RESISTANCE TO MALATHION IN HEAT-SHOCKED DAPHNIA-MAGNA

A 34 degrees C heat shock has been shown to induce a transient toleran ce to malathion in the cladoceran Daphnia magna. HSP70, a common heat- shock protein, is induced by the same heat shock. Various possible mec hanisms 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 genera l esterase, carboxylesterase (Aliesterase, AliE), and glutathione-S-tr ansferase (GST) activities were examined as mechanisms to explain this phenomenon. General esterase activity was not affected by a 34 degree s C heat shock, but was reduced to nearly zero in lethal malathion con centrations, with or without heat shock. Conversely, although AliE act ivity was reduced by heat shock, there was no further effect of malath ion on AliE in either the heat-shocked or control animals. A heat-indu cible high molecular mass protein complex with GST activity was observ ed in the resistant daphnids. There was no corresponding increase in G ST activity, however. These data suggest that the transient protection against malathion is not caused by an increase in the amount of enzym e or enzyme activity.