Earlier, we have shown that rat hepatic and pancreatic
fatty acid ethyl
ester (FAEE) synthases are structurally and functionally similar to rat liver
carboxylesterase (CE) and
pancreatic cholesterol esterase (ChE), respectively. We have also reported that only hepatic
FAEE synthase is inhibited by tri-o-tolylphosphate (
TOTP) in vivo and in human
hepatocellular carcinoma (HepG2) cells. The metabolism of
TOTP is a prerequisite for the inhibition of hepatic
FAEE synthase as well as
esterase activity. To further elucidate the mechanism of such differential inhibition by inhibitors of
serine esterases, we synthesized two metabolites of
TOTP, 2-(o-cresyl)-4H-1:3:2-benzodioxaphosphoran-2-one (
CBDP; cyclic
saligenin phosphate) and di-o-tolyl-o-( proportional, variant -hydroxy)tolylphosphate (HO-
TOTP), and one ChE inhibitor,
3-benzyl-6-chloro-2-pyrone (3-BCP). The inhibitory effect of
CBDP, HO-
TOTP, and
3-BCP on
FAEE synthase and
esterase activity was studied using rat hepatic and pancreatic postnuclear (PN) fractions, commercial porcine hepatic CE and pancreatic ChE, and in HepG2 and rat pancreatic
tumor (AR42J) cell lines. Only HO-
TOTP and
CBDP inhibited
FAEE synthase as well as
esterase activity of hepatic PN fraction and commercial CE and ChE in a concentration-dependent manner, and the inhibition was found to be irreversible. However, no inhibition was found in pancreatic PN fraction by both
TOTP metabolites and
3-BCP. Although
3-BCP inhibited only the
esterase activity of commercial ChE in a concentration-dependent manner, the activity was reversible within 30 min of incubation. Studies with HepG2 cells also showed a significant inhibition of
FAEE synthase-
esterase activity by
CBDP and HO-
TOTP within 15 min of incubation, while no inhibition was observed in AR42J cells.
3-BCP did not inhibit
FAEE synthase-
esterase activity either in HepG2 or AR42J cells. Such differential inhibitory effect of the
TOTP metabolites on hepatic and pancreatic
FAEE synthase-
esterase is supported by our earlier in vivo and in vitro studies. Further investigations are needed to understand the biochemical mechanism(s) of inactivation of
TOTP metabolites and
3-BCP in the pancreas and AR42J cells towards
FAEE synthase-
esterase activities.