Induction of ethoxyresorufin-O-deethylase activity in frog tadpoles (Rana arvalis) by exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin was confirmed experimentally. In a field study on a polluted site, the Volgermeer, the ethoxyresorufin-O-deethylase activities in juvenile green frogs (Rana esculenta) were inhibited, despite an elevated Toxic Equivalency Quotienttcdd, indicative of exposure to dioxin-like contaminants. Aromatase activities in the frogs from the Volgermeer were inhibited, which is in line with exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin in in vitro studies reported elsewhere. It is hypothesized that a non-identified (group of) contaminant(s) inhibits ethoxyresorufin-O-deethylase activity and so interferes with the P450 enzyme system. The combined results suggest that dioxin-like contaminants may affect the reproductive performance of frogs in the Volgermeer. This study shows that the combined deployment of various biomarkers is needed in order to elucidate the possible risks that contaminants pose to amphibians. Enzyme activities in frogs appear suitable indicators, provided that a combined set of enzyme markers is investigated.

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