Tissue distribution and metabolism of benzo[a]pyrene in embryonic and larval medaka (Oryzias latipes)

The need to understand chemical uptake, distribution, and metabolism in embryonic and larval fish derives from the fact that these early life stages often exhibit greater sensitivity to xenobiotic compounds than do adult animals. In this study, a 6-h acute waterborne exposure immediately after fertilization was used to quickly load the egg with benzo[a]pyrene (BaP). This exposure was used to mimic the initial egg concentration of a persistent bioaccumulative toxicant that could result from maternal transfer. We used multiphoton laser scanning microscopy (MPLSM) in combination with conventional analytical chemistry methods to characterize the tissue distribution of BaP and its principal metabolites in medaka embryos and post-hatch larvae. Embryonic metabolism of BaP was evident by MPLSM prior to liver formation or heart development. A major product of this metabolism was identified by liquid chromatography/mass spectrometry as BaP-3-glucuronide. MPLSM showed that metabolites were sequestered within the yolk, biliary system, and gastrointestinal tract. When the gastrointestinal tract became patent a few days after hatch, the metabolites were rapidly eliminated. These findings indicate that some of the earliest embryonic tissues are metabolically competent and that redistribution of BaP and its metabolic products occurs throughout development. Rapid metabolism of BaP substantially reduces the body burden of parent chemical in the developing embryo, potentially reducing toxicity. It remains unclear whether metabolism of BaP in medaka embryos leads to the formation of DNA adducts associated with genotoxic effects or yields metabolites that later lead to other toxicity in juveniles or adults.