Role of paraoxonase 1 in organophosphate G-series nerve agent poisoning and future therapeutic strategies.

Chemical warfare nerve agents (CWNA) are neurotoxic chemicals unethically used as agents of mass destruction by terrorist outfits and during war. The available antidote against CWNA-mediated toxicity is not sufficiently effective and possesses several limitations. As a countermeasure, paraoxonase 1 (PON1), a catalytic bioscavenger, is being developed as a prophylactic treatment. However, the catalytic activity and substrate specificity of human PON1 are insufficient to be used as a potential antidote. Several laboratories have made different approaches to enhance the CWNA hydrolytic activity against various nerve agents. This review explores the holistic view of PON1 as a potential prophylactic agent against G-series CWNA poisoning, from its initial development to recent advancements and limitations. Apart from this, the review also provides an overview of all available PON1 variants that could be used as a potential prophylactic agent and discusses several possible ways to counteract immunogenicity.

Expression and purification of biologically active recombinant human paraoxonase 1 from inclusion bodies of Escherichia coli.

Human PON1 (h-PON1) is a Ca(2+)-dependent serum enzyme and can hydrolyze (and inactivate) a wide range of substrates. It is a multifaceted enzyme and exhibit anti-inflammatory, anti-oxidative, anti-atherogenic, anti-diabetic, anti-microbial, and organophosphate (OP)-detoxifying properties. Thus, h-PON1 is a strong candidate for the development of therapeutic intervention against these conditions in humans. Insufficient hydrolyzing activity of native h-PON1 against desirable substrate affirms the urgent need to develop improved variant(s) of h-PON1 having enhanced activity. Production of recombinant h-PON1 (rh-PON1) using an Escherichia coli expression system is a key to develop such variant(s). However, generation of rh-PON1 using E. coli expression system has been elusive until now because of the aggregation of over-expressed rh-PON1 protein in inactive form as inclusion bodies (IBs) in the bacterial cells. In this study, we have over-expressed rh-PON1(wt) and rh-PON1(H115W;R192K) proteins as IBs in E. coli, and refolded the inactive enzymes present in the IBs to their active form using in vitro refolding. The active enzymes were isolated from the refolding mixture by ion-exchange chromatography. The catalytic properties of the refolded enzymes were similar to their soluble counterparts. Our results show that the pure and the active variant of rh-PON1 enzyme having enhanced hydrolyzing activity can be produced in large quantities using E. coli expression system. This method can be used for the industrial scale production of rh-PON1 enzymes and will aid in developing h-PON1 as a therapeutic candidate.

Expression, purification and immobilization of recombinant AiiA enzyme onto magnetic nanoparticles.

AiiA is a "28-kDa lactonase" from Gram-positive Bacillus sp. 240B1. The enzyme can hydrolyze and inactivate a variety of acyl homoserine lactones (AHLs), quorum sensor molecules involve in bacterial quorum sensing (QS). AiiA is a strong candidate for the development of bio-decontaminating agent that can disrupt QS in industrial and environmental samples. However, commercial application of AiiA suffer from several limitations including high cost of production of enzyme and lack of efficient recovery mean(s) of enzyme from the application environment for its reuse. In this study we have cloned, expressed and purified recombinant AiiA (r-AiiA) enzyme. The purified enzyme was covalently immobilized onto magnetic nanoparticles (MNPs) and the quorum quenching ability of r-AiiA-MNP nanobiocatalyst was evaluated in aqueous buffer. Our results show that r-AiiA-MNPs (a) can hydrolyze 3O-C10AHL and inhibit QS in aqueous buffer, (b) can be recovered from the reaction mixture using external magnetic field, and (c) can be reused multiple times to hydrolyze 3O-C10AHL in aqueous buffer. Results of this study can be used to develop a formulation of AiiA enzyme for industrial applications.

Improving storage stability of recombinant organophosphorus hydrolase.

Organophosphorus hydrolase (OPH) is a ∼38kDa enzyme encoded by opd gene of Flavobacterium sp. The enzyme can hydrolyze and inactivate variety of organophosphate (OP)-compounds, including chemical warfare nerve agents. Thus, OPH is a strong candidate for the development of therapeutic intervention against OP-poisoning in humans and other animals. It is also a promising bio-decontaminating agent for clean-up of OP-contaminated objects and areas. For successful commercial application, long-term storage stability of purified OPH enzyme is important. In this study we have cloned and expressed recombinant OPH (r-OPH) in Escherichia coli and the effect of different excipients on the long-term storage stability of purified enzyme was analyzed. The enzyme was stored in either aqueous solution or in lyophilized form at 25°C for 60days in the presence or absence of different excipients and the stability of the enzyme was determined by monitoring the paraoxon-hydrolyzing activity. Our results suggest that, (a) maltose, trehalose, arginine and proline were most effective in stabilizing the enzyme when stored in aqueous buffer at 25°C, and (b) maltose, trehalose, and mannose exerted maximum stabilization effect when the enzyme was stored in lyophilized form at 25°C for 60days. The study shows that common excipients can be used to stabilize purified OPH enzyme in order to store it for long period of time under different storage conditions. The results of this study can be used to develop formulation(s) of OPH enzyme for commercial use.

Oxidized-phospholipids in reconstituted high density lipoprotein particles affect structure and function of recombinant paraoxonase 1.

Paraoxonase 1 (PON1) is an HDL-associated enzyme and exhibits anti-inflammatory, anti-diabetic, and anti-atherogenic properties. Association of PON1 to HDL particles increases the stability and activity of PON1 and is important for the normal functioning of the enzyme. HDL particles are made up of lipid and protein constituents and apolipoprotein A-I (apoA-I) is a principal protein constituent of HDL that facilitates various biological activities of HDL. In many disease conditions the oxidized phospholipid (Ox-PL) content of HDL is found to be increased and an inverse correlation between the activity of PON1 and oxidation of the HDL is observed. However, the molecular details of the inhibitory action of the Ox-PL-containing HDL on the function of PON1 are not clear yet. In this study we have assembled reconstituted HDL (rHDL) particles with and without Ox-PL and compared their effect on the structure and function of (13)C-labeled recombinant PON1 ((13)C-rPON1) by employing attenuated total reflectance Fourier transformed infrared (ATR-FTIR) spectroscopy and enzymatic assay. Our results show that the presence of the Ox-PL in the rHDL particles alters the structure of rPON1 and decreases its lactonase activity.

Human paraoxonase 1 as a pharmacologic agent: limitations and perspectives.

Human PON1 (h-PON1) is a multifaceted enzyme and can hydrolyze (and inactivate) a wide range of substrates. The enzyme shows anti-inflammatory, antioxidative, antiatherogenic, ant-diabetic, antimicrobial, and organophosphate (OP)-detoxifying properties. However, there are certain limitations regarding large-scale production and use of h-PON1 as a therapeutic candidate. These include difficulties in producing recombinant h-PON1 (rh-PON1) using microbial expression system, low hydrolytic activity of wild-type h-PON1 towards certain substrates, and low storage stability of the purified enzyme. This review summarizes the work done in our laboratory to address these limitations. Our results show that (a) optimized polynucleotide sequence encoding rh-PON1 can express the protein in an active form in E. coli and can be used to generate variant of the enzyme having enhanced hydrolytic activity, (b) in vitro refolding of rh-PON1 enzyme can dramatically increase the yield of an active enzyme, (c) common excipients can be used to stabilize purified rh-PON1 enzyme when stored under different storage conditions, and (d) variants of rh-PON1 enzyme impart significant protection against OP-poisoning in human blood (ex vivo) and mouse (in vivo) model of OP-poisoning. The rh-PON1 variants and their process of production discussed here will help to develop h-PON1 as a therapeutic candidate.

Molecular and functional insight into anti-EGFR nanobody: Theranostic implications for malignancies.

Targeted therapy and imaging are the most popular techniques for the intervention and diagnosis of cancer. A potential therapeutic target for the treatment of cancer is the epidermal growth factor receptor (EGFR), primarily for glioblastoma, lung, and breast cancer. Over-production of ligand, transcriptional up-regulation due to autocrine/paracrine signalling, or point mutations at the genomic locus may contribute to the malfunction of EGFR in malignancies. This exploit makes use of EGFR, an established biomarker for cancer diagnostics and treatment. Despite considerable development in the last several decades in making EGFR inhibitors, they are still not free from limitations like toxicity and a short serum half-life. Nanobodies and antibodies share similar binding properties, but nanobodies have the additional advantage that they can bind to antigenic epitopes deep inside the target that conventional antibodies are unable to access. For targeted therapy, anti-EGFR nanobodies can be conjugated to various molecules such as drugs, peptides, toxins and photosensitizers. These nanobodies can be designed as novel immunoconjugates using the universal modular antibody-based platform technology (UniCAR). Furthermore, Anti-EGFR nanobodies can be expressed in neural stem cells and visualised by effective fluorescent and radioisotope labelling.

Towards development of biobetter: L-asparaginase a case study.

BACKGROUND: L-asparaginase (ASNase) has played a key role in the management of acute lymphoblastic leukaemia (ALL). As an amidohydrolase, it catalyzes the hydrolysis of L-asparagine, a crucial step in the treatment of ALL. Various ASNase variants have evolved from diverse sources since it was first used in paediatric patients in the 1960s. This review describes the available ASNase and approaches being used to develop ASNase as a biobetter candidate. SCOPE OF REVIEW: The review discusses the Glycosylation and PEGylation techniques, which are frequently used to develop biobetter versions of the majority of the therapeutic proteins. Further, it explores current ASNase biobetters in therapeutic use and discusses the protein engineering and chemical modification approaches that were employed to reduce immunogenicity, extend protein half-life, and enhance protease stability of ASNase. Emerging strategies like immobilization and encapsulation are also highlighted as potential pathways for improving ASNase properties. MAJOR CONCLUSIONS: The purpose of the development of ASNase biobetter is to achieve a novel therapeutic candidate that could improve catalytic efficiency, in vivo stability with minimum glutaminase (GLNase) activity and toxicity. Modification of ASNase by immobilization and encapsulation or by fusion technologies like Albumin fusion, Fc fusion, ELP fusion, XTEN fusion, etc. can be exploited to develop a novel biobetter candidate suitable for therapeutic approaches. GENERAL SIGNIFICANCE: This review emphasizes the importance of biobetter development for therapeutic proteins like ASNase. Improved ASNase molecules have the potential to significantly advance the treatment of ALL and have broader implications in the pharmaceutical industry.

Oxidized phospholipid content destabilizes the structure of reconstituted high density lipoprotein particles and changes their function.

High density lipoprotein (HDL) particles are made up of lipid and protein constituents and apolipoprotein A-I (apoA-I) is a principal protein component that facilitates various biological activities of HDL particles. Increase in Ox-PL content of HDL particles makes them 'dysfunctional' and such modified HDL particles not only lose their athero-protective properties but also acquire pro-atherogenic and pro-inflammatory functions. The details of Ox-PL-induced alteration in the molecular properties of HDL particles are not clear. Paraoxonase 1 (PON1) is an HDL-associated enzyme that possesses anti-inflammatory and anti-atherogenic properties; and many of the athero-protective functions of HDL are attributed to the associated PON1. In this study we have characterized the physicochemical properties of reconstituted HDL (rHDL) particles containing varying amounts of Ox-PL and have compared their PON1 stimulation capacity. Our results show that increased Ox-PL content (a) modifies the physicochemical properties of the lipid domain of the rHDL particles, (b) decreases the stability and alters the conformation as well as orientation of apoA-I molecules on the rHDL particles, and (c) decreases the PON1 stimulation capacity of the rHDL particles. Our data indicate that the presence of Ox-PLs destabilizes the structure of the HDL particles and modifies their function.

The chemical nature of the polar functional group of oxidized acyl chain uniquely modifies the physicochemical properties of oxidized phospholipid-containing lipid particles.

Oxidative modification of phospholipids generates a variety of oxidized phospholipid (Ox-PL) species which differ considerably in their chemical compositions and molecular structures. Recent results suggest that even closely related Ox-PL species can have considerably different biological effects. However, the molecular mechanism for this is not yet clear. In truncated Ox-PLs (tOx-PLs) the fatty acyl chain is shorter in length than the parent nonoxidized phospholipid molecules and contains a polar functional group(s). In a previous study we showed that two closely related tOx-PL species having a similar polar functional group and differing only in the length of the oxidized fatty acyl chain exerts significantly different effects on the physicochemical properties of the nonoxidized phospholipid particles containing these lipids (Kar et al., Chem Phys Lipids 164:54-61, 2011). In this study we have characterized the effect of polar functional groups of oxidized fatty acyl chain on the physicochemical properties of the nonoxidized phospholipid particles containing these lipids. Our results show that Ox-PL species differing only in the chemical nature of polar functional groups in their oxidized fatty acyl chain modify the properties of nonoxidized phospholipid particles containing them in a distinctive way. These results indicate that different species of Ox-PLs induce unique changes in the physicochemical properties of lipid particles/membranes containing them and that this may lead to their different biological effects.

Oxidatively modified fatty acyl chain determines physicochemical properties of aggregates of oxidized phospholipids.

In vivo oxidation of glycerophospholipid generates a variety of products including truncated oxidized phospholipids (tOx-PLs). The fatty acyl chains at the sn-2 position of tOx-PLs are shorter in length than the parent non-oxidized phospholipids and contain a polar functional group(s) at the end. The effect of oxidatively modified sn-2 fatty acyl chain on the physicochemical properties of tOx-PLs aggregates has not been addressed in detail, although there are few reports that modified fatty acyl chain primarily determines the biological activities of tOx-PLs. In this study we have compared the properties of four closely related tOx-PLs which differ only in the type of modified fatty acyl chain present at the sn-2 position: 1-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine (PazePC), 1-palmitoyl-2-(9'-oxo-nonanoyl)-sn-glycero-3-phosphocholine (PoxnoPC), 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphocholine (PGPC), and 1-palmitoyl-2-(5'-oxo-valeroyl)-sn-glycero-3-phosphocholine (POVPC). Aggregates of individual tOx-PL in aqueous solution were characterized by fluorescence spectroscopy, size exclusion chromatography, native polyacrylamide and agarose gel electrophoresis. The data suggest that aggregates of four closely related tOx-PLs form micelle-like particles of considerably different properties. Our result provides first direct evidence that because of the specific chemical composition of the sn-2 fatty acyl chain aggregates of particular tOx-PL possess a distinctive set of physicochemical properties.

Membrane surface charge modulates lipoprotein complex forming capability of peptides derived from the C-terminal domain of apolipoprotein E.

Apolipoprotein E (apoE) plays a major role in the transport and metabolism of lipid by acting as a ligand for low density lipoprotein-receptors. The amphipathic helical regions of its C-terminal domain are necessary for the lipoprotein binding and assembly of nascent lipoprotein particles. Lipoproteins in the plasma are known to possess a net negative charge, determined by both its protein and lipid components, which regulates the metabolism of lipoproteins. The role of membrane surface charge on the interaction of apoE has not been studied previously. Also the importance of individual amphipathic helical regions of its C-terminal domain in binding to negatively charged lipid membrane is not addressed. In this study we have compared the interaction of four peptide segments of apoE C-terminal domain (apoE((202-223)), apoE((223-244)), apoE((245-266)), and apoE((268-289))) with zwitterionic and negatively charged model membranes by employing UV-visible and fluorescence spectroscopy, circular dichroism, and native PAGE analysis. Our results show that the peptide sequence 202-223, 245-266 and 268-289 of apoE has higher affinity towards negatively charged lipid membrane and are independently capable of forming lipoprotein particles of 17+/-2 nm Stokes diameter. The results suggest that surface charge of lipoprotein regulates its metabolism possibly by modulating the recruitment of apoE on its surface.

Preferential binding of apolipoprotein E derived peptides with oxidized phospholipid.

The physiological function of apolipoprotein E (apoE) includes transport and metabolism of lipids and its C-terminal domain harbors high affinity lipid-binding sites. Although the binding of apoE with non-oxidized phospholipid containing membranes has been characterized earlier, the interaction of apoE or its fragments with oxidized phospholipid containing membrane has never been studied. In this study we have compared the interaction of amphipathic helical peptide sequences derived from the C-terminal domain of apoE with membrane vesicles containing oxidized phospholipid, 1-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine (PazePC), with membrane vesicles without PazePC. The interaction was studied by monitoring (a) fluorescence emission maxima of the peptides, (b) acrylamide quenching of the peptides tryptophan residues and (c) by measuring the equilibrium binding constants by resonance energy transfer (RET) analysis. Our result shows that peptide sequence 202-223, 245-266 and 268-289 of apoE has higher affinity towards membrane containing PazePC, compared to membrane without PazePC. Presence of 1mM divalent cation or 50 mM NaCl in the buffer decreased the binding of peptides to PazePC containing membrane vesicles suggesting possible involvement of the electrostatic interaction in the binding. These observations suggest that the preferential binding of apoE to oxidized phospholipid containing membrane may play a role in the anti-oxidative properties of apoE.

Antibiotic-free expression system for the production of human interferon-beta protein.

Recombinant human interferon-ß (rhIFN-ß), a therapeutic protein, is produced using both prokaryotic and eukaryotic expression systems. However, instability of recombinant plasmid during cultivation of Escherichia coli results in low yield of the recombinant proteins. In addition, use of antibiotics during the cultivation imposes a major concern. In this study, we have compared the expression yield of rhIFN-ß in E. coli BL21 (DE3) and E coli SE1 cells. Gene-encoding rhIFN-ß was expressed in E. coli BL21 (DE3) and SE1 cells and the cultivation of recombinant E. coli cells was done in a laboratory scale bioreactor. Our results suggest that, compared to BL21(DE3) cells, the SE1 cells expressing rhIFN-ß protein can be cultivated in the medium without antibiotic and provide increased stability of recombinant plasmid and higher expression yield of rhIFN-ß protein. This system can be used for the production of rhIFN-ß proteins for biomedical applications.

Towards Understanding the Catalytic Mechanism of Human Paraoxonase 1: Experimental and In Silico Mutagenesis Studies.

Human paraoxonase 1 (h-PON1) is a ~45-kDa serum enzyme that can hydrolyze a variety of substrates, including organophosphate (OP) compounds. It is a potential candidate for the development of antidote against OP poisoning in humans. However, insufficient OP-hydrolyzing activity of native enzyme affirms the urgent need to develop improved variant(s) having enhanced OP-hydrolyzing activity. The crystal structure of h-PON1 remains unsolved, and the molecular details of how the enzyme catalyses hydrolysis of different types of substrates are also not clear. Understanding the molecular details of the catalytic mechanism of h-PON1 is essential to engineer better variant(s) of enzyme. In this study, we have used a random mutagenesis approach to increase the OP-hydrolyzing activity of recombinant h-PON1. The mutants not only showed a 10-340-fold increased OP-hydrolyzing activity against different OP substrates but also exhibited differential lactonase and arylesterase activities. In order to investigate the mechanistic details of the effect of observed mutations on the hydrolytic activities of enzyme, molecular docking studies were performed with selected mutants. The results suggested that the observed mutations permit differential binding of substrate/inhibitor into the enzyme's active site. This may explain differential hydrolytic activities of the enzyme towards different substrates.

Refolded Recombinant Human Paraoxonase 1 Variant Exhibits Prophylactic Activity Against Organophosphate Poisoning.

Organophosphate (OP) compounds are neurotoxic chemicals, and current treatments available for OP-poisoning are considered as unsatisfactory and inadequate. There is an urgent need for the development of more effective treatment(s) for OP-poisoning. Human paraoxonase 1 (h-PON1) is known to hydrolyze a variety of OP-compounds and is a leading candidate for the development of prophylactic and therapeutic agent against OP-poisoning in humans. Non-availability of effective system(s) for the production of recombinant h-PON1 (rh-PON1) makes it hard to produce improved variant(s) of this enzyme and analyze their in vivo efficacy in animal models. Production of recombinant h-PON1 (rh-PON1) using an Escherichia coli expression system is a key to develop variant(s) of h-PON1. Recently, we have developed a procedure to produce active rh-PON1 enzymes by using E. coli expression system. In this study, we have characterized the OP-hydrolyzing properties of refolded rh-PON1(wt) and rh-PON1(H115W;R192K) variant. Our results show that refolded rh-PON1(H115W;R192K) variant exhibit enhanced OP-hydrolyzing activity in in vitro and ex vivo assays and exhibited prophylactic activity in mouse model of OP-poisoning, suggesting that refolded rh-PON1 can be developed as a therapeutic candidate.

Toward Understanding the Catalytic Mechanism of Human Paraoxonase 1: Site-Specific Mutagenesis at Position 192.

Human paraoxonase 1 (h-PON1) is a serum enzyme that can hydrolyze a variety of substrates. The enzyme exhibits anti-inflammatory, anti-oxidative, anti-atherogenic, anti-diabetic, anti-microbial and organophosphate-hydrolyzing activities. Thus, h-PON1 is a strong candidate for the development of therapeutic intervention against a variety conditions in human. However, the crystal structure of h-PON1 is not solved and the molecular details of how the enzyme hydrolyzes different substrates are not clear yet. Understanding the catalytic mechanism(s) of h-PON1 is important in developing the enzyme for therapeutic use. Literature suggests that R/Q polymorphism at position 192 in h-PON1 dramatically modulates the substrate specificity of the enzyme. In order to understand the role of the amino acid residue at position 192 of h-PON1 in its various hydrolytic activities, site-specific mutagenesis at position 192 was done in this study. The mutant enzymes were produced using Escherichia coli expression system and their hydrolytic activities were compared against a panel of substrates. Molecular dynamics simulation studies were employed on selected recombinant h-PON1 (rh-PON1) mutants to understand the effect of amino acid substitutions at position 192 on the structural features of the active site of the enzyme. Our results suggest that, depending on the type of substrate, presence of a particular amino acid residue at position 192 differentially alters the micro-environment of the active site of the enzyme resulting in the engagement of different subsets of amino acid residues in the binding and the processing of substrates. The result advances our understanding of the catalytic mechanism of h-PON1.

Improving Properties of Recombinant SsoPox by Site-Specific Pegylation.

SsoPox, a ~35 kDa enzyme from Sulfolobus solfataricus, can hydrolyze and inactivate a variety of organophosphate (OP)-compounds. The enzyme is a potential candidate for the development of prophylactic and therapeutic agent against OP-poisoning in humans. However, the therapeutic use of recombinant SsoPox suffers from certain limitations associated with the use of recombinant protein pharmaceuticals. Some of these limitations could be overcome by conjugating SsoPox enzyme with polyethylene glycol (PEG). In this study, we report generation and in vitro characterization of N-terminal mono-PEGylated rSsoPox(2p) (a variant of rSsoPox(wt) having enhanced OP-hydrolyzing activity). The enzyme was PEGylated with mPEG-propionaldehyde and the PEGylated protein was isolated using ion-exchange chromatography. Compared with the unmodified enzyme, mono-PEGylation of rSsoPox results in improvement in the thermostability and protease resistance of the enzyme. PEGylated rSsoPox(2p) can be developed as a candidate for the prevention / treatment of OP-poisoning.

Interplay between amino acid residues at positions 192 and 115 in modulating hydrolytic activities of human paraoxonase 1.

Human paraoxonase 1 (h-PON1) is a Ca(2+)-dependent serum enzyme that catalyzes the hydrolysis of different types of substrates. The crystal structure of h-PON1 is not solved yet and the molecular details of how the enzyme catalyzes different types of reactions are not clear. Literature suggests that the amino acid residues at positions 192 and 115 are important for various hydrolytic activities of h-PON1. It is proposed that catalytic residue H115 (and H134) mediates the lactonase and the arylesterase activities of the enzyme while the amino acid residue at position 192 modulates various other hydrolytic activities of the enzyme. However, the relationship between these two residues in the hydrolytic activities of h-PON1 is not studied in detail. In this study, we have expressed and purified the wild-type recombinant h-PON1 (rh-PON1(wt)) and its point mutants differing in the amino acid residues at positions 192 and/or 115 using an Escherichia coli expression system. The hydrolytic activities of the purified enzymes were compared using enzymatic assays. Our results, for the first time, show that (a) the presence of a particular amino acid residue at position 192 differentially alters the effect of the H115W substitution, and (b) H115 residue is not always needed for the lactonase and arylesterase activities of the enzyme. The results also suggest that the amino acid residues at position 192 and 115 act in conjunction in modulating the hydrolytic activities of the enzyme.

Characterization of human paraoxonase 1 variants suggest that His residues at 115 and 134 positions are not always needed for the lactonase/arylesterase activities of the enzyme.

Human paraoxonase 1 (h-PON1) hydrolyzes variety of substrates and the hydrolytic activities of enzyme can be broadly grouped into three categories; arylesterase, phosphotriesterase, and lactonase. Current models of the catalytic mechanism of h-PON1 suggest that catalytic residues H115 and H134 mediate the lactonase and arylesterase activities of the enzyme. H-PON1 is a strong candidate for the development of catalytic bioscavenger for organophosphate poisoning in humans. Recently, Gupta et al. (Nat. Chem. Biol. 2011. 7, 120) identified amino acid substitutions that significantly increased the activity of chimeric-PON1 variant (4E9) against some organophosphate nerve agents. In this study we have examined the effect of these (L69G/S111T/H115W/H134R/R192K/F222S/T332S) and other substitutions (H115W/H134R and H115W/H134R/R192K) on the hydrolytic activities of recombinant h-PON1 (rh-PON1) variants. Our results show that the substitutions resulted in a significant increase in the organophosphatase activity of all the three variants of rh-PON1 enzyme while had a variable effect on the lactonase/arylesterase activities. The results suggest that H residues at positions 115 and 134 are not always needed for the lactonase/arylesterase activities of h-PON1 and force a reconsideration of the current model(s) of the catalytic mechanism of h-PON1.