Effect of selected induction of microsomal and nuclear aryl hydrocarbon monooxygenase and epoxide hydrolase as well as cytoplasmic glutathione S-epoxide transferase on the covalent binding of the carcinogen benzo(a)pyrene to rat liver DNA in vivo

Sodium cholate extraction of rat liver nuclear xenobiotic-metabolizing enzymes

DNA is the purported target of several carcinogenic and mutagenic agents. Nuclear enzymes which could generate or detoxify reactive metabolites are of major concern. Several such enzymes have been identified within nuclei, but obtaining samples with enriched content or activity is difficult, time-consuming, and uses harsh isolation techniques. Extraction of rat liver nuclear suspensions with cholate-containing buffer results in solubilization of 25-30% of the protein. Linear extraction was obtained for total protein and cytochromes P-450 and b5, NADPH-cytochrome P-450 reductase, NADH-cytochrome b5 reductase, DT-diaphorase, and microsomal-like epoxide hydrolase with specific activities comparable to values reported for isolated nuclear membrane, while the yield was five to ten times greater. Detergent extracts of rat liver nuclei were employed to study the comparative response of microsomal and nuclear enzymes to chemical treatment. While the responses to acute inductive (phenobarbital and 3-methylcholanthrene) and toxic (carbon tetrachloride and dibromochloropropane) treatments were qualitatively similar, an initiation-promotion protocol (diethylnitrosamine with phenobarbital promotion) resulted in divergent responses between the enzymes in the two subcellular fractions. Detergent extracts of nuclei offer an efficient means of recovering xenobiotic-metabolizing enzymes from rat liver nuclei, and have been utilized to demonstrate a differential response of nuclear enzymes during preneoplastic development.

Polysubstrate monooxygenases in Drosophila, mammals and man

There is overwhelming evidence that polysubstrate monooxygenases play a central role in the metabolism of endogenous compounds as well as in the biotransformation of xenobiotics. These enzyme systems are of great importance in such diverse fields as insecticide resistance, mutagenicity, carcinogenicity, drug metabolism, etc. The constitutive and, in particular, the induced forms represent various products from a multigene family. This has first been shown for the mouse, but evidence is accumulating that this is also true for other mammals and for man. Also in insects a similar picture is emerging. If the regulation of cytochrome P-450 induction resembles in any way the other methods by which prokaryotes and eukaryotes cope genetically with the many forms of environmental selective pressures, it is very likely that most organisms have the genetic capacity to produce not only hundreds but probably thousands of inducible forms of cytochrome P-450 (Nebert et al., 1981). Doubtless, many fields from pest control to cancer prevention to drug safety will profit from the elucidation of the genetic mechanisms involved.

Differential induction of cytosolic epoxide hydrolase, microsomal epoxide hydrolase, and glutathione S-transferase activities

Three major enzyme systems have been shown to metabolize epoxidized xenobiotics in vertebrate tissues, and this study demonstrates that these enzyme systems can be differentially induced. The cytosolic epoxide hydrolase activity was routinely monitored with trans-beta-ethylstyrene oxide, the microsomal epoxide hydrolase activity with benzo(a)pyrene, 4,5-oxide, and the glutathione S-transferase activity with 2,4-dichloro-4-nitrobenzene. Commonly used inducers of microsomal mixed-function oxidase, microsomal epoxide hydrolase, and cytosolic glutathione S-transferase activities failed to cause significant induction of the cytosolic epoxide hydrolase while leading to the expected induction of the other epoxide metabolizing enzymes. The compounds tested by ip injection into male Swiss-Webster mice included phenobarbital, 3-methylcholanthrene, Aroclor 1254, trans- and cis-stilbene oxides, pregnenolone-16 alpha-carbonitrile, chalcone, and 4-bromochalcone. To determine if there were strain, sex, or species differences, the enzymes were monitored in male C57BL/6 mice, female Swiss-Webster mice, and male Sprague-Dawley rats following ip injection of phenobarbital, 3-methylcholanthrene, and/or pregnenolone-16 alpha-carbonitrile. The time dependence of enzyme induction was followed in Sprague-Dawley rats following trans-stilbene oxide administration. Male Swiss-Webster mice were additionally exposed to dietary alpha-naphthoflavone and 2(3)-tert-butyl-4-hydroxyanisole while male Sprague-Dawley rats were fed 2,6-di-tert-butyl-4-methylphenol. In no case was significant induction of cytosolic epoxide hydrolase activity observed. Dietary di-(2-ethylhexyl)phthalate, 2-ethyl-l-hexanol, and clofibrate proved to be potent inducers of the cytosolic epoxide hydrolase in male Swiss-Webster mice while probucol (a nonperoxisome proliferating hypolipidemic drug) failed to cause significant induction. Data from isoelectric focusing experiments and other data are consistent with the epoxide hydrolase activities induced by 2-ethyl-l-hexanol and clofibrate being due to the same protein that is present in control animals. The lack of induction of the cytosolic epoxide hydrolase by a variety of compounds which were selected to demonstrate induction of other xenobiotic metabolizing enzymes, may indicate that the cytosolic epoxide hydrolase has a constitutive role whereas its induction by clofibrate could be related to some of the pharmacological and/or carcinogenic actions of this drug.

Epoxide hydrolase activity in small biopsy samples of preneoplastic and neoplastic rat liver

Epoxide hydrolase (EH) activity was measured in serial fine-needle aspiration biopsy (FNAB) samples of male rats chronically treated with the hepatocarcinogen, N-nitrosomorpholine (NNM, 10 mg/100 ml drinking water). The development of neoplasia was confirmed histopathologically at the end of the experiment. The enzyme activity was significantly increased after 2 days of NNM administration and reached a 3 to 4-fold increase after 2 weeks that remained stable throughout the experiment. Although EH is not related to the metabolism of NNM, the induction of enzyme as readily recognized with the FNAB method might be a useful marker in the attempt to characterize hepatocarcinogenic substances in rats.