Enzymological differences of AChE and diazinon hepatic metabolism: correlation of in vitro data with the selective toxicity of diazinon to fish species.

The in vitro hepatic metabolism of diazinon, as well as the sensitivity of the brain acetylcholine esterase, to diazoxon inhibitory action have been studied in order to explain the different toxicity of diazinon to Oncorhynchus mykiss (rainbow trout), Poecilia reticulata (guppy), Brachydanio rerio (zebra fish) and Cyprinus carpio (carp). In spite of a very sensitive acetylcholine esterase the carp is very resistant to diazinon toxicity because of its very low rate of bioactivation and relatively high activity of detoxicating enzymes. The trout is very sensitive towards diazinon in spite of its low activity of bioactivation, because of its lack of detoxicating enzymes and a very sensitive acetylcholine esterase. Diazinon is very toxic for the guppy, because this fish combines a relatively sensitive acetylcholine esterase with a high rate of bioactivation. The zebra fish has the most insensitive acetylcholine esterase, associated with a limited activation rate, thus resulting a rather resistant species. The results obtained indicate that diazinon toxicity differences among the fish species studied can largely be explained in relation to metabolic balances in the liver and with the features of the target enzyme.