzyme activity is cardio-protective. We investigated whether vitamin C prevented loss of PON activity from HDL during oxidant stress. HDL was incubated with either hydrophilic or lipophilic peroxyl radical initiators in the absence (control) or presence of vitamin C (50 and 100 micromol/L). Regardless of the type of radical, accumulation of lipid oxidation products in HDL was similar in incubations lacking vitamin C. Loss of PON activity was greater in HDL exposed to hydrophilic, in contrast to lipophilic, radicals, but addition of vitamin C maintained enzyme activity. Vitamin C's capacity to attenuate loss of the HDL ability to prevent atherogenic modification of LDL (assessed as electrophoretic mobility) was, however, modest, and appeared limited only to those incubations in which HDL was exposed to lipophilic radicals. Our results indicate that vitamin C may, under some conditions, prevent loss of cardio-protective function from HDL during oxidant stress.

Apolipoprotein A-I-mimetic peptides with antioxidant actions

To augment antioxidant action of apolipoprotein A-I (Apo A-I)-mimetic peptide, the peptide F3,6,14,18 18A (DWFKAFYDKVAEKFKEAF) was modified by incorporating antioxidant amino acid residues. Introduction of His residue at position 2 or 3 at N-terminal of the peptide remarkably enhanced antioxidant action against Cu2+ oxidation of LDL and the capability of sequestering Cu2+. Likewise, the substitution of Ala for Cys residue at position 12 increased antioxidant action against Cu2+ oxidation of LDL. Additionally, the Cys substitution contributed to enhanced capabilities in the removal of hypochlorous acid (HOCl) and 13-hydroperoxyoctadecadienoic acid. Furthermore, the combined incorporation of His and Cys residues enhanced antioxidant actions in preventing Cu2+ oxidation and reducing HOCl and hydroperoxide levels. Separately, in solubilizing phosphatidylcholine, either peptides with His residue at N-terminal position 2 or 3, or those containing Cys residue at position 11 or 12 were equipotent to peptide F3,6,14,18 18A. Further, the lipid-solubilizing ability of those containing both His and Cys residues was comparable to that of peptide F3,6,14,18 18A. In support of this, a similar structural importance was observed with Trp fluorescence study illustrating the penetration of peptides in phosphatidylcholine liposome. Besides, the modified peptides were also comparable to peptide F3,6,14,18 18A in restoring phosphatidylserine-induced loss of PON1 activity. These results indicate that the insertion of His or Cys residue into peptide F3,6,14,18 18A at appropriate positions could lead to enhanced antioxidant action with no significant change of lipid-solubilizing action.

Tandem purification of two HDL-associated partner proteins in human plasma, paraoxonase (PON1) and phosphate binding protein (HPBP) using hydroxyapatite chromatography

Human plasma paraoxonase (PON1) is calcium-dependent enzyme that hydrolyses esters, including organophosphates and lactones, and exhibits anti-atherogenic properties. Human phosphate binding protein (HPBP) was discovered as contaminant during crystallization trials of PON1. This observation and uncertainties for the real activities of PON1 led us to re-evaluate the purity of PON1 preparations. We developed a hydroxyapatite chromatography for the separation of both HDL-associated proteins. We confirmed that: (1) HPBP is strongly associated to PON1 in HDL, and generally both proteins are co-purified; (2) standard purification protocols of PON1 lead to impure enzyme; (3) hydroxyapatite chromatography allows the simultaneous purification of PON1 and HPBP.

Enhanced stereoselective hydrolysis of toxic organophosphates by directly evolved variants of mammalian serum paraoxonase

We addressed the ability of various organophosphorus (OP) hydrolases to catalytically scavenge toxic OP nerve agents. Mammalian paraoxonase (PON1) was found to be more active than Pseudomonas diminuta OP hydrolase (OPH) and squid O,O-di-isopropyl fluorophosphatase (DFPase) in detoxifying cyclosarin (O-cyclohexyl methylphosphonofluoridate) and soman (O-pinacolyl methylphosphonofluoridate). Subsequently, nine directly evolved PON1 variants, selected for increased hydrolytic rates with a fluorogenic diethylphosphate ester, were tested for detoxification of cyclosarin, soman, O-isopropyl-O-(p-nitrophenyl) methyl phosphonate (IMP-pNP), DFP, and chlorpyrifos-oxon (ChPo). Detoxification rates were determined by temporal acetylcholinesterase inhibition by residual nonhydrolyzed OP. As stereoisomers of cyclosarin and soman differ significantly in their acetylcholinesterase-inhibiting potency, we actually measured the hydrolysis of the more toxic stereoisomers. Cyclosarin detoxification was approximately 10-fold faster with PON1 mutants V346A and L69V. V346A also exhibited fourfold and sevenfold faster hydrolysis of DFP and ChPo, respectively, compared with wild-type, and ninefold higher activity towards soman. L69V exhibited 100-fold faster hydrolysis of DFP than the wild-type. The active-site mutant H115W exhibited 270-380-fold enhancement toward hydrolysis of the P-S bond in parathiol, a phosphorothiolate analog of parathion. This study identifies three key positions in PON1 that affect OP hydrolysis, Leu69, Val346 and His115, and several amino-acid replacements that significantly enhance the hydrolysis of toxic OPs. GC/pulsed flame photometer detector analysis, compared with assay of residual acetylcholinesterase inhibition, displayed stereoselective hydrolysis of cyclosarin, soman, and IMP-pNP, indicating that PON1 is less active toward the more toxic optical isomers.

High-throughput screening of enzyme libraries: thiolactonases evolved by fluorescence-activated sorting of single cells in emulsion compartments

Single bacterial cells, each expressing a different library variant, were compartmentalized in aqueous droplets of water-in-oil (w/o) emulsions, thus maintaining a linkage between a plasmid-borne gene, the encoded enzyme variant, and the fluorescent product this enzyme may generate. Conversion into a double, water-in-oil-in-water (w/o/w) emulsion enabled the sorting of these compartments by FACS, as well as the isolation of living bacteria cells and their enzyme-coding genes. We demonstrate the directed evolution of new enzyme variants by screening >10(7) serum paraoxonase (PON1) mutants, to yield 100-fold improvements in thiolactonase activity. In vitro compartmentalization (IVC) of single cells, each carrying >10(4) enzyme molecules, in a volume of <10 femtoliter (fl), enabled detection and selection despite the fast, spontaneous hydrolysis of the substrate, the very low initial thiolactonase activity of PON1, and the use of difusable fluorescent products.