COMPARATIVE METABOLISM OF 2-ACETYLAMINOFLUORENE BY RAINBOW-TROUT AND RAT-LIVER SUBCELLULAR-FRACTIONS

In order to assess the cause of the resistance of rainbow trout (Oncor hynchus mykiss), Shasta strain, to the hepatocarcinogenic effects of 2 -acetylaminofluorene (AAF), we have investigated the metabolism of thi s model chemical by subcellular fractions from trout liver. Treatment of trout with 3-methylcholanthrene (3-MC) enhanced the rate of metabol ism of AAF by liver microsomes, The major AAF metabolites produced by trout liver microsomes at saturating substrate concentration (34.3 and 19.5 mu M AAF for control and 3-MC-induced microsomes, respectively) were 7-hydroxy-AAF and 5-hydroxy-AAF which represented 75 and 21% of t otal AAF metabolites, respectively for control microsomes, and 79 and 18% of total metabolites, respectively for induced microsomes. With bo th control and 3-MC-induced microsomes, N-hydroxy-AAF (the proximate c arcinogen of AAF) was a minor metabolite accounting for less than 1% o f total AAF metabolites. At a low substrate concentration (0.18 mu M A AF), 7-hydroxy-AAF formation accounted for 95 and 91% of total metabol ites for control and 3-MC induced microsomes, respectively, and a slig ht relative increase in N-hydroxy-AAF production (1.5-2.5%) was observ ed. Liver microsomes from untreated male Sprague-Dawley rats (a specie s susceptible to hepatocarcinogenesis by AAF) formed N-hydroxy-AAF in greater proportions (14-fold) and ring-hydroxylated metabolites in sma ller proportions compared to trout liver microsomes. The activities of cytosolic N-hydroxy-AAF sulfotransferase and acyltransferase in trout liver (24 and 17 pmol/mg h(-1), respectively) were found to be substa ntially lower than those reported for Sprague-Dawley rats. The low act ivity of hepatic enzymes implicated in the metabolic activation of AAF (AAF-N-hydroxylase, N-hydroxy-AAF-sulfotransferase, and N-hydroxy-AAF acyltransferase) coupled with a relatively high capacity of the trout liver for ring-hydroxylation (detoxification) of AAF may explain, in part, the resistance of rainbow trout to the hepatocarcinogenic action of AAF.