MOLECULAR DOSIMETRY OF DNA-ADDUCTS IN THE MEDAKA SMALL FISH MODEL

Small fish models are being used with increasing frequency for carcino genicity testing and comparative cancer research in the US, Canada and Europe. However, there is a need to further define the early biochemi cal events of carcinogenesis in these species. Identification and quan titation of DNA adducts can integrate all of the various factors invol ved in chemical exposure, uptake, distribution and biotransformation o f a putative carcinogen. In the present study, Japanese medaka (Oryzia s latipes) were exposed to the alkylating agent, diethylnitrosamine (D EN), in the ambient water. Liver DNA was analyzed for O-6-ethylguanine (O(6)EG), O-4-ethylthymidine ((OET)-E-4) and O-2-ethylthymidine ((OET )-E-2) by the immune-slot-blot technique, using monoclonal antibodies against each adduct of interest. While fish exposed to 10 p.p.m. DEN h ad liver DNA adduct concentrations at or only slightly higher than bac kground levels, those exposed to 100 p.p.m. DEN averaged 34 and 53 pmo l O(6)EG/mu mol guanine, 15 and 41 pmol (OET)-E-2/mu mol thymidine and 2 and 6 pmol (OET)-E-4/mu mol thymidine at 0 and 24 h post-exposure, respectively. The results of this study show that, under these short-t erm exposure conditions, ethyl-DNA adducts appear to accumulate in med aka liver tissue in a sublinear (i.e. non-linear) fashion after aqueou s exposure to DEN. Thus, critical DNA repair enzymes such as O-6-alkyl guanine DNA alkyltransferase, which are relatively efficient at lower carcinogen levels, are probably saturated at the 100 p.p.m. concentrat ion level of DEN.