Resistance to malathion in heat-shocked Daphnia magna

A 34 degrees C heat shock has been shown to induce a transient tolerance to malathion in the cladoceran Daphnia magna. HSP70, a common heat-shock protein, is induced by the same heat shock. Various possible mechanisms exist to explain this heat-inducible response. Several enzyme systems are known to be involved in the detoxification of pesticides, including the esterases, transferases, and oxidases. Changes in general esterase, carboxylesterase (Aliesterase, AliE), and glutathione-S-transferase (GST) activities were examined as mechanisms to explain this phenomenon. General esterase activity was not affected by a 34 degrees C heat shock, but was reduced to nearly zero in lethal malathion concentrations, with or without heat shock. Conversely, although AliE activity was reduced by heat shock, there was no further effect of malathion on AliE in either the heat-shocked or control animals. A heat-inducible high molecular mass protein complex with GST activity was observed in the resistant daphnids. There was no corresponding increase in GST activity, however. These data suggest that the transient protection against malathion is not caused by an increase in the amount of enzyme or enzyme activity.