Modulation of substrate-specific glutathione S-transferase activity in Daphnia magna with concomitant effects on toxicity tolerance.

Glutathione S-transferase (GST) activity was measured in Daphnia magna and Ceriodaphnia reticulata using 1-chloro-2,4-dinitrobenzene (CDNB) and ethacrynic acid (EA) as conjugation substrates. Levels of GST activity were comparable between species with CDNB; however, D. magna had nearly twice the GST activity with EA as compared to C. reticulata. GST activity with CDNB was elevated from exposure of daphnids to either CDNB or sodium pentachlorophenate (PCP), but not from exposure to EA. GST activity with EA could not be modulated from exposure to CDNB or EA. GST activity towards CDNB and EA was biochemically separated into different protein fractions suggesting the existence of two distinct isozymes. Preexposure of daphnids to CDNB or PCP increased the organisms' tolerance to the toxic effects of PCP, but not CDNB.