Dietary (R)(+)-salithion was a more potent insecticide (LC50=9.3 ppm) than the (S)(-)-enantiomer (LC50=63 ppm), correlating acetylcholinesterase (AChE) inhibition in vivo (I50=7.7 and 63 ppm) against Tribolium castaneum larvae. (S)(-)-Salioxon derived from (R)(+)-salithion had a stronger inhibitory activity (I50=8.8 μM) than the (R)(+)-enantiomer (I50=22 μM) in vitro against larval AChE. However, against Musca domestica female adults, a reversed tendency was observed: (S)(-)-salithion and (R)(+)-salioxon were more potent than (R)(+)-salithion and (S)(-)-salioxon in topical insecticidal and anticholinesterase activities in vivo and in vitro, respectively. Hence, the reversed stereospecificity between the T. castaneum larvicidal and M. domestica insecticidal activities of salithion enantiomers is due to a stereospecific difference in the intrincic potency of salioxon enantiomers as an AChE inhibitor with the two insects. A suppression of M. domestica larval growth, gut amylase and invertase activities by dietary 0.5, 1.0 and 1.25 ppm of (S)(-)-salithion was observed 2 days after treatment. A similar tendency was recognized when 1.25 and 2.50 ppm of (R)(+)-salithion was used. However, against M. domestica female adults, no appreciable effect on these enzymes was recognized 1-72 hr after topical application of 0.05 and 0.10 μg/fly of (S)(-)-salithion, and 0.01 and 0.05 μg/fly of (R)(+)-salithion. The whole-body levels of cyclic adenosine 3′, 5′-monophosphate (cAMP) were slightly increased in T. castaneum larvae 3 days after a dietary 60 and 80 ppm of (S)(-)-salithion treatment, and in M. domestica larvae 2 days after treating by dietary 1.25 and 2.50 ppm of (R)(+)-salithion, respectively. Neither of the enantiomers of salithion activated adenylate cyclase prepared from Periplaneta americana ventral nerve cords, but suppressed octopamine potency, suggesting that salithion could act as an antagonist to the octopamine-receptor and that the increased level of whole-body cAMP could be due to the inhibition of phosphodiesterase.
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