Selective inhibition of rat liver carboxylesterases by various organophosphorus diesters in vivo and in vitro

Five purified carboxylesterases/amidases of rat liver are susceptible to inhibition by various organophosphorus diesters and triesters. The inhibitory effect of these esters generally decreases in the order paraoxon>bis(4-nitrophenyl)phosphate>4-nitrophenyl-phenyl-phosphate>bis(4-cyanophenyl) phosphate>2-nitrophenyl-phenyl-phosphate. After in vivo application of radioactive bis(4-nitrophenyl) phosphate, all microsomal carboxylesterases of rat liver—including the five hydrolases investigated in vitro—are irreversibly labeled. We conclude that all carboxylic este hydrolases of rat liver that act at neutral pH can be classified as serine hydrolases. The hydrolases pI 6.2 and 6.4 that have the highest monoleylglycerol-cleaving activities are relatively insensitive to inhibition by organophosphorus diesters as compared to the hydrolases with lower pI. The very similar hydrolases pI 6.2 and 6.4 may be distinguished in vitro by the action of 0.1 mM HgCl₂, which inhibits hydrolase pI 6.4 almost completely. Among the organophosphorus diesters investigated, bis (4-cyanophenyl)phosphate is a surprisingly specific inhibitor for the acetanilide-cleaving hydrolase pI 5.6 both in vitro and in vivo. This inhibitor allows a clear discrimination between the hydrolases pI 5.2 and 5.6 that have similar substrate specificity and chemical properties. After intraperitoneal injection into mice, bis(4-cyanophenyl) phosphate exhibits extremely low toxicity.