Abstract
Esterases in human liver microsomes hydrolysed fluazifop-butyl ( V max 9.8 ± 1.6 μmol/min/g tissue), paraoxon ( V max 47.4 ± 7.5 nmol/min/g tissue) and phenylacetate ( V max 57 ± 8 μnol/min/g tissue), whereas esterases found in the human liver cytosol hydrolysed fluazifop-butyl ( V max 10.0 ± 0.5 μmol/min/g tissue) and phenylacetate ( V max 37 ± 2.9 μmol/min/g tissue) but not paraoxon. Human plasma esterase hydrolysed fluazifop-butyl ( V max 0.09 ± 0.006 μmol/min/mL), paraoxon ( V max 210 ± 14 nmol/min/mL) and phenylacetate ( V max 250 ± 17 μmol/min/mL). Inhibitory studies using paraoxon, bis-nitrophenol phosphate and mercuric chloride indicated fluazifop-butyl hydrolysis involved carboxylesterase in liver microsomes and cytosol, and cholinesterase and carboxylesterase in plasma. Phenylacetate hydrolysis involved arylesterase in plasma, both arylesterase and carboxylesterase in liver microsomes and carboxylesterase in liver cytosol. Plasma hydrolysis is less important and overall esterase activity is lower in humans than in the rat which is therefore a poor model.
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