Date Awarded

1998

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Morris H. Roberts, Jr

Abstract

When animals are exposed to mixtures of environmental pollutants, it is generally assumed that the toxic effects of the individual components are additive. However, examples of synergistic and antagonistic effects have been described. to study the mechanisms of interaction between a metal and a polycyclic aromatic hydrocarbon, mummichog (Fundulus heteroclitus) were injected with combinations of cadmium (Cd) and benzo (a) pyrene (BP). Measured effect parameters were: mortality, BP-metabolite production in isolated hepatocytes and microsomes, hepatic induction of CYP1A (the BP metabolizing enzyme) and metallothionein (the Cd binding protein), and biliary excretion of BP-metabolites. The mortality data demonstrated that both synergistic and antagonistic effects can occur. A Cd dose of 0.32 mg/kg significantly reduced the expected mortality caused by BP. In contrast, a BP dose of 10 mg/kg significantly increased the toxicity of Cd above the expected mortality. to study the mechanisms of these interactive effects, liver cells (hepatocytes) were isolated from fish that were previously injected with combinations of Cd and BP. These cells were exposed to radiolabeled BP to study the rate of BP metabolism, and the formation of BP-metabolites. Cadmium exposure had an overall inhibiting effect on the metabolism of BP. No effects of Cd were observed on the formation of individual metabolites. to distinguish between direct interference of Cd with CYP1A at the active site versus indirect interference by inhibiting CYP1A induction, microsomal preparations were evaluated for enzyme activity and enzyme concentration. While there was no direct effect of Cd on enzyme catalytic activity, there was an effect on CYP1A production. The demonstrated inhibition of BP metabolism by Cd would suggest a reduced excretion of BP-metabolites. However, analysis of bile and water samples after fish were injected with radiolabeled BP demonstrated an enhanced biliary excretion of conjugated BP-metabolites under influence of Cd. Cadmium exposure caused a significant induction of hepatic metallothionein in the fish. When BP was dosed together with Cd, the induction of MT was inhibited. The hypothesis that reactive BP metabolites would compete with Cd for binding sites on MT could not be confirmed. There was no measurable binding of BP to MT.

DOI

https://dx.doi.org/doi:10.25773/v5-0qat-jq35

Rights

© The Author

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