Effect of enzyme inhibitors on enhanced carbofuran metabolism in treated soil

Abstract

AbstractThe rate of loss of [14C]carbofuran, and the rate of formation of metabolites from it, were studied in a ‘treated’ soil, in which the insecticide was rapidly degraded. Metabolic inhibitors were added to the soil to study their effects upon these processes. The organophosphorus compounds paraoxon, chlorfenvinphos, and disulfoton, which, in their active forms, are inhibitors of ‘B’ esterases, all caused a temporary reduction of the rate of loss of carbofuran from the soil. However, within a period of 3–21 days, depending on the compound, the soil recovered its ability to break down carbofuran rapidly. The incorporation of both piperonyl butoxide (an inhibitor of oxidations catalysed by monooxygenases which contain cytochrome P450) and paraoxon into soil reduced the rate of loss to a greater extent and over a longer period than was found with paraoxon alone. Where soils were treated with paraoxon alone, or with paraoxon plus piperonyl butoxide, larger quantities of the primary oxidative metabolites 3‐keto‐carbofuran (< 4.1% of applied carbofuran) and 3‐hydroxy‐carbofuran (< 1.2% of applied carbofuran) were found than in control soils in which the corresponding values were < 0.54% and < 0.43% respectively.The results were consistent with the hypothesis that inhibition of microbial enzymes was the main reason for the stabilization of carbofuran in this soil following treatment with these compounds. The evidence suggested that esterase hydrolysis provided a major rate of breakdown, whilst oxidative metabolism provided a minor one. The higher levels of oxidative metabolites in soils treated with paraoxon and paraoxon plus piperonyl butoxide were thought to be due to the inhibition of their further metabolism. The possible use of certain of these compounds as ‘extenders’ for carbofuran in problem soils is discussed.