The fate of organic xenobiotics in aquatic ecosystems: quantitative and qualitative differences in biotransformation by invertebrates and fish.

Biotransformation of natural and man-made foreign compounds (xenobiotics) proceeds via introduction of a functional group (phase I metabolism) and subsequent attachment of a polar moiety to the group (phase II metabolism). The biotransformation fate of xenobiotics depends on the activities, complement and inducibility of the biotransformation enzymes. Previous analysis of the dependence of in vivo rates of biotransformation on tissue parent compound concentration for marine invertebrates revealed that hydrocarbons are metabolised more slowly than xenobiotics already containing functional groups, and crustaceans metabolise both types of xenobiotics faster than molluscs (Livingstone D.R., Persistent pollutants in marine ecosystems, pp. 3-34, Pergamon, Oxford). Use of the same approach showed that fish metabolise pentachlorophenol (PCP) and benzo[a]pyrene (BaP) faster than certain aquatic invertebrates, viz. rates of biotransformation to total metabolites (pmol min-1 g-1 wet wt.) at a tissue parent compound concentration of 10 nmol g-1 were, respectively, 19.2 +/- 3.7 (Carassius auratus) and 4.8 +/- 6.6 (molluscan species) (PCP), and 19.1 +/- 6.3 (fish species) and 2.1 +/- 0.2 (crustacean species) (BaP). The higher rate of biotransformation of BaP in fish is consistent with higher levels of total cytochrome P450 and inducible cytochrome P4501A (CYP1A) activity. The similar rate of metabolism of a hydrocarbon (BaP) (requires initial metabolism by cytochrome P450) and a functional group compound (PCP) by fish may also be due to the high levels of cytochrome P450, compared with the situation in invertebrates where rate-limiting cytochrome P450 may be responsible for the lower rates of hydrocarbon compared with functional group compound metabolism.