Effect of 3-methylcholanthrene on monooxygenase, epoxide hydrolase, and glutathione S-transferase activities in small estuarine and freshwater fish

Abstract

Monooxygenase activities, epoxide hydrolase, and glutathione S-transferase activities were measured in fractions prepared from pooled livers of three small estuarine fish species, Fundulus grandis, Cyprinodon variegatus, and Poecilia latipinna, and three freshwater species, Oryzias latipes, Peocilia reticulata, and Pimephales promelas. Interspecies differences in xenobiotic metabolizing activities of up to 10-fold were observed, with the smaller freshwater species generally having lower activities than the estuarine species. Groups of each species were injected with microliter volumes of corn oil or a solution of 3-methylcholanthrene (3MC) in corn oil, and enzyme activities measured 4–5 days later in pooled liver fractions from control or 3MC-treated fish. Although statistical analysis of induction effects was not possible, monooxygenase activities with ethoxyresorufin and benzo( a)pyrene were 2.5–15 times higher in 3MC-treated fish than in corn oil or untreated control groups. In 3MC-treated C. variegatus and F. grandis, microsomal cytochrome P-450 content was doubled and the spectral maximum of the CO-reduced microsomes was shifted from 449.6 to 448.5 nm. Microsomes from 3MC-treated C. variegatus and F. grandis metabolized benzo( a)pyrene 5–6 times faster than controls (per mg protein), but the proportion of 9,10- and 7,8-dihydrodiol metabolites was lower in 3MC-treated fish, suggesting that epoxide hydrolase may be rate limiting for production of dihydrodiols. 3MC treatment did not alter styrene oxide hydrolase or glutathione S-transferase activities at the time point tested.