THE IMPORTANCE OF BIOTRANSFORMATION IN THE TOXICITY OF XENOBIOTICS TO FISH .1. TOXICITY AND BIOACCUMULATION OF DIAZINON IN GUPPY (POECILIA RETICULATA) AND ZEBRA FISH (BRACHYDANIO RERIO)

The 96-h LC50 value of the organophosphorus pesticide diazinon (O,O-Diethyl-O-2-isopropyl-4-methyl-6-pyrimidyl thiophosphate) was about 0.8 mg/l for the guppy and about 8 mg/l for the zebra fish. Both the guppy and the zebra fish metabolized diazinon. The detected metabolite was 2-isopropyl-6-methyl-4-pyrimidinol (pyrimidinol). Diazoxon (O,O-diethyl-O-2-isopropyl-4-methyl-6-pyrimidylphosphate) could not be found as a metabolite. After short exposures (up to 48 h) to low sublethal diazinon cocentrations the body concentration of pyrimidinol was linearly dependent on the body concentration of diazinon. The ratio of pyrimidinol to diazinon body concentrations was about 5 times higher in the guppy than in the zebra fish. In both species however, the pyrimidinol level reached a maximum at concentrations corresponding to 1/4 to 1/2 of the LC50 value and these levels were remarkably similar. Kinetic experiments at diazinon concentrations of 0.1 and 0.4 ppm pesticide in water showed that an apparent steady state was reached after 24 to 48 h. After longer exposures an increased accumulation of diazinon was observed concomitantly with the inhibition of pyrimidinol formation. The inhibition of the brain acetylcholinesterase reached 73% in the guppy and 83% in the zebra fish. The maximum of the inhibition was already reached when the concentration of diazinon in water was 25% of the LC50 value. The factor of bioconcentration (BCF) was dependent on the level of the pesticide in the water and varied between 39 (in the guppy, at a water concentration of 0.1 ppm) and more than 300 (in the zebra fish, at a water concentration of 4 ppm). The results indicate that the metabolism plays a key role in the bioaccumulation and in the species specific toxicity of diazinon in fish.