Elsa Reiner

Dialkylphosphorus metabolites in the urine and activities of esterases in the serum as biochemical indices for human absorption of organophosphorus pesticides

Ninety-seven agricultural workers were monitored for absorption of the organophosphorus pesticides methidathion, vamidothion, and azinphos-methyl, which were sprayed in an orchard during two seasons. Low levels of only one dialkylphosphorus metabolite (dimethyl phosphorothioate) were found in only eight workers in pre-exposure urine samples. More than one dialkylphosphorus metabolite was detected in almost all exposed individuals in after-exposure urine samples. The highest concentrations were measured after exposure to azinphos-methyl; the median concentrations of dimethyl phosphorodithioate and dimethyl phosphorothioate were 0.92 and 0.78 nmol/mg creatinine with a concentration range up to 14.3 and 53.7, respectively. Three diethylphosphorus metabolites were also detected in some samples, but at lower concentrations. Cholinesterase activities were decreased (31–48%) in the serum of 12 workers; four of those workers had no dialkylphosphorus metabolites in the urine. Paraoxonase and arylesterase activities in the serum were unaffected by the absorption of pesticides, and there was no correlation between the activities of these esterases and the metabolite concentrations in the urine. This study confirmed that dialkylphosphorus metabolites in the urine are a more sensitive index of absorption than cholinesterase inhibition in the serum but lack of correlation between cholinesterase inhibition and metabolite concentration indicates that both parameters should be monitored.

Serum paraoxonase and cholineseterase activities in individuals with lipid and glucose metabolism disorders

In patients with hyperlipaemia, serum paraoxonase activities were polymodally distributed with 75% individuals in the low activity mode. In the same patients the distribution of serum cholinesterase activities was unimodal, but asymmetrical. Patients with impaired glucose tolerance or non-insulin-dependent diabetes mellitus had slightly higher cholinesterase activities than patients with hyperlipaemia only.

Differentiation of esterases reacting with organophosphorus compounds.

The hydrolysis of paraoxon (POX), phenylacetate (PA) and beta-naphthylacetate (BNA) was studied in human serum. Based upon correlations between enzyme activities, upon reversible inhibition by EDTA and upon progressive inhibition by iso-OMPA, tabun, eserine and bis-4 nitrophenylphosphate, the following conclusions were drawn about the number and specificity of enzymes involved in the hydrolysis. Two paraxonases hydrolyse paraoxon: one sensitive and the other insensitive to EDTA. The EDTA-sensitive paraoxonase also hydrolysed BNA. The EDTA-insensitive hydrolysis of BNA and PA was attributed to a serine esterase. The EDTA-sensitive hydrolysis of PA is probably due to more than one enzyme, which might be an arylesterase and a carboxylesterase.

Cholinesterases in rabbit serum

1. Rabbit serum was shown to contain two cholinesterases which hydrolysed acetylthiocholine and butyrylthiocholine and one cholinesterase which hydrolysed only butyrylthiocholine. 2. The three enzymes were identified by the kinetics of heat inactivation and kinetics of phosphorylation by the organophosphate VX. 3. Using selective inhibitors (iso-OMPA, eserine, BNPP and BW-284C51) it was shown that the hydrolysis of acetylthiocholine and butyrylthiocholine in untreated native serum had properties of acetylcholinesterase (EC 3.1.1.7), butyrylcholinesterase (EC 3.1.1.8) and also some properties of carboxylesterase (EC 3.1.1.1). 4. Separation of proteins (on PAA-gels) in untreated native serum gave four bands with acetylthiocholine and three with butyrylthiocholine. 5. The two cholinesterases hydrolysing both substrates corresponded to the slow moving bands on the gel. 6. The fastest moving band hydrolysing only butyrylthiocholine could be attributed to the cholinesterase least sensitive to VX.

Hydrolysis of some organophosphorus dichlorophenyl esters by hen brain homogenates and rabbit serum compared with hydrolysis of paraoxon

The hydrolysis of four organophosphorus dichlorophenyl esters and of paraoxon was studied in hen brain homogenates and in rabbit serum. All compounds were hydrolysed by both preparations, but the rates were different in the two preparations. EDTA inhibited the hydrolysis almost completely in rabbit serum, but had only a small effect on the hydrolysis in hen brain homogenates.

Interaction of imidazolium and pyridinium dioximes with human erythrocyte acetylcholinesterase.

Two pyridinium and two imidazolium dioximes were tested as reversible inhibitors of human erythrocyte acetylcholinesterase (AChE), as protectors of the enzyme against phosphorylation and as reactivators of the phosphorylated AChE. All four dioximes reversibly inhibited AChE, protected the enzyme against phosphorylation by soman and tabun and reactivated AChE after phosphorylation by sarin, VX and tabun. From the experimental results the enzyme/dioxime dissociation constants were evaluated for the catalytically active enzyme and for phosphorylated enzyme. The evaluation constants have shown that all four dioximes have about the same affinity for the catalytically active as for the phosphylated AChE. Obtained results also indicate that imidazolium dioximes probably bind only to the allosteric, while pyridinium dioximes bind to both, the catalytic and the allosteric site of the enzyme.

Catalytic Properties and Classification of Phosphoric Triester Hydrolases: Edta-Insensitive and Edta-Sensitive Paraoxonases in Sera of Non-Diseased and Diseased Population Groups

Phosphoric triester hydrolases (EC 3.1.8) are defined as enzymes acting on organophosphorus compounds, including esters of phosphonic and phosphinic acids, and on phosphorus anhydride bonds; the enzymes are subdivided into paraoxonase (EC 3.1.8.1) and DFP-ase (EC 3.1.8.2) [1].

Kinetics and Mechanisms of Reversible and Progressive Inhibition of Acetylcholinesterase

This paper concerns the interaction of reversible ligands, particularly oximes, with acetylcholinesterase (AChE) and their effect upon the catalytic properties of the enzyme.

Inhibition of serum cholinesterase by trialkylphosphorothiolates

The kinetics of inhibition of horse serum cholinesterase (EC 3.1.1.8) by six trialkylphosphorothiolates was studied (25° C, pH 7.4). The compounds were: OOS-trimethylphosphorothiolate (OOS-Me), OSS-trimethylphosphorodithiolate (OSS-Me), SSS-trimethylphosphorotrithiolate (SSS-Me) and their corresponding ethyl analogues (OOS-Et, OSS-Et, SSS-Et). The second order rate constants of inhibition ranged from 7.2 to 2128 mol−1 1 min−1, and the enzyme/inhibitor dissociation constants from 0.079 to 1.5 mM. The ethyl esters were better inhibitors than their methyl analogues and the OSS-compounds were better inhibitors than the OOS-or SSS-compounds. The same structure-activity relationship is known to hold for the reaction of the compounds with acetylcholinesterase (EC 3.1.1.7).

The inhibitory power of 2-isopropoxyphenyl-N-methylcarbamate against serum cholinesterase of various individuals

SummaryThe inhibition of serum cholinesterase by 2-isopropoxyphenyl-N-methylcarbamate in the sera of 10 subjects was measuredin vitro at a pH of 7.4 and at 25‡ C. Using two different methods, spectrophotometric and titrimetric, the same rate constantki=0.95·103 M−1 min−1 for the inhibition of cholinesterase was obtained. No individual variations in the rate constantki were observed.