Gill EROD Activity in Fish : A Biomarker for Waterborne Ah-receptor Agonists

Sammanfattning: Induction of the cytochrome P450(CYP)1A protein and the connected increase in 7-ethoxyresorufin O-deethylase (EROD) activity are common biomarkers in fish. Enhanced activity of this protein signals exposure to Ah-receptor agonists such as chlorinated dioxins, co-planar polychlorinated biphenyls (PCBs) and certain polycyclic aromatic hydrocarbons (PAHs). The EROD biomarker is commonly analyzed in liver microsomes. However, the gill is directly exposed to waterborne pollutants, and in this thesis the gill filament EROD assay was therefore evaluated as a monitoring tool for waterborne CYP1A inducers in fish. Originally developed in rainbow trout (Oncorhynchus mykiss), the assay was here applied in various limnic and marine species. Following exposure to low waterborne concentrations of the readily metabolized CYP1A inducers benzo(a)pyrene (BaP) and indigo, a strong EROD induction was observed in the gill but not in the liver. This likely reflected metabolic clearance of the inducers in gill and other extrahepatic tissues. The high sensitivity of the gill was confirmed in studies of fish caged in waters in urban and rural areas in Sweden where the gill consistently showed a more pronounced EROD induction compared with the liver and the kidney. Fish caged in the reference waters showed surprisingly strong gill EROD induction and CYP1A immunostaining. Consequently, there may be CYP1A inducers present in the aquatic environment that are not yet identified. The assay was further applied in Atlantic cod (Gadus morhua) as a biomarker of exposure to crude oil and produced water (PW) from oil fields in the North Sea. The assay was finally adapted to detect inhibiting compounds, and an imidazole, a triazole and a plant flavonoid turned out to be potent gill EROD inhibitors. The overall conclusion from the studies of this thesis is that the gill filament EROD assay is a practical and sensitive biomarker of exposure to waterborne CYP1A inducers in various fish species. The induction of gill EROD activity in fish also at the reference sites in the field studies calls for further studies on background contamination in Swedish waters.