A seasonal study of microsomal mixed-function oxidase components in insecticide-resistant and susceptible mosquitofish, Gambusia affinis

Abstract

The mixed-function oxidase components which are involved in xenobiotic hydroxylations were studied in liver microsomes from two populations of mosquitofish ( Gambusia affinis) for 26 months. One population is highly resistant to organochlorine insecticides and is exposed periodically to agricultural chemical contamination; the other population is susceptible to these chemicals and is not exposed to such pollutants. The following parameters were measured: cytochromes P-450 and b 5; NADPH-cytochrome c, NADPH-dichloroindophenol, NADH-cytochrome c, and NADH-cytochrome b 5 reductases; liver weight to body weight ratios; and microsomal protein. Most parameters demonstrated seasonal cycles with the highest values occurring during the late fall and winter. The cyclic nature of the parameters investigated may be the result of the relative magnitude of microsomal hydroxylations during the year in relationship to other microsomal processes such as biosynthesis. The range of concentrations or specific activities of most of the components was about the same for both fish populations, although the resistant fish possessed relatively larger livers and, therefore, greater amounts of these microsomal components than the susceptible fish. The results suggest the induction of these microsomal oxidase components in the resistant population during their annual exposure to environmental pollutants in the agricultural runoff.