Inhibition and aging of neuropathy target esterase by the stereoisomers of a phosphoramidate related to methamidophos

Cu2+-dependent hydrolysis of O-hexyl 2,5-dichlorophenyl phosphoramidate by reptile sera

This study shows the EDTA-resistant, Ca2+ and Cu2+-dependent hydrolysis of O-hexyl 2,5-dichlorophenyl phosphoramidate (HDCP) compound in reptiles sera determined by spectrophotometry UV/Vis and chiral chromatography. Samples of ten reptile species were incubated with aliquot of 100 or 400 μM HDCP in presence of 100 or 300 μM Cu2+, or 2.5 mM Ca2+ or 5 mM EDTA at 37 °C for 30-60 min. The results shown an activator effect of Cu2+ on HDCP hydrolysis in freshwater turtles sera (Trachemys scripta, Chelydra serpentina and Macrochelys temminckii) because the levels of 2,5-dichlorophenol (DCP; product hydrolysis) were similar (∼37 μM DCP) to chicken serum (positive control group). The marine turtles (Chelonia mydas and Eretmochelys imbricata) and crocodiles (Crocodylusacutus and Crocodylus moreletii) showed ∼50% less HDCPase activity (13-17 μM DCP) compared to the HDCPase activity of the freshwater turtle species. Terrestrial reptile species (snakes and lizards) showed around 25% of activity (7-13 μM DCP) with both copper concentrations. These Cu2+-dependent hydrolysis were stereospecific to R(+)-HDCP (p˂0.05) in the three freshwater turtle species that showed similar hydrolysis to the chicken serum. However, the Ca2+ did not show a significant activating effect on the HDCPase activity (1-8 μM DCP) in any reptile serum. Their hydrolysis levels were very similar to those of EDTA-resistant activity. The present study demonstrates a Cu2+-dependent A-esterase (HDCPase) activity in turtles and points serum albumin as the cuproprotein responsible for this activity, reinforcing its N-terminal sequence (DAEH) as a catalytic center.

DAEH N-terminal sequence of avian serum albumins as catalytic center of Cu (II)-dependent organophosphorus hydrolyzing A-esterase activity

O-hexyl O-2,5-dichlorophenyl phosphoramidate (HDCP) induces delayed neuropathy. The R (+)-HDCP inhibits and caused the so call "aging reaction" on inhibited-NTE. This enantiomer is not hydrolyzed by Ca(II)-dependent A-esterases in mammal tissues but is hydrolyzed by Cu(II)-dependent chicken serum albumin (CSA). With the aim of identifying HDCP hydrolysis by other vertebrate albumins, we incubated albumin with 400 μM racemic HDCP in the presence of 100 μM copper sulfate. HDCPase activity was assessed by measurement of HDCP with chiral chromatography. Human, sheep, dog, pig, lamprey or cobra serum albumin did not show a significant activity (~10%). Rabbit and bovine albumins hydrolyzed both enantiomers of HDCP (25% and 50% respectively). Turkey serum albumin had more HDCPase activity (~80 μM remaining) than the chicken albumin (~150 μM remaining). No animal albumins other than chicken showed stereoselective hydrolysis. Preincubation of chicken albumin with 1 mM the histidine modifying agents, 100 μM N-bromosuccinimide (NBS) and Zn(II), inhibited its Cu(II)-dependent R (+)-HDCPase activity, where as other mM amino acids modifiers had no inhibitory effects. . These results confirm that the stereoselective hydrolysis of (+)-HDCP is a specific A-esterase catalytic property of chicken albumin. The higher HDCPase activity by turkey albumin suggests the amino-terminal sequence of avian albumins (DAEHK) is the active center of this Cu(II)-dependent A-esterase activity.

Biochemical, histopathological and clinical evaluation of delayed effects caused by methamidophos isoforms and TOCP in hens: ameliorative effects using control of calcium homeostasis

This work evaluated the potential of the isoforms of methamidophos to cause organophosphorus-induced delayed neuropathy (OPIDN) in hens. In addition to inhibition of neuropathy target esterase (NTE) and acetylcholinesterase (AChE), calpain activation, spinal cord lesions and clinical signs were assessed. The isoforms (+)-, (±)- and (-)-methamidophos were administered at 50mg/kg orally; tri-ortho-cresyl phosphate (TOCP) was administered (500mg/kg, po) as positive control for delayed neuropathy. The TOCP hens showed greater than 80% and approximately 20% inhibition of NTE and AChE in hen brain, respectively. Among the isoforms of methamidophos, only the (+)-methamidophos was capable of inhibiting NTE activity (approximately 60%) with statistically significant difference compared to the control group. Calpain activity in brain increased by 40% in TOCP hens compared to the control group when measured 24h after dosing and remained high (18% over control) 21 days after dosing. Hens that received (+)-methamidophos had calpain activity 12% greater than controls. The histopathological findings and clinical signs corroborated the biochemical results that indicated the potential of the (+)-methamidophos to be the isoform responsible for OPIDN induction. Protection against OPIDN was examined using a treatment of 2 doses of nimodipine (1mg/kg, i.m.) and one dose of calcium gluconate (5mg/kg, i.v.). The treatment decreased the effect of OPIDN-inducing TOCP and (+)-methamidophos on calpain activity, spinal cord lesions and clinical signs.

An alternative in vitro method for detecting neuropathic compounds based on acetylcholinesterase inhibition and on inhibition and aging of neuropathy target esterase (NTE)

Organophosphorus-induced delayed polyneuropathy (OPIDP) is a syndrome induced by certain organophosphorus compounds (OPs) through a mechanism based on the inhibition and further modification (aging) of neuropathy target esterase (NTE). OECD guidelines for testing the capability of OPs to trigger OPIDP include two in vivo tests with hens. Activities of acetylcholinesterase and NTE found in SH-SY5Y human neuroblastoma cells were inhibited by 10 different OPs with kinetics similar to those found with chicken brain enzymes (model system for in vivo and in vitro-ex vivo assays). NTE in SH-SY5Y cells inhibited by these OPs aged and reactivated similarly to that described for hen brain NTE ex vivo. In short, we have developed an alternative methodology for predicting the capability of OPs to induce OPIDP based on the inhibition kinetics of acetylcholinesterase and NTE and on the capability of OPs to age the inhibited NTE from SH-SY5Y cell line. The results obtained always agreed with the previously reported ex vivo results with hen brain. The developed methodology correctly predicted the neuropathic potential of the tested OPs in eight cases. The in vivo-in vitro discrepancies with two of the tested compounds can be explained on the basis of differences between in vivo and in vitro biotransformation.

Stereospecific hydrolysis of a phosphoramidate as a model to understand the role of biotransformation in the neurotoxicity of chiral organophosphorus compounds

Calcium-dependent and EDTA-resistant hydrolyses of R and S isomers of O-hexyl O-2,5-dicholorophenyl phosphoramidate (HDCP) were observed in serum and subcellular fractions of liver, kidney and brain from hen, rat and rabbit. In serum, the Ca(2+)-dependent hydrolysis was much higher in rabbit than in other species. Liver showed a higher activity than kidney and brain. The S-HDCP isomer was hydrolysed to a higher extent than the other isomer. The fact that this stereospecificity favours the S-isomer is more clearly observed in rabbit serum, and in rat and rabbit liver particulate fractions. In such tissues and species, the EDTA-resistant hydrolysis was not stereospecific. Soluble fractions of rat brain and of hen liver, kidney and brain, showed a lower total activity but with a higher proportion of EDTA-resistant activity and a higher hydrolysis of the R-HDCP isomer. The Ca(2+)-dependent stereoselective biodegradation of S-HDCP is dominant in the most active tissues in rabbit and rat. It can therefore be concluded that S-HDCP would be biodegraded faster than R-HDCP. Furthermore, R-HDCP is the isomer that will remain at a higher proportion to be available for interaction with the target of neurotoxicity.

Recovery of neuropathy target esterase activity after inhibition with mipafox and O-hexyl O-2,5-dichlorophenyl phosphoramidate in bovine chromaffin cell cultures

Neuropathy target esterase (NTE) is a membrane protein present in various tissues whose physiological function has been recently suggested to be the maintenance of phosphatidylcholine homeostasis. Inhibition and further modification of NTE by certain organophosphorus compounds (OPs) were related to the induction of the "organophosphorus induced delayed neuropathy". Bovine chromaffin cells were cultured at 75,000cells/well in 96-well plates and exposed to 25microM mipafox or 3microM O-hexyl O-2,5-dichlorophenyl phosphoramidate (HDCP) for 60min. Inhibitors were removed by washing cells three times with Krebs solution. Then NTE activity was assayed at 0, 24, 48 and 120h after exposure using the Biomek 1000 workstation. Immediately after mipafox treatment NTE activity represented 3% of the control (6.7+/-1.9mU/10(6) cells). At 24, 48 and 120h after removing inhibitor, recorded activities were 33%, 42% and 111% of their respective controls (5.7+/-3.1; 5.7+/-1.9; 5.4+/-0.0mU/10(6) cells, respectively). Treatment with HDCP also displayed a time-dependent pattern of NTE recovery. As NTE inhibited by phosphoramidates is not reactivated in homogenized tissues, these results confirm a time-dependent regeneration of NTE after inhibition by neuropathic OPs.

Enzymes involved in the detoxification of organophosphorus, carbamate and pyrethroid insecticides through hydrolysis

The most employed insecticides for indoor and agriculture purposes belong to carbamates, pyrethroid or organophosphates. The chemical structures of these three groups correspond to carbamic, carboxylic and triphosphoric esters. Technical monographs suggest that the hydrolysis of ester bonds of carbamates and pyrethroids plays an important role in the detoxification of these compounds. However, detailed studies about enzymes hydrolysing carbamates and pyrethroids in vertebrates are not available. Certain carbamate hydrolysing activities are associated to serum albumin. Phosphotriesterases, being of an unknown physiological role, hydrolyse (in some cases stereospecifically) organophosphorus insecticides (OP). Phosphotriesterases have been found in a multitude of species, from mammals to bacteria. A phosphotriesterase activity, EDTA-resistant, has been detected in serum albumin. Phosphotriesterases in serum of mammals display polymorphisms. Phosphotriesterases offer applications in therapy of organophosphorus poisonings, in biodegradation and bioremedation of organophosphates. Similar studies should be developed with enzymes hydrolysing pyrethroids and carbamate insecticides. Such studies will improve the knowledge of the detoxification routes in non-target species and will help to design specific and safer carbamate and pyrethroid insecticides.

Dichlorophenyl phosphoramidates as substrates for avian and mammalian liver phosphotriesterases: activity levels, calcium dependence and stereospecificity

The present study shows the existence of both Ca2+-dependent and EDTA-resistant hydrolysing activities against HDCP and paraoxon in the particulate and soluble fractions of hen, rat and rabbit liver. HDCP was more extensively hydrolysed than paraoxon in both subcellular fractions and each of three individuals of the three animal species under study in spite of wide interindividual variations. However the ratio of HDCP versus paraoxon hydrolysing activity (HDCPase/paraoxonase), although within the same order of magnitude, cannot be considered as constant as it ranges one- to seven-fold between individuals of the same species. Also there is no constant ratio of Ca2+-dependent/EDTA-resistant activities. Rabbit liver showed the highest rates of Ca2+-dependent hydrolysis for both organophosphorus compounds whereas the hen paraoxonase activity was not inhibited by EDTA. The stereospecific hydrolysis of HDCP was mostly a Ca2+-dependent one, the S-HDCP isomer being hydrolysed faster than the R-HDCP one. The suggestion is made that HDCP could be conveniently used to measure PTE activity in the liver.

The role of phosphotriesterases in the detoxication of organophosphorus compounds

The enzymes that hydrolyze organophosphorus compounds are called phosphotriesterases. The presence of phosphotriesterases has been described in a variety of tissues. The physiological role of these enzymes is not known, although a clear correlation exists between the levels of phosphotriesterases and susceptibility of the species to the toxic effects of organophosphorus compounds. Thus, mammals that possess high levels of phosphotriesterases in serum and liver are more tolerant to the toxic effects of these compounds than birds and insects - these being species considered lacking of phosphotriesterases. Because most of these enzymes are not well characterized, they are usually differentiated according to their different patterns of response to activators and/ or inhibitors. Phosphotriesterases usually depend of divalent cations and therefore EDTA usually inhibits them. A peculiar EDTA-resistant phosphotriesterase has been described in serum albumin. The biotechnological and therapeutical applications of phosphotriesterases are currently subject to study.