Levels of plasma B-esterases and glutathione-S-transferase activities in three South American toad species

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We determined the normal levels of butyrylcholinesterase (BChE), carboxylesterase (CbE), and glutathione S-transferases (GST) activities in three South American toad species in order to establish reference values for field pesticide monitoring purposes. Interspecies variations in B-esterase and GST activities were examined according to body mass. In addition, comparative inhibition of BChE and CbE activities using malaoxon, and chemical reactivation of malaoxon-inhibited BChE activity using pyridine-2-aldoxime methochloride (2-PAM) were investigated. Bufo fernandezae had average activity values for BChE: 17.31 mmol min−1 ml−1; CbE: 621.49 nmol min−1 ml−1 and GST: 1.94 mmol min−1 ml−1 while B. arenarum enzymatic average activities were BChE: 9.51 mmol min−1 ml−1; CbE: 270.07 nmol min−1 ml−1, and GST: 1.59 mmol min−1 ml−1; finally Bufo schneideri had enzymatic mean values of BChE: 2.08 mmol min−1 ml−1; CbE: 301.95 nmol min−1 ml−1, and GST: 1.60 mmol min−1 ml−1. Moreover, we found an allometric relationship between plasma BChE and CbE activities and body size for the three toad species. We suggest that B. fernandezae would be the species with a higher tolerance capacity to organophosphorous insecticides compared to the other toad species, while B. schneideri may be the most vulnerable toad species to field pesticide exposure, although some other factors (e.g., brain AChE sensitivity or pesticide metabolism by phosphotriesterases) should be also taken into account. The malaoxon-inhibited BChE activity of the three toad species was reactivated in the presence of 2-PAM, and it is recommended as a specific and sensitive methodology in the assessment of field exposure to OP insecticides together to compare BChE activity levels between OP-exposed and nonexposed individuals.