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Glutathione‐dependent detoxifying enzymes in rainbow trout liver: Search for specific biochemical markers of chemical stress

Activities of trout liver microsomal glutathione S‐transferase (GST) and a series of cytosolic glutathione‐dependent detoxifying enzymes were determined after a single intraperitoneal treatment with phenobarbital, 2,2‐bis (p‐chlorophenyl)‐1,1‐dichloroethane (p,p′‐DDE), 2,3,‐dimethoxynaphthoquinone (NQ), or 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD). This study aimed to find xenobiotic‐specific parameters applicable as biochemical markers of the impacts of the prototypal xenobiotics. The effects of xenobiotics on cytosolic GST activities were substrate dependent. The rate of conjugation of p‐nitrobenzyl chloride was significantly induced by higher doses of p,p′‐DDE or NQ. The conjugation of ethacrynic acid was enhanced by phenobarbital, p,p′‐DDE, and NQ (i.e., by xenobiotics that do not induce cytochrome P4501A forms). The GST activity against 1,2‐epoxy‐3‐(p‐nitrophenoxy)propane was induced only by phenobarbital and by lower doses of p,p′‐DDE. The cytosolic GST activity, measured with 1‐chloro‐2,4‐dinitrobenzene as a substrate, was only weakly increased by phenobarbital, TCDD, higher doses of p,p′‐DDE, or by NQ at the lowest dose of 1 mg/kg. Although the latter activity is frequently used as a biomarker in ecotoxicology, various factors (including its weak inducibility) indicate that this biochemical parameter is probably not a suitable indicator of contamination in fish. Similarly, cytosolic glutathione peroxidase was not affected by the prototypal xenobiotics and appeared to be an unsuitable bioindicator of oxidative impacts of the tested compounds. On the other hand, microsomal GST activity was nonspecifically increased by phenobarbital, NQ, TCDD, and high doses of p,p′‐DDE. Glutathione reductase, another potential biomarker of oxidative stress, was induced by phenobarbital, NQ, and, to a lesser extent, p,p′‐DDE; therefore it appeared to be a less sensitive indicator to the exposure to prototypal xenobiotics than the microsomal GST. We conclude that the increase of microsomal GST and cytosolic glutathione reductase activities could become useful biochemical markers of oxidative stress, while the induction of cytosolic GST activities toward ethacrynic acid and probably also toward p‐nitrobenzyl chloride appear to hold promise as biochemical markers of specific impacts of p,p′‐DDE and redox cycling quinones in trout liver.