Elevation of hepatic microsomal epoxide hydrolase activity by 2-acetylaminofluorene: role of metabolism

Hepatic N-acetyltransferases: selective inactivation in vivo by a carcinogenic N-arylhydroxamic acid

N-Hydroxy-2-acetamidofluorene (N-OH-AAF), a carcinogenic N-arylhydroxamic acid, is a selective and irreversible inhibitor of arylamine N-acetyltransferase (NAT) activity in vitro. The present study demonstrates that intraperitoneal administration of N-OH-AAF to hamsters caused an irreversible reduction of the hepatic transacetylase activity that catalyzes the transfer of the acetyl group from N-OH-AAF to 4-aminoazobenzene (AAB), but did not affect the acetyl coenzyme A (CoASAc) dependent NAT that is responsible for acetylation of p-aminobenzoic acid (PABA). A 40% loss of N-OH-AAF:AAB transacetylase activity occurred 4 hr after administration of 50 mg/kg of N-OH-AAF. To determine whether biotransformation of N-OH-AAF is a factor in determining its ability to inactivate N-OH-AAF:AAB transacetylase activity in vivo, the enzyme-inducing agent phenobarbital and the esterase/acylamidase inhibitor bis(p-nitrophenyl)phosphate (BNPP) were administered to the animals prior to the administration of N-OH-AAF. The loss of N-OH-AAF:AAB transacetylase activity was prevented by treatment of the animals with either phenobarbital or with BNPP. The ability of the esterase/acylamidase inhibitor, BNPP, to prevent the N-OH-AAF-mediated loss of transacetylase activity indicates that, in contrast to the inactivation process in vitro, esterase-catalyzed deacetylation of N-OH-AAF may be required for transacetylase inactivation in vivo. It is proposed that in vivo the endogenous acetyl donor, CoASAc, acetylates the enzyme and prevents the deacetylation of N-OH-AAF by NAT, thereby impeding the N-OH-AAF-mediated inactivation process, but facilitating enzyme inactivation by N-hydroxy-2-aminofluorene. The latter proposal was supported by the demonstration that CoASAc inhibited the in vitro inactivation of N-OH-AAF:AAB transacetylase activity by N-OH-AAF.

Detection of epoxide hydrolase in rat hepatoma cell lines and primary rat liver cells by immunoblotting

Rat epoxide hydrolase (EH) (EC 3.3.2.3) is elevated in cells of premalignant liver lesions, and variable EH activity has been reported for hepatocellular carcinomas. To facilitate detection of altered EH levels in liver cells, an immunoblotting method was devised. Rabbit antiserum specific for rat EH was prepared and used to detect EH extracted from suspensions of normal liver cells and from hepatoma cell lines. Compared with normal liver cells, 3 rat hepatoma cell lines, 7777, HTC and 17X, showed virtually undetectable EH levels by immunoblotting. The immunoblotting results rule out the possibility that very low EH enzymatic activity in the hepatoma cells results from production of normal amounts of non-functional enzyme protein.

Induction of liver microsomal epoxide hydrolase, UDP-glucuronyl transferase and cytosolic glutathione transferase in different rodent species by 2-acetylaminofluorene or 3-methylcholanthrene

Control activities vary 12-fold for microsomal epoxide hydrolase, two-fold for UDP-glucuronyl transferase and five-fold for cytosolic glutathione (GSH) transferase among the different rodents (rat, hamster, guinea-pig, mouse) examined. For all three enzymes the activities in rat liver are towards the lower values. In these rodents, except for a 100% increase in microsomal epoxide hydrolase in guinea-pig liver, 2-acetylaminofluorene induces the three phase 2 enzymes only in rat. Treatment with 3-methylcholanthrene also produces the largest effects on these three enzyme activities in rat liver; exceptions are its failure to induce microsomal epoxide hydrolase in female rat and the large induction of cytosolic GSH transferase in hamster liver. Quantitatively, hepatic microsomal epoxide hydrolase, UDP-glucuronyl transferase and cytosolic GSH transferase activities, and their inducibility by 2-acetylaminofluorene or 3-methylcholanthrene, in male Sprague-Dawley rats are not representative for other rodent species or even, in all cases, for female rat.