Enzyme Nanoreactor for In Vivo Detoxification of Organophosphates

A nanoreactor containing an evolved mutant of Saccharolobus solfataricus phosphotriesterase (L72C/Y97F/Y99F/W263V/I280T) as a catalytic bioscavenger was made for detoxification of organophosphates. This nanoreactor intended for treatment of organophosphate poisoning was studied against paraoxon (POX). Nanoreactors were low polydispersity polymersomes containing a high concentration of enzyme (20 μM). The polyethylene glycol-polypropylene sulfide membrane allowed for penetration of POX and exit of hydrolysis products. In vitro simulations under second order conditions showed that 1 μM enzyme inactivates 5 μM POX in less than 10 s. LD₅₀-shift experiments of POX-challenged mice through intraperitoneal (i.p.) and subcutaneous (s.c.) injections showed that intravenous administration of nanoreactors (1.6 nmol enzyme) protected against 7 × LD₅₀ i.p. in prophylaxis and 3.3 × LD₅₀ i.p. in post-exposure treatment. For mice s.c.-challenged, LD₅₀ shifts were more pronounced: 16.6 × LD₅₀ in prophylaxis and 9.8 × LD₅₀ in post-exposure treatment. Rotarod tests showed that transitory impaired neuromuscular functions of challenged mice were restored the day of experiments. No deterioration was observed in the following days and weeks. The high therapeutic index provided by prophylactic administration of enzyme nanoreactors suggests that no other drugs are needed for protection against acute POX toxicity. For post-exposure treatment, co-administration of classical drugs would certainly have beneficial effects against transient incapacitation.

Environmental Decontamination of a Chemical Warfare Simulant Utilizing a Membrane Vesicle-Encapsulated Phosphotriesterase

While technologies for the remediation of chemical contaminants continue to emerge, growing interest in green technologies has led researchers to explore natural catalytic mechanisms derived from microbial species. One such method, enzymatic degradation, offers an alternative to harsh chemical catalysts and resins. Recombinant enzymes, however, are often too labile or show limited activity when challenged with nonideal environmental conditions that may vary in salinity, pH, or other physical properties. Here, we demonstrate how phosphotriesterase encapsulated in a bacterial outer membrane vesicle can be used to degrade the organophosphate chemical warfare agent (CWA) simulant paraoxon in environmental water samples. We also carried out remediation assays on solid surfaces, including glass, painted metal, and fabric, that were selected as representative materials, which could potentially be contaminated with a CWA.

[Engineering of catalytic bioscavengers of organophosphorus compounds]

Catalytic bioscavengers show promise for the prevention and treatment of organophosphate poisoning. Catalytic bioscavengers are enzymes capable of binding and hydrolyzing organophosphorus compounds (OPs). These enzymes could be used to degrade OPs before they reach their biological targets. As such, they could be administered as antidotes before or after exposure, and be used for skin protection and decontamination. Protein engineering can yield enzymes with improved catalytic properties, that are stable during storage and in the bloodstream, and that are immunologically compatible. Large-scale production must be feasible and reasonably inexpensive.

Atomic resolution crystal structure of squid ganglion DFPase

Diisopropylfluorophosphatases (DFP-ases) are capable of detoxifying chemical warfare agents like diisopropylfluorophosphate (DFP) by hydrolysis. The protein reported here was recombinantely expressed in E. coli. The X-ray crystal structure of this enzyme has been refined to a resolution of 0.85 A and a crystallographic R value of 9.4%. Reversible flash-cooling improved both, mosaicity and resolution of the crystals considerably. The overall structure of this protein represents a six-bladed beta-propeller with two calcium ions bound in a central water filled tunnel. 496 water, 2 glycerol, 2 MES-buffer molecules, and 18 PEG fragments of different lengths could be refined in the solvent region. The 208 most reliable residues, without disorder or reduced occupancy in their side-chains, were finally refined without restraints. A subsequent full-matrix refinement cycle for the positional parameters yielded estimated standard deviations (esds) by matrix inversion. The herewith calculated bond lengths and bond-esds were used to obtain averaged bond lengths, which have been compared to the restraints used in preceding refinement cycles.

High-activity enzyme-polyurethane coatings

The synthesis of water-borne polyurethane coatings in the presence of diisopropylfluorophosphatase (DFPase, E.C. 3.8.2.1) enabled the irreversible attachment of the enzyme to the polymeric matrix. The distribution of immobilized DFPase as well as activity retention are homogeneous within the coating. The resulting enzyme-containing coating (ECC) film hydrolyzes diisopropylfluorophosphate (DFP) in buffered media at high rates, retaining approximately 39% intrinsic activity. Decreasing ECC hydrophilicity, via the use of a less hydrophilic polyisocyanate during polymerization, significantly enhanced the intrinsic activity of the ECC. DFPase-ECC has biphasic deactivation kinetics, where the initial rapid deactivation of DFPase-ECC leads to the formation of a hyperstable and active form of enzyme.

Sequence-specific assignment of histidine and tryptophan ring 1H, 13C and 15N resonances in 13C/15N- and 2H/13C/15N-labelled proteins

Methods are described to correlate aromatic 1H(delta)2/13C(delta)2 or 1H(epsilon)1/15N(epsilon)1 with aliphatic 13C(beta) chemical shifts of histidine and tryptophan residues, respectively. The pulse sequences exclusively rely on magnetization transfers via one-bond scalar couplings and employ [15N, 1H]- and/or [13C, 1H]-TROSY schemes to enhance sensitivity. In the case of histidine imidazole rings exhibiting slow HN-exchange with the solvent, connectivities of these proton resonances with beta-carbons can be established as well. In addition, their correlations to ring carbons can be detected in a simple [15N, 1H]-TROSY-H(N)Car experiment, revealing the tautomeric state of the neutral ring system. The novel methods are demonstrated with the 23-kDa protein xylanase and the 35-kDa protein diisopropyl-fluorophosphatase, providing nearly complete sequence-specific resonance assignments of their histidine delta-CH and tryptophan epsilon-NH groups.

Thermoinactivation of diisopropylfluorophosphatase-containing polyurethane polymers

The thermoinactivation of native diisopropylfluorophosphatase (DFPase, EC 3.8.2.1) is highly calcium dependent, first-order kinetic. Deactivation is coupled with a simultaneous reduction in beta-sheet content. We report herein our attempts to enhance the thermostability of DFPase by irreversibly incorporating the enzyme into polyurethane polymers. Immobilized DFPase has biphasic deactivation kinetics. Our data demonstrate that the initial rapid deactivationof immobilized DFPase leads to the formation of a hyperstable and still active form of enzyme. Like native DFPase, DFPase-containing polyurethanes exhibit a calcium-dependent thermostability. Since bioplastics cannot be analyzed by spectroscopy, the structural mechanisms involved in thermoinactivation of immobilized DFPase were determined using PEG-modified DFPase. The thermoinactivation profile of highly modified DFPase mirrors the stepwise deactivation pattern of bioplastics. Spectroscopic studies enable a structural analysis of the hyperstable intermediate.

Irreversible immobilization of diisopropylfluorophosphatase in polyurethane polymers

The synthesis of polyurethane polymers in the presence of diisopropylfluorophosphatase (DFPase) has enabled the irreversible attachment of the enzyme to the polymeric matrix. The resulting bioplastic hydrolyzes diisopropylfluorophosphate (DFP) in buffered media up to 67% of the rate for the same amount of soluble enzyme. Above a DFPase concentration of approximately 0.1 mg/gfoam, the rate of the reaction catalyzed by the enzyme-containing polymer was controlled by internal mass transfer. Increasing foam hydrophilicity, via the use of nonionic surfactants during polymerization, significantly affected the structural properties of matrix, thereby enhancing the intrinsic and apparent efficiency of modified DFPase. The resulting reduction in internal mass transfer limitations was explained morphologically with electron microscopy.

Crystal structure of diisopropylfluorophosphatase from Loligo vulgaris

BACKGROUND

Phosphotriesterases (PTE) are enzymes capable of detoxifying organophosphate-based chemical warfare agents by hydrolysis. One subclass of these enzymes comprises the family of diisopropylfluorophosphatases (DFPases). The DFPase reported here was originally isolated from squid head ganglion of Loligo vulgaris and can be characterized as squid-type DFPase. It is capable of hydrolyzing the organophosphates diisopropylfluorophosphate, soman, sarin, tabun, and cyclosarin.

RESULTS

Crystals were grown of both the native and the selenomethionine-labeled enzyme. The X-ray crystal structure of the DFPase from Loligo vulgaris has been solved by MAD phasing and refined to a crystallographic R value of 17.6% at a final resolution of 1.8 A. Using site-directed mutagenesis, we have structurally and functionally characterized essential residues in the active site of the enzyme.

CONCLUSIONS

The crystal structure of the DFPase from Loligo vulgaris is the first example of a structural characterization of a squid-type DFPase and the second crystal structure of a PTE determined to date. Therefore, it may serve as a structural model for squid-type DFPases in general. The overall structure of this protein represents a six-fold beta propeller with two calcium ions bound in a central water-filled tunnel. The consensus motif found in the blades of this beta propeller has not yet been observed in other beta propeller structures. Based on the results obtained from mutants of active-site residues, a mechanistic model for the DFP hydrolysis has been developed.

Insights into the reaction mechanism of the diisopropyl fluorophosphatase from Loligo vulgaris by means of kinetic studies, chemical modification and site-directed mutagenesis

Kinetic measurements, chemical modification and site-directed mutagenesis have been employed to gain deeper insights into the reaction mechanism of the diisopropyl fluorophosphatase (DFPase) from Loligo vulgaris. Analysis of the kinetics of diisopropyl fluorophosphate hydrolysis reveals optimal enzyme activity at pH >/=8, 35 degrees C and an ionic strength of 500 mM NaCl, where k(cat) reaches a limiting value of 526 s(-1). The pH rate profile shows that full catalytic activity requires the deprotonation of an ionizable group with an apparent pK(a) of 6.82, DeltaH(ion) of 42 kJ/mol and DeltaS(ion) of 9.8 J/mol K at 25 degrees C. Chemical modification of aspartate, glutamate, cysteine, arginine, lysine and tyrosine residues indicates that these amino acids are not critical for catalysis. None of the six histidine residues present in DFPase reacts with diethyl pyrocarbonate (DEPC), suggesting that DEPC has no accessibility to the histidines. Therefore, all six histidine residues have been individually replaced by asparagine in order to identify residues participating in catalysis. Only substitution of H287 renders the enzyme catalytically almost inactive with a residual activity of approx. 4% compared to wild-type DFPase. The other histidine residues do not significantly influence the enzymatic activity, but H181 and H274 seem to have a stabilizing function. These results are indicative of a catalytic mechanism in which H287 acts as a general base catalyst activating a nucleophilic water molecule by the abstraction of a proton.

Role of calcium ions in the structure and function of the di-isopropylfluorophosphatase from Loligo vulgaris

Di-isopropylfluorophosphatase (DFPase) is shown to contain two high-affinity Ca(2+)-binding sites, which are required for catalytic activity and stability. Incubation with chelating agents results in the irreversible inactivation of DFPase. From titrations with Quin 2 [2-([2-[bis(carboxymethyl)amino]-5-methylphenoxy]-methyl)-6-methoxy-8-[bis(carboxymethyl)-amino]quinoline], a lower-affinity site with dissociation constants of 21 and 840 nM in the absence and the presence of 150 mM KCl respectively was calculated. The higher-affinity site was not accessible, indicating a dissociation constant of less than 5.3 nM. Stopped-flow experiments have shown that the dissociation of bound Ca(2+) occurs in two phases, with rates of approx. 1.1 and 0.026 s(-1) corresponding to the dissociation from the low-affinity and high-affinity sites respectively. Dissociation rates depend strongly on temperature but not on ionic strength, indicating that Ca(2+) dissociation is connected with conformational changes. Limited proteolysis, CD spectroscopy, dynamic light scattering and the binding of 8-anilino-1-naphthalenesulphonic acid have been combined to give a detailed picture of the conformational changes induced on the removal of Ca(2+) from DFPase. The Ca(2+) dissociation is shown to result in a primary, at least partly reversible, step characterized by a large decrease in DFPase activity and some changes in enzyme structure and shape. This step is followed by an irreversible denaturation and aggregation of the apo-enzyme. From the temperature dependence of Ca(2+) dissociation and the denaturation results we conclude that the higher-affinity Ca(2+) site is required for stabilizing DFPase's structure, whereas the lower-affinity site is likely to fulfil a catalytic function.

High-yield expression, purification, and characterization of the recombinant diisopropylfluorophosphatase from Loligo vulgaris

Organophosphate degrading enzymes are of great interest in light of their ability to detoxify chemical warfare agents. The diisopropylfluorophosphatase (DFPase) from Loligo vulgaris is characterized by its high stability and broad substrate specifity. Here we report the production of large amounts of active, recombinant DFPase using an Escherichia coli expression system. The enzyme was purified to homogeneity using a combination of immobilized metal affinity and ion exchange chromatography. CD-spectroscopy indicates a well folded protein with a high amount of beta-sheet structure. Limited proteolysis was used to gain a deeper insight into the structural organization of the protein. DFPase possesses an internal protease-sensitive region located between amino acids 146 and 149. The two proteolytic fragments remain as a tight complex retaining a DFPase activity comparable to the intact enzyme. Overexpression clones for each fragment were constructed with the expression resulting in the formation of inclusion bodies. Upon isolation and refolding active protein is only formed when both fragments are present. Thus, the two proteolytic fragments are probably part of a stable single-domain protein with multiple contacts between them and only one protease accessible surface loop.

Crystallization and preliminary X-ray crystallographic analysis of DFPase from Loligo vulgaris

'Squid-type' diisopropylfluorophosphatases (DFPases), a subclass of the phosphotriesterases, are enzymes capable of hydrolysing organophosphorus nerve agents. To date, no three-dimensional structure of a 'squid-type' DFPase is known. Here, the crystallization of the DFPase originally isolated from head ganglion of the squid Loligo vulgaris is reported. The protein has been heterologously expressed in Escherichia coli, purified to homogeneity and subsequently crystallized. The protein crystals belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 43.1, b = 82.1, c = 86.6 A and one monomer per asymmetric unit. Under cryoconditions (120 K) the crystals diffracted beyond 2.0 A using a Cu rotating-anode X-ray generator.

Purification and properties of soman-hydrolyzing enzyme from human liver

A soman-hydrolyzing enzyme (somanase) was purified from human liver. The human somanase is capable of hydrolyzing pinacolyl methylphosphonofluoridate (soman), diisopropylphosphorofluoridate (DFP), and ethyl-N-dimethyl phosphoramidocyanidate (Tabun) with P-F or P-CN bonding, but not ethyl (S-2-diisopropylaminoethyl) methylphosphonothiolate (VX) and diethyl-p-nitro-phosphenylphosphate (paraoxon) with P-S or P-O bonding. The somanase has been purified 1570-fold with a specific activity of 41.4 mumol/min/mg protein. Its molecular weight is around 58 kDa determined by SDS-PAGE. The somanase could be stimulated by the divalent cations Mn+2, Mg+2, and Co+2, where CO+2 activation is the highest. The requirement of disulfide bonds for the enzyme activity was demonstrated by the inhibition effect of DTT.

Characterization of a soluble mouse liver enzyme capable of hydrolyzing diisopropyl phosphorofluoridate

A novel mouse liver soluble fraction DFPase which has organophosphatase activities with sarin, soman and tabun, was purified and characterized. However, it lacks paraoxonase and arylesterase activities with paraoxon and phenyl acetate, respectively. This DFPase closely resembles and may be identical with the one purified by Little et al. in 1989 from the soluble fraction of rat liver, based on its substrate specificity, size (approximately 39 kDa) and its stimulation by several metal ions, namely magnesium, manganese and cobalt. Sequencing of our purified mouse liver DFPase showed it to be identical in its amino acid sequence with the recently identified senescence marker protein-30 (SMP-30) by Fujita et al. in 1996. Other senescence marker proteins possessing high structural homology with the mouse SMP-30 have also been found and sequenced from human and rat livers. There is no structural homology between the senescence marker protein family and the group of mammalian paraoxonases. Thus, it is clear that there are at least two distinct, unrelated families of mammalian liver enzymes that share DFPase activity.

Organophosphorus acid anhydride hydrolase activity in human butyrylcholinesterase: synergy results in a somanase

Organophosphorus acid anhydride (OP) "nerve agents" are rapid, stoichiometric, and essentially irreversible inhibitors of serine hydrolases. By placing a His near the oxyanion hole of human butyrylcholinesterase (BChE), we made an esterase (G117H) that catalyzed the hydrolysis of several OP, including sarin and VX [Millard et al. (1995) Biochemistry 34, 15925-15930]. G117H was limited, however, because it was irreversibly inhibited by pinacolyl methylphosphonofluoridate (soman); soman is among the most toxic synthetic poisons known. This limitation of G117H has been overcome by a new BChE (G117H/E197Q) that combines two engineered features: spontaneous dephosphonylation and slow aging (dealkylation). G117H/E197Q was compared with the single mutants BChE G117H and E197Q. Each retained cholinesterase activity with butyrylthiocholine as substrate, although kcat/Km decreased 11-, 11- or 110-fold for purified G117H, E197Q, or G117H/E197Q, respectively, as compared with wild-type BChE. Only G117H/E197Q catalyzed soman hydrolysis; all four soman stereoisomers as well as sarin and VX were substrates. Phosphonylation and dephosphonylation reactions were stereospecific. Double mutant thermodynamic cycles suggested that the effects of the His and Gln substitutions on phosphonylation were additive for PSCR or PRCR soman, but were cooperative for the PSCS stereoisomer. Dephosphonylation limited overall OP hydrolysis with apparent rate constants of 0.006, 0.077, and 0.128 min-1 for the PR/SCR, PSCS, and PRCS soman stereoisomers, respectively, at pH 7.5, 25 degrees C. We conclude that synergistic protein design converted an archetypal "irreversible inhibitor" into a slow substrate for the target enzyme.

Polyclonal antibody generation in rabbit by administration of an organophosphorus acid anhydrolase (OPAA) from squid

When a nerve gas hydrolyzing enzyme [organophosphorus acid anhydrolase (OPAA), formerly DFPase] purified from squid hepatopancreas was injected into rabbits, the resulting sera (RAS) inhibited OPAA purified from either squid hepatopancreas or squid optic ganglia. The inhibition was non-competitive, with 50% inhibition at a 1:1,000 serum dilution, and with the limit of inhibition (in effect, a "titer") at approximately 1:10,000. This RAS did not inhibit the distinctly different OPAAs from a mammalian and two bacterial sources. The hepatopancreas-generated RAS also reacted positively to the appropriate enzyme-linked immunosorbent assay (ELISA) at a titer of 1:100,000. In marked contrast, when OPAA purified from squid optic ganglion was injected into rabbits, the resulting sera did not inhibit squid OPAA, and did not give a positive ELISA. Control sera taken from the same rabbits prior to any injection (RS) did not inhibit the OPAAs. These results show another major difference between squid type OPAAs and the OPAAs from other sources, sometimes termed "Mazur type" OPAAs.

Purification and properties of a diisopropyl-fluorophosphatase from squid Todarodes pacificus steenstrup

A diisopropyl-fluorophosphatase (DFPase) was purified from brain and ganglia of squid Todarodes pacificus steenstrup. The DFPase had a preference in hydrolysis toward diisopropylphosphorofluoridate (DFP). It also was able to hydrolyze O-1,2,2-trimethylpropyl methylphosphofluoridate (soman) and O-isopropyl methylphosphonofluoridate (sarin) at nearly equal hydrolytic rates but only 1/10 that of DFP. The hydrolytic activity toward diethyl-p-nitrophenylphosphate (paraoxon) was very low compared with DFP, soman, and sarin. The DFPase was purified 330-fold to a specific activity of 18,300 n mol/min/mg protein. Its molecular weight was 34,000 dalton determined by gel-filtration chromatography. Mn2+ stimulation of the DFPase was not observed when DFP and soman were the substrates, but with sarin, the rate increased onefold in the presence of 1.0 mM of Mn2+. Ethylenediamine tetraacetic acid disodium (EDTA-Na2) at 0.05 M inhibited the DFPase activity about 30%. It could be concluded that this DFPase belongs to the squid-type DFPase.

The encapsulation of squid diisopropylphosphorofluoridate-hydrolyzing enzyme within mouse erythrocytes

This study describes the entrapment of squid-type diisopropylphosphorofluoridate-hydrolyzing enzyme (DFPase) within mouse red blood cells. These erythrocytes thereby gain the ability to rapidly hydrolyze alkylphosphate cholinesterase (ChE) inhibitors such as diisopropyl fluorophosphate (DFP). DFPase rapidly hydrolyzes DFP to diisopropyl phosphate. Resealed erythrocytes provide a stable carrier system that can preserve the activity of encapsulated enzymes against otherwise rapid in vivo degradation; thus, ChE inhibitors can be degraded to relatively nontoxic metabolites by these erythrocyte carriers. Squid DFPase was purified from the hepatopancreas of Atlantic squid and DFPase activity was determined by measuring changes in fluoride ion concentration using a fluoride ion selective electrode. Mouse erythrocytes in suspension with excess squid DFPase were dialyzed against hypotonic buffer to allow the encapsulation of the enzyme to occur. Cells were then resealed by returning the suspension to isosmotic with saline. Rate of DFP hydrolysis observed with these cells was much greater than the rate of nonenzymatic hydrolysis and was directly proportional to the amount of the erythrocyte suspension added to the assay solution. The rate of hydrolysis was first order in substrate. Erythrocyte controls showed no endogenous DFPase activity. These results suggest that enzyme entrapment may be developed as a method to prevent and antagonize organophosphate poisoning.

The classification of esterases which hydrolyse organophosphates: recent developments

In the IUB classification of 1984, enzymes which hydrolyse paraoxon and other organophosphorous triesters were included in the category of arylesterases--enzymes which hydrolyse phenylacetate (EC 3.1.1.2). With the discovery that some forms of paraoxonase do not hydrolyse phenylacetate, a new entry was made in the revised classification of 1989, Aryldialkylphosphatase (EC 3.1.8.1) under phosphoric triester hydrolases (EC 3.1.8), to distinguish these enzymes from arylesterases. Also some enzymes that hydrolyse phenylacetate do not hydrolyse paraoxon, whereas other enzymes do. Additionally, there is growing evidence for the existence of a number of enzymes which hydrolyse P-F or P-CN bonds of organophosphorous diesters e.g., the nerve gases tabun and soman. These enzymes are in effect organophosphorous acid anhydrolases, and it has been proposed that the earlier entry of (EC 3.8.2.1) now be deleted, and a new entry diisoprophylfluorophosphatase (EC 3.1.8.2) put in its place. Within this category, there is evidence of several enzymes showing different substrate specificities, and different requirements for divalent cations as cofactors, which presents further problems of classification and nomenclature.

Requirements for extracellular metabolism of pulmonary surfactant: tentative identification of serine protease

Pulmonary alveolar surfactant is secreted by the alveolar epithelium in the form of lamellar bodylike structures that evolve sequentially into tubular myelin and vesicular forms that can be separated by centrifugation. Using an in vitro procedure by which the extracellular metabolism of pulmonary surfactant can be mimicked, namely cyclic variation in surface area, we previously reported that serine protease activity, which we called "convertase," was required for the conversion of tubular myelin to the vesicular form. In the present studies we explored the biochemical requirements of this activity and sought the enzyme in alveolar products. Convertase activity has unusual requirements; in addition to being dependent on repetitive variations in surface area (cycling), it requires the presence of a high g fraction of lung secretions that is heat stable and not inhibitable by diisopropyl fluorophosphate (DFP) or alpha 1-antitrypsin, both typical serine protease inhibitors. The enzyme does not require calcium ions and has a pH optimum of 7.4. Convertase appears to be a component of surfactant itself because the ability of purified surfactant to convert to the vesicular form on cycling is impaired by pretreating it with DFP. A protein of Mr 75,000 that reacts with DFP and is heat sensitive was found in alveolar lavage, lamellar body preparations, and lung homogenate. It copurifies with lung surfactant in sucrose gradients. A similar DFP-reactive protein was observed in stable human neoplastic peripheral airway cell lines that express type II properties, suggesting that it may be a product of type II cells. We tentatively conclude that surfactant convertase is a 75,000 serine protease that is closely associated with surfactant phospholipid and that may be a product of alveolar type II cells.

Inhibition of cholinesterases by DRP and induction of organophosphate-detoxicating enzymes in rats

1. The inhibition of cholinesterase and carboxylesterase activities in the diisopropyl fluorophosphate (DFP) intoxication, and the inducibility of organophosphate (OP) detoxicating enzymes was studied in rats. 2. In phenobarbital (PB)-, but not in beta-naphthoflavone (NF)-pretreated rats, the activities of DFP-inhibited cholinesterases were 70-120% higher than in non-pretreated rats. Also the inhibition of the microsomal and cytosolic carboxylesterase activity in liver was efficiently antagonized by BP, but not by NF. 3. In vitro the microsomes from PB-treated rats detoxicated DFP probably by O-dealkylation, since no fluoride was released from DFP. Glutathione S-transferase did not detoxicate DFP. 4. 7-Pentoxyresorufin O-dealkylase, a specific enzyme of cytochrome P450IIB subfamily, was induced by PB, flumecinol, isosafrole and NF by 167- 61-, 26- and 1.6-fold, respectively. 7-Ethoxyresorufin O-deethylase, a marker enzyme of cytochrome P450IA subfamily, was induced by those agents 5-, 4-, 31- and 94-fold, given in the same order. Glutathione S-transferase, paraoxonase and DFPase activities were increased 0-72% by the tested inducers. 5. The results suggest that the cytochrome P450IIB subfamily, inducible by PB, participates in DFP detoxication by O-dealkylation. Its induction probably causes the protection against the cholinesterase inhibition by OPs.

Organofluorophosphate-hydrolyzing activity in an estuarine clam, Rangia cuneata

1. The bivalve Rangia cuneata can enzymatically detoxify the organophosphorus acetylcholinesterase inhibitors DFP and soman. 2. Digestive gland homogenates contained Mazur-type DFPases based on response to Mn2+ ions, and relative rates of DFP: soman hydrolysis. Squid-type DFPase contributed little to the total organophosphate acid (OPA) anhydrase activity of these preparations. 3. The natural substrate(s) and physiological role(s) of OPA anhydrase in R. cuneata has yet to be determined; however, DFPase specific activity was pronounced in the digestive gland, the primary organ involved in bioconcentration and biotransformation of xenobiotics, and in the gills, which are in continuous contact with water-borne chemicals.

Immunohistochemical investigation on the hog kidney di-isopropyl-fluorophosphate fluorohydrolase (E.C.3.8.2.1) in hog kidney and heart

Using a polyclonal antibody raised in rabbits against the highly purified di-isopropyl-fluorophosphate fluorohydrolase (DFPase, E.C. 3.8.2.1) of hog kidney, DFPase-immunoreactivity could be demonstrated by the unlabelled antibody peroxidase-antiperoxidase immunohistochemical method in the hog kidney in the brush border of epithelial cells in the proximal segment of the nephron. In the heart DFPase-immunoreactivity was found at the plasma membranes of the cardiac muscle cells.

Bacterial detoxification of diisopropyl fluorophosphate

The ability of 18 gram-negative bacterial isolates to detoxify diisopropyl fluorophosphate, a structural analog of the agents soman and sarin, was investigated. Detoxification by both frozen cell sonicates and acetone powders was assayed by two methods, i.e., the hydrolytic release of fluoride, measured by a fluoride-specific ion electrode, and the disappearance of acetylcholinesterase inhibition in vitro. Frozen cell sonicates for all strains exhibited some activity (F- ion release). In general, acetone powder preparations produced higher activity than frozen cell sonicates did, and the highest activities were exhibited by strains with known parathion hydrolase activity. Two ranges in activity were observed, low level, ranging from 0.1 to 7.0 mumol/min per g of protein, and high level, detected only in parathion hydrolase-producing strains, from 47 to greater than 300 mumol/min per g of protein. Results indicate that parathion hydrolase was nonspecific in phosphoesterase activity. Also, it was an effective detoxicant at low concentrations and near-neutral pH.

The effects of blood flow and detoxification on in vivo cholinesterase inhibition by soman in rats

The in vivo time course of cholinesterase inhibition was measured in brain, lung, spleen, hind limb skeletal muscle, diaphragm, intestine, kidney, heart, liver, and plasma of rats receiving 90 micrograms/kg soman, im. This dose of soman produced severe respiratory depression and transient hypertension, but no significant changes in the cardiac output or heart rate of anesthetized rats. The rate and maximal extent of in vivo cholinesterase inhibition by soman varied widely among the tissues. Although cardiac output was unchanged by soman administration, the blood flow in heart, brain, and lung (bronchial arterial flow and arteriovenous shunts) was increased, whereas blood flow in spleen, kidney, and skeletal muscle was decreased. The relative importance of tissue blood flow, tissue levels of cholinesterase and acetylcholinesterase, and tissue levels of soman-detoxifying enzymes (diisopropyl-fluorophosphatase and carboxylesterase) in determining the in vivo rate and maximal extent of cholinesterase inhibition was examined by multiple regression analysis. The best multiple regression model for the maximal extent of cholinesterase inhibition could explain only 63% of the observed variation. The best multiple regression model for the in vivo rate of cholinesterase inhibition contained three independent variables (blood flow, carboxylesterase, and cholinesterase) and could account for 94% of the observed variation. Of these three variables blood flow was the most important, accounting for 79% of the variation in the in vivo rate of cholinesterase inhibition. This suggests that it may be possible to use a flow-limited physiological pharmacokinetic model to describe the kinetics of in vivo cholinesterase inhibition by soman.

Discovery of multiple organofluorophosphate hydrolyzing activities in the protozoan Tetrahymena thermophila

Recently it has been found that homogenates of Tetrahymena thermophila can hydrolyze the potent acetylcholinesterase inhibitors O,O-diisopropylphosphofluoridate (DFP) and O-1,2,2-trimethylpropylmethylphosphonofluoridate (soman). Upon purification of the DFP hydrolyzing activity 10-fold it had been noted that the soman hydrolyzing activity increased only 2-3 fold. Treatment with manganous ion and comparison of the soman and DFP hydrolysis rates of the homogenate indicated that a mixture of the squid-type and Mazur-type DFPases may be present. Subsequent purification of the enzymatic activities within the Tetrahymena-homogenate demonstrated that there are at least five functioning proteins of molecular weights 67,000 to 96,000. None are directly homologous to the DFPases found in hog kidney or squid. The enzymatic activities are designated DFPase-1 through DFPase-5. A hypothesis is presented that the functions of DFPases are in the normal metabolism of organophosphates naturally synthesized by T. thermophila.

Kinetics of the DFPase activity in Tetrahymena thermophila

Crude homogenates of the ciliate protozoon, Tetrahymena thermophila, can hydrolyze the potent acetylcholinesterase inhibitors O,O-diisopropylphosphorofluoridate (DFP) and O-1,2,2-trimethylpropylmethylphosphonofluoride (soman). Characterization of the enzymatic activity of the homogenate has been performed. The DFPase operates over a pH range of 4 to 10 and an ionic range of 0-500 mM NaCl. Rate of reaction increases three- to four-fold from 25 degrees C to 40 degrees C and is still present at 55 degrees C. These results indicate that the enzymatic activity operates over a broad range of environmental conditions, making it an attractive material for use in the detoxification and detection of organofluorophosphates. DFPases may be important in the metabolism of naturally occurring organophosphates.

The in vitro hydrolysis of diisopropyl fluorophosphate during penetration through human full-thickness skin and isolated epidermis

Skin may play an important role in the detoxification of certain substances during their passage into the body. The degree of hydrolysis of diisopropyl fluorophosphate, DFP, in skin suspensions and during penetration through isolated epidermis and full-thickness skin from humans was investigated in vitro. When isolated sheets of epidermis were used, 11% of the penetrated amount of DFP was hydrolyzed whereas 46% was hydrolyzed during penetration through full-thickness skin. A comparison is made between the degree of hydrolysis during penetration as obtained from direct measurements and that calculated from kinetic data of the enzyme (Kappm and Vappmax), the half-life of DFP, the skin concentration, and the lag time. The concentration of DFP in the skin was not measured, but the concentration of DFP equivalents (DFP and metabolites formed during penetration) was determined at different times. At steady state, the amount of DFP equivalents in the skin corresponded to the amount that had penetrated into the skin during the lag time. This indicates that the penetration rate corresponded to the uptake via the skin and that no diffusion barrier existed between the skin and the receptor medium. It was also found that the concentration in the skin was proportional to the penetration rate, thus indicating that the enzymatic degree of hydrolysis depends upon the penetration rate.

An organofluorophosphate-hydrolyzing activity in Tetrahymena thermophila

An enzymatic activity that hydrolyzes O,O-diisoproplyphosphofluoridate (DFP) and O-1,2,2-trimethylpropylmethylphosphonofluoridate (Soman) was discovered in the ciliate protozoan Tetrahymena thermophila. The enzymatic activity classifies the protein as Mazur-type similar to that found in hog kidney and Escherichia coli. The rate of hydrolysis of Soman by the Tetrahymena-extract is the highest, on a per gram of extract basis, of any eucaryote. The molecular weight is approximately 75,400 as determined by Sephacryl column chromatography. A maximum fifteen-fold purification has been achieved. Potential exists for the detoxification and one-step detection of common organofluorophosphate pollutants. Additionally, Tetrahymena should prove an easier subject for manipulation than mammalian or squid sources. Protozoa may be a potentially important source of detoxification and degradation enzymes for other environmental contaminants.

Inhibition of a soman- and diisopropyl phosphorofluoridate (DFP)-detoxifying enzyme by Mipafox

Mipafox, N,N'-diisopropylphosphordiamidofluoridate, has been found to be a reversible competitive inhibitor of a diisopropyl phosphorofluoridate hydrolyzing enzyme (DFPase) isolated from hog kidney and Escherichia coli. Heretofore, this DFPase was characterized by its more rapid hydrolysis of Soman (1,2,2-trimethylpropyl methylphosphonofluoridate), its stimulation by Mn2+, and its wide distribution. In sharp contrast, Mipafox did not inhibit the DFPase found only in cephalopod nerve, hepatopancreas, and saliva, and further characterized by its more rapid hydrolysis of DFP than of Soman, and its indifference to Mn2+. Neither of these two DFPases hydrolyzed Mipafox.

Two enzymes for the detoxication of organophosphorus compounds--sources, similarities, and significance

An enzyme in E. coli that hydrolyzes diisopropylphosphorofluoridate (DFP) has now been found to hydrolyze the nerve gas 1,2,2- trimethylpropylmethylphosphonofluoridate (soman) many times faster. With either substrate the E. coli enzyme is stimulated manyfold by 10(-3) M Mn2+. These criteria are combined and applied to this, and to a superficially similar but distinctly different, enzyme found in squid nerve. The results suggest that while several tissues of the squid contain only this second kind of DFP hydrolyzing enzyme, termed squid type DFPase , many other sources including E. coli contain a mixture of squid type DFPase (the name not strictly indicative of source) and the other DFP hydrolyzing enzyme, now termed Mazur type DFPase . Procedures for the purification of Mazur type DFPase from hog kidney, while increasing the specific activity for DFP hydrolysis may actually have been enriching the purified material in the squid type DFPase . Because E. coli has the highest soman hydrolyzing capacity of any source so far examined, this organism is a promising source for the development of new purification procedures for Mazur type DFPase .

Characterization of a DFP-hydrolyzing enzyme in squid posterior salivary gland by use of Soman, DFP and manganous ion

1. A phosphorus-fluorine splitting enzyme (DFPase) from squid nerve hydrolyzes DFP 5-10 times faster than it hydrolyzes another P-F compound, Soman, whereas a superficially similar enzyme from rat kidney hydrolyzes Soman 20-40 times faster than it hydrolyzes DFP, all under comparable conditions. 2. The DFPase from rat kidney is stimulated 2- to 3-fold by 4 X 10(-4) M Mn2+, whereas the DFPase from squid nerve is unaffected or slightly inhibited by 4 X 10(-4) M Mn2+. 3. These observations form the basis for distinguishing between a squid type DFPase and a mammalian DFPase, the names not being rigorously indicative of enzyme source or substrate. 4. When these criteria are applied to a P-F splitting enzyme found in squid saliva, the enzyme is identifiable as squid type DFPase. There is a significantly higher level of this enzyme in whole saliva from female squids than in whole saliva from male squids. This squid type DFPase is different from the proteinous toxin also found in squid saliva.

Organophosphate detoxicating hydrolases in different vertebrate species

Phosphorylphosphatase activities in various organs of vertebrate species from different classes were determined using a spectrophotometric assay for paraoxonase (EC 3.1.1.2) and a potentiometric assay with a fluoride sensitive electrode for DFPase (EC 3.8.2.1). Temperature-dependent inactivation experiments, an extended interpretation of mixed substrate studies and activity distribution patterns confirm that in vertebrate tissue at least two different enzymes are responsible for hydrolytic detoxication of paraoxon and DFP. Total organophosphate detoxicating phosphorylphosphatase activity of a certain animal species is shown to be the major determinant for differences between the inhibitory potency of organophosphorus compounds on the animal's target enzymes in vitro and organophosphate toxicity in vivo.