Interactive effects of cadmium and benzo[a]pyrene on metallothionein induction in mummichog (Fundulus heteroclitus)

Previous experiments demonstrated that exposure of mummichog to cadmium (Cd ) in combination with benzo[a]pyrene (BaP) caused a higher mortality than w ould be expected from simple additive effects. Experiments are described he re that investigated whether BaP exposure inhibits the induction of metallo thionein (MT), a major detoxifying protein for Cd, or if reactive BaP metab olites compete with Cd for binding sites on MT. Fish were injected with or without BaP (18 mg/kg) in combination with a low (1 mg/kg) or high (3.2 mg/ kg) dose of Cd, and in one treatment BP was dosed 4 days after Cd. The resu lts showed a rapid induction of MT to 1.5 mg/g wet weight liver, 1 day afte r injecting the low Cd dose. Simultaneous BaP exposure significantly delaye d the induction of MT, for both low and high Cd doses, and BaP temporarily lowered the induced MT concentration when dosed 4 days after induction by C d. To test if binding of BaP metabolites to MT reduces the detoxification p otential for Cd, microsomes of CYP1A-induced fish were incubated with MT an d radiolabeled BaP. Active metabolism of BaP was observed by high-performan ce liquid chromatography analysis, but no association of BaP metabolites wi th MT was found. Neither could this be demonstrated in vivo, in liver MT is olated from mummichog dosed with H-3-BaP and Cd. These results suggest that increased toxicity of Cd in combination with BaP exposure is likely to be caused by inhibited MT synthesis, rather than by interference of BaP metabo lites with Cd binding on MT. (C) 2000 Elsevier Science Ltd. All rights rese rved.