Effects of Rac-Metalaxyl and R-Metalaxyl on Juvenile Zebrafish (Danio rerio)

Chiral pesticides are used widely in the world, and at present, older racemic products are being replaced by enantiopure products. Pesticides as xenobiotic released into environment impose a great stress on nontarget organisms. Traditionally it is a necessary procedure for pesticides to have a registration based on toxicological data from nontarget organism, but until now ecological risk assessment about metalaxyl only depend on racemic products. Hence, in our work, acute toxicity of rac-metalaxyl and R-metalaxyl to juvenile zebrafish was studied, and the results showed R-metalaxyl was about 2-fold toxic than rac-metalaxyl. The reason for death of juvenile zebrafish was possibly the oxidative stress because pericardial edema and tissue canker for juvenile zebrafish exposed were visible physiological phenomena. Based on this, under sublethal exposure, SOD and CAT activity in juvenile zebrafish were measured at 24h, 48h, 72h, and 96h. The results proved that R- and rac-metalaxyl were totally different in the pattern of inducing or inhibiting SOD and CAT activity at the same exposure dose, which implied that each one might be metabolized or transformed differently in the chiral environments of juvenile zebrafish in spite of identical chemical structure. These evidences emphasized that enantiopure pesticide should be paid more attention due to enantioselective effects on untarget organisms.