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.