Concomitant induction of microsomal epoxide hydrolase and UDP-glucuronosyltransferase activities by dipyridine...

The coordinated response of the major rat hepatic phase II xenobiotic-metabolizing enzymes following 3-day exposure to diaryl compounds was investigated. Four diaryl compounds containing heterocyclic nitrogen atoms elevated microsomal epoxide hydrolase activity from 2- to 4-fold. Equivalent compounds lacking the heteroatom, when given in the same dosing regimen (75 mg/kg, ig, daily for 3 days), did not induce this or any other drug-metabolizing enzyme activity. Epoxide hydrolase activity closely paralleled UDP-glucuronosyltransferase activity toward three aglycones: 4-nitrophenol (r = 0.87), morphine (r = 0.84), and 1-naphthol (r = 0.78). There was less correlation (r = 0.60) between epoxide hydrolase activity and both UDP-glucuronosyltransferase activity toward testosterone and cytosolic glutathione S-transferase activity. There was no correlation between microsomal epoxide hydrolase activity and cytochrome P-450 or the monooxygenase reaction (4-nitrophenol hydroxylase) preferentially induced by pyridine-containing compounds. Induction of rat hepatic microsomal epoxide hydrolase activity by some pyridine-containing compounds appears coordinately regulated with glucuronidation rather than oxidation enzymes.