XENOBIOTIC BIOTRANSFORMATION SYSTEMS IN FISHES

Early speculations that fish would not possess detoxification systems because of the excretory activity of the gill have been shown to be wrong. Fish are capable of xenobiotic metabolism by microsomal oxidation, reduction and conjugation. Enzyme characteristics appear to be similar between fish and mammals, with the exception of typically lower temperature optima in fish which may reflect the usual lower operating temperatures of heterotherms as compared to homeotherms. Early reports of little or no enzyme activity in fish may have reflected a lack of knowledge of the lower temperature optima in fish. Even under optimized conditions, fish demonstrate lower enzyme activities than mammals. Although reports are fragmentary, evolutionary trends can be observed among fish with some biotransformation systems absent or less developed in the more primitive fish as compared with the more advanced fish. The less effective induction system in fish as compared to mammals might reflect the fact that fish live in an environment in which toxicants are diluted, thereby reducing the response of enzymes to a xenobiotic challenge. The real significance of the xenobiotic biotransformation enzymes to the fish from an in vivo standpoint has not been extensively investigated. In a few cases the metabolism of a xenobiotic by fish is known to affect its toxicity; the lower enzyme activities that fish possess may be sufficient for the disposition of the concentrations of toxicants that fish ordinarily encounter. However, the aquatic environment is becoming more contaminated than ever before. It cannot be predicted whether fish have the capability to adapt to and process these higher concentrations of toxicants. Further study of fish biotransformation systems is essential for an evaluation of fish-toxicant interactions.