Seasonal variation of mixed-function oxidase activity in a population of Yellow-Legged herring gull: relationship to sexual cycle and pollutants

Induction of the mixed-function oxidase (MFO) system has commonly been used as a biochemical marker of xenobiotic contamination in birds. The present study considers the influence of intrinsic (e.g. sexual cycle) and extrinsic (xenobiotic) factors in determining the levels of MFO activity in a population of Yellow-legged Herring gull (Larus cachinnans). Aldrin epoxidase undergoes marked seasonal variation, reaching a peak in the breeding season; on the other hand 7-ethoxyresorufin dealkylation activity is more closely related to certain organochlorine residues (polychlorinated biphenyls. Lindane, hexachlorobenzene). Seasonal variations in NADPH-and NADH-cytochrome c reductase and NADH-ferricyanide reductase activities were also found. These results suggest that seasonal variations in MFO activity, which may be related to the sexual cycle, need to be taken into account when considering biochemical effects of pollutants.