Kinetics of heat inactivation of phenyl valerate hydrolases from hen and rat brain

Human ferredoxin-2 displays a unique conformational change

Human ferredoxin-1 (hFd1) and human ferredoxin-2 (hFd2) share high sequence similarity but serve on distinct cellular pathways. A unique conformational change is observed when holo hFd2 is warmed to physiological temperatures, or higher. Enzymatic studies show that this conformational change causes the increase of affinity between hFd2 and adrenodoxin reductase. No such change was observed for hFd1, which may contribute to the distinct cellular functions of hFd1 and hFd2 under physiological conditions.

Influence of lysophospholipid hydrolysis by the catalytic domain of neuropathy target esterase on the fluidity of bilayer lipid membranes

Neuropathy target esterase (NTE) is an integral membrane protein localized in the endoplasmic reticulum in neurons. Irreversible inhibition of NTE by certain organophosphorus compounds produces a paralysis known as organophosphorus compound-induced delayed neuropathy. In vitro, NTE has phospholipase/lysophospholipase activity that hydrolyses exogenously added single-chain lysophospholipids in preference to dual-chain phospholipids, and NTE mutations have been associated with motor neuron disease. NTE's physiological role is not well understood, although recent studies suggest that it may control the cytotoxic accumulation of lysophospholipids in membranes. We used the NTE catalytic domain (NEST) to hydrolyze palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (p-lysoPC) to palmitic acid in bilayer membranes comprising 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and the fluorophore 1-oleoyl-2-[12-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]dodecanoyl]-sn-glycero-3-phosphocholine (NBD-PC). Translational diffusion coefficients (D(L)) in supported bilayer membranes were measured by fluorescence recovery after pattern photobleaching (FRAPP). The average D(L) for DOPC/p-lysoPC membranes without NEST was 2.44 microm(2)s(-1)+/-0.09; the D(L) for DOPC/p-lysoPC membranes containing NEST and diisopropylphosphorofluoridate, an inhibitor, was nearly identical at 2.45+/-0.08. By contrast, the D(L) for membranes comprising NEST, DOPC, and p-lysoPC was 2.28+/-0.07, significantly different from the system with inhibited NEST, due to NEST hydrolysis. Likewise, a system without NEST containing the amount of palmitic acid that would have been produced by NEST hydrolysis of p-lysoPC was identical at 2.26+/-0.06. These results indicate that NTE's catalytic activity can alter membrane fluidity.

Bovine chromaffin cells in culture show carboxylesterase activities sensitive to organophosphorus compounds

Carboxylesterase activities are widely distributed in a great variety of tissues; however, the biological function of these enzymes remains unclear. Some organophosphorus compounds induce a neurodegenarative syndrome related to the covalent modification of a carboxylesterase known as neuropathy target esterase. We investigated the expression of neuropathy target esterase and related carboxylesterase in bovine chromaffin cells with the aim of developing a potential in vitro model for studying the cellular function of carboxylesterase enzymes and toxic effects of organophosphorus compounds. Total phenyl valerate esterase exhibited an activity of 1.27 +/- 0.19 mU/10(5) cells (SD, n = 15). From the phenyl valerate esterase paraoxon and mipafox inhibition curves the following activities have been determined: B-activity (resistant to 40 microM paraoxon), 1.05 +/- 0.08 mU/10(5) cells (n = 8); C-activity (resistant to 40 microM paraoxon plus 250 microM mipafox), 0.12 +/- 0.05 mU/10(5) cells (n = 8); and neuropathy target esterase, calculated by the difference between B- and C-activities, 0.93 +/- 0.08 mU/10(5) cells (n = 8). All of these activities increased linearly with the number of cells and time of incubation with the substrate. Most of the phenol product of the reaction was released and detected in the extracellular medium. None of the components of the reaction were shown to affect cell viability when assessed by trypan blue exclusion. The study shows that bovine chromaffin cells possess carboxylesterase activities and respond to inhibition by paraoxon and mipafox, thus facilitating the discrimination of neuropathy target esterase. In conclusion, bovine chromaffin cells are appropriate as an in vitro cell model for studying toxic effects of organophosphorus compounds.

Organophosphorus inhibition and heat inactivation kinetics of particulate and soluble forms of peripheral nerve neuropathy target esterase

Neuropathy target esterase (NTE) is the proposed target site for the mechanism of initiation of the so-called organophosphorus-induced delayed polyneuropathy (OPIDP). NTE is operationally defined in this article as the phenylvalerate esterase activity which is resistant to inhibition by 40 microM paraoxon and sensitive to 250 microM mipafox. Soluble (S-NTE) and particulate (P-NTE) forms of NTE had first been identified in hen sciatic nerve [E. Vilanova, J. Barril, V. Carrera, and M. C. Pellín (1990). J. Neurochem., 55, 1258-1265]. P-NTE and S-NTE showed different sensitivities to the inhibition by several organophosphorus compounds over a range of inhibitor concentrations for a 30 or 120 minute fixed inhibition time at 37 degrees C. S-NTE was less sensitive to the inhibition by O,O'-diisopropyl phosphorofluoridate (DFP), hexyl 2,5-dichlorophenyl phosphoramidate (H-DCP), and mipafox than P-NTE and brain NTE, while the opposite was true for O,S-dimethyl phosphoroamidothioate (methamidophos). For each of the four inhibitors assayed, S-NTE showed two components of different sensitivity according to the inhibition curves fitted with exponential models. However, the inhibition of P-NTE by mipafox, DFP, and HDCP did not show the presence of a considerable proportion of a second component. The kinetics of heat inactivation showed that P-NTE inactivated faster and to a greater extent than S-NTE. It is concluded that (1) sciatic nerve S-NTE is more different from brain NTE than P-NTE; (2) P-NTE and S-NTE have different sensitivities to the inhibition by the studied organophosphorous compounds; (3) the inhibition curves suggest that S-NTE has two different enzymatic components while these are not so evident for P-NTE.

Partial characterization of neuropathy target esterase and related phenyl valerate esterases from bovine adrenal medulla

The mechanism by which organophosphorus-induced delayed polyneuropathy is induced relates to the specific inhibition and subsequent modification ("aging") of a protein known as neuropathy target esterase (NTE), operatively defined as paraoxon-resistant and mipafox-sensitive phenyl valerate (PV) esterase activity. This protein has fundamentally been investigated in hen brain, the latter being the habitually employed OPIDP study model. In the present article, a partial characterization is made of the NTE and other related PV esterases in the bovine adrenal medulla and brain; NTE sensitivity to the neurotoxic organophosphorus compound mipafox is investigated, and its subcellular distribution is studied. The NTE activity of the adrenal medulla was found to be the highest of those among the tissues studied to date (5000 +/- 1400 mU/g tissue; +/- SD, n = 12). This activity represented 93% of the PV esterase activity resistant to 40 microM paraoxon in the particulate fraction of the adrenal medulla and approximately 50% of total PV esterase activity. In the bovine brain, these proportions were 72 and 26%, respectively, i.e., similar to those described in hen brain. The mipafox inhibition curve of PV esterase activity resistant to 40 microM paraoxon in the particulate fraction of the adrenal medulla suggests that NTE activity fundamentally comprises a mipafox-sensitive component with an I50 of 6.39 microM at 30 minutes, which is similar to the value reported in hen brain. NTE activity in the bovine adrenal medulla is almost exclusively limited to the particulate fraction, the microsomal fraction, plasma membrane, and chromaffin granule-enriched fractions being the highest in terms of specific activity.(ABSTRACT TRUNCATED AT 250 WORDS)

The pathogenesis of organophosphate polyneuropathy

This review discusses the facts regarding organophosphate-induced delayed polyneuropathy (OPIDP) as they are related to its pathogenesis rather than being a comprehensive review of all available data. Neuropathy target esterase (NTE) is considered to be the molecular target for OPIDP which is affected by several esterase inhibitors. Such inhibitors are ranked according to their toxicological effects as follows: 1. Phosphates, phosphoroamidates, and phosphonates cause OPIDP when high amounts of NTE are inhibited. In most cases 70 to 80% inhibition is enough, whereas in others much more is required. 2. Phosphinates, carbamates, and sulfonyl halides cause either protection from or promotion of OPIDP when given before or after a neuropathic OP, respectively. Both effects are related to doses that inhibit NTE. Neuropathy is also caused by the combined treatment with a carbamate and a sulfonyl fluoride. The potency of a given NTE inhibitor to cause OPIDP is related to the chemistry of the residue left attached to NTE, in addition to its affinity for the enzyme. The capability of inhibited NTE to undergo the aging process distinguishes inhibitors with high from those with negligible or very low potency to cause OPIDP. Therefore, protection from neuropathic doses of effective OPs is obtained when NTE is mostly inhibited with nonageable inhibitors. Promotion of OPIDP is likely to involve another site besides NTE because it might occur when almost all NTE is affected. Promotion affects either progression or expression of OPIDP after the initial biochemical lesion on NTE. Since only NTE inhibitors have been proven to be promoters, it is possible that this site is made available after the initiation of OPIDP and that it may have biochemical properties indistinguishable from those of NTE of naïve birds. Age-related resistance to OPIDP also seems to be related to either progression or expression of OPIDP and/or to the different physiology of NTE at a given age. Previously reported resistance of rats to clinical OPIDP seems also to be age-dependent. The physiological function(s) of NTE is unknown, but some practical gains have been obtained from its identification, including OPIDP risk assessment and biomonitoring.

Organophosphates and delayed neuropathy--is NTE alive and well?

Neuropathy target esterase (NTE) is a membrane-bound protein with high esterase catalytic activity. The physiological function of the protein is not known and the catalytic activity is not essential to health of nerve axons. Nevertheless there is overwhelming evidence that modification of the structure of NTE by covalent binding of some organophosphorus esters initiates an irreversible polyneuropathy: this event can be monitored. The experimental evidence for this conclusion is reviewed and some conceptual objections are resolved. Studies of NTE have generated successful predictions concerning (1) prophylaxis; (2) structure-activity relationships including stereospecificity; (3) the effects of prolonged low-level administration of neurotoxicants; and (4) extrapolations from (a) NTE responses seen after low doses to enzyme and clinical effects seen after high doses, (b) from in vitro to in vivo, and (c) from hen to human responses. The relationship of initiation on NTE to subsequent events in development of neuropathy is considered. Purification of NTE is reaching the point where antibodies may be obtained for neurobiological study. No single rigid protocol can be devised for incorporation of NTE assays into toxicological evaluations. A proposed two-stage procedure requires interpretation of Stage 1 to influence the design of Stage 2.

In vivo and in vitro regional differential sensitivity of neuropathy target esterase to di-n-butyl-2,2-dichlorovinyl phosphate

Organophosphate-induced delayed polyneuropathy (OPIDP) is initiated by inhibition/aging of more than 70-75% of neuropathy target esterase (NTE). Di-n-butyl-2,2-dichlorovinyl phosphate (DBDCVP) (1 mg/kg s.c.) inhibited 96%, 86% and 83% of NTE in brain, spinal cord and peripheral nerve, respectively, and induced a typical central peripheral distal axonopathy in hens. A lower dose (0.45 mg/kg s.c.) caused 90%, 83% and 54% NTE inhibition in the same organs; by contrast, hens developed a spastic ataxia with axonal degeneration in spinal cord but not in peripheral nerve. With a dose of 0.2 mg/kg s.c., a suprathreshold inhibition of NTE was produced in brain (78%) but not in spinal cord (56%) and peripheral nerve (33%) and no morphological or clinical signs of neuropathy developed in hens. With doses up to 4.0 mg/kg s.c., acetylcholinesterase (AChE) inhibition was similar throughout the nervous system. In vitro time-course inhibition studies showed a different sensitivity to DBDCVP of NTE from peripheral nerve (ka = 5.4 x 10(6)) relative to that from spinal cord (ka = 13.9 x 10(6)) or brain (ka = 20.6 x 10(6)). In vitro I50s of DBDCVP for AChE were similar in brain, spinal cord and peripheral nerve (11-17 nM). These data support the hypothesis that the critical target for initiation of OPIDP is located in the nerve fiber, possibly in the axon and also suggest that peripheral nerve NTE has a different sensitivity to DBDCVP than the brain enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Prophylaxis and the mechanism for the initiation of organophosphorous compound-induced delayed neurotoxicity

Recent work concerned with the mechanism underlying the development of organophosphorous compound-induced delayed neurotoxicity (OPIDN) is reviewed. Topics covered include the prophylaxis of OPIDN by phenylmethylsulfonyl fluoride and other agents, neurotoxic esterase (NTE) as measured using either labelled di-isopropyl phosphorofluoridate or an esterase assay, and the relationship between NTE and the development of OPIDN. There is considerable evidence that NTE has the biochemical properties which should be expected for the initiation site for OPIDN. However, the in vitro assays as currently performed may not entirely reflect the behavior of organophosphorous compounds in vivo, or the assays may not be sensitive enough to identify the actual target. It is argued that prophylaxis is a distinguishing characteristic of OPIDN which is not necessarily related to NTE inhibition, although it does provide evidence that NTE is involved. It is concluded that the NTE hypothesis could be furthered by additional studies with peripheral nerve, more sensitive methods for the detection of potential binding sites, and the establishment of a physiological role for NTE which relates it to the neuropathy.