Hepatic microsomal mixed‐function oxidase activity in the japanese medaka (Oryzias latipes)

Growing interest in the use of the Japanese medaka Oryzias latipes, a small aquarium fish, for carcinogenicity and toxicological studies necessitates an understanding of their xenobiotic metabolizing capabilities. Historically, hepatic mixed‐function oxidase activity has been characterized using inducing agents such as beta‐naphthoflavone (BNF). Hepatic cytochrome P_450, 7‐ethoxy‐resorufin o‐deethylase (EROD) activity, and NADPH‐cytochrome c reductase activity were increased in male and female adult medaka following dietary exposure to BNF. In noninduced male and female adult medaka, EROD activity was not detectable. The EROD activity of the male medaka increased to 1.05 nmol/min/mg protein on day 4. Female EROD activity reached a maximum of 0.65 nmol/min/mg protein on day 2. NADPH‐cytochrome c reductase activity in the males increased from a control level of 33.02 to 81.9 nmol/min/mg protein by day 4 of BNF exposure. NADPH‐cytochrome c reductase activity in females reached a maximum rate of 63.4 nmol/min/mg protein within 48 h of exposure, but returned to noninduced levels (49.6 nmol/min/mg protein) by day 4. Cytochrome P ₄₅₀ concentration, liver‐to‐body‐weight ratios, and microsomal protein concentration increased in a time‐dependent fashion for both sexes following BNF exposure.