Zoran Radić

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.

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.

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).