Enzyme Concentration as an Important Factor in the In Vitro Testing of the Stereospecificity of the Enzymatic Hydrolysis of Organophosphorus Compounds

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.

Chiral and isotope analyses for assessing the degradation of organic contaminants in the environment: Rayleigh dependence

The Rayleigh equation is frequently used to describe isotope fractionation as a function of conversion. In this article we propose to draw a parallel between isotope and enantiomeric enrichments and derive a set of conditions that allow the use of the Rayleigh approach to describe the enantiomeric enrichment-conversion dependencies. We demonstrate an implementation of the Rayleigh equation for the enantioselective enzymatic hydrolysis of Mecoprop-methyl, Dichlorprop-methyl, and dimethyl-methylsuccinate by lipases from Pseudomonas fluorescens, Pseudomonas cepacia, and Candida rugosa. The data obtained for all the studied reactions gave good fits to the Rayleigh equation, with a linear regression R(2) > 0.96. In addition to that, our analysis of four microcosm studies on the hydrolysis of the individual enantiomers of Dichloroprop methyl, Lactofen, Fenoxaprop-ethyl, and Metalaxyl reported in the literature by other research groups revealed a suitability of the Rayleigh dependence. Two dimensional plots describing the isotope fractionation versus enantiomeric enrichment are demonstrated for all studied cases. Processes not accompanied by enantiomeric enrichment (acid and base hydrolysis) and by isotope enrichment (transesterification) are demonstrated, their 2-D plots are either horizontal or vertical which can illuminate concealed degradation pathways.

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.

Comparative hydrolysis of O-hexyl O-2,5-dichlorophenyl phosphoramidate and paraoxon in different tissues of vertebrates

The Ca(2+)-dependent and EDTA-resistant hydrolysis of O-hexyl O-2,5-dichlorophenyl phosphoramidate (HDCP) and paraoxon was studied in serum and subcellular fractions of liver, kidney and brain of hen, rat and rabbit. HDCP was the best substrate among all the tissues studied, except that of rabbit serum which showed the highest Ca(2+)-dependent paraoxon hydrolysing activity (paraoxonase). High HDCP hydrolysing activity (HDCPase) was detected in the brain tissue of the three species studied, whereas low or no paraoxonase was found. The HDCPase/paraoxonase ratio of Ca(2+)-dependent hydrolysing activities ranged from 0.5 to 83 for tissues of the same species. EDTA-resistant HDCPase activity was more than 50% of the total activities in hen tissues, with an almost undetectable Ca(2+)-dependent paraoxonase activity in most organs. The same response was observed in rat tissues, except for serum where the Ca(2+)-dependent HDCPase and paraoxonase activities were higher (70 and 25% of total activities, respectively). EDTA-resistant HDCPase and paraoxonase activities represented less than 25% of all activities in rabbit tissues. Paraoxon has traditionally been the substrate for measuring organophosphorus hydrolysing activities. However, HDCP could be a good substrate in addition to paraoxon for monitoring other phosphotriesterases in biological tissues.

Influence of kaolinite on chiral hydrolysis of methyl dichlorprop enantiomers

The effect of kaolinite on the enzymatic chiral hydrolysis of methyl dichlorprop enantiomers ((R,S)-methyl-2-(2,4- dichlorophenoxy) propanoic acid, 2,4-DPM) was investigated using chiral gas chromatography. Compared with the control without kaolinite, the enantiomeric ratio (ER) increased from 1.35 to 8.33 and the residual ratio of 2,4-DPM decreased from 60.89% to 41.55% in the presence of kaolinite. Kaolinite likely had emotion influence on lipase activity and its enantioselectivity. Moreover, the amount of kaolinite added was also found to be a sensitive factor affecting the enantioselective hydrolysis of 2,4-DPM. Fourier transform infrared (FTIR) spectroscopy studies of the interaction of lipase with kaolinite provided insight into the molecular structure of the complex and offered explanation of the effects of kaolinite on enzymatic hydrolysis of 2,4-DPM. Spectra showed that the effect of kaolinite on the hydrolysis of 2,4-DPM was affected by adsorption of lipase on kaolinite and changes of adsorbed lipase conformation, which led to the modified enantioselectivity.