Eight of the twenty-two agents of greatest concern at Superfund sites are toxic metals, which at present at substantial levels in over 60% of all Superfund sites, and which also represent a significant problem at other waste sites and in the environment in general. The mechanisms by which these metals elicit their adverse biological effects are still poorly understood, particularly the effects of low dose, chronic exposures such as occur for most human exposures near these sites. Ultimately, the toxicity of metals can be traced to their specific interactions with biological macromolecules, in particular with cellular proteins. However, there are still only a few examples of specific metal-protein interactions that have been directly associated with subsequent toxic effects. The overall leading to subsequent toxic events based upon previous work by this program and others. We will focus in particular on the protein interactions of arsenic, nickel and chromium. All three of these metals are human carcinogens, and all three have a high affinity for or react with the thiol of cysteines, yet they are not readily detoxified by metallothionein. We hypothesize that these cysteine interactions in key target proteins may mediate or contribute to their mechanisms of action as toxins and carcinogens. Model protein and peptide systems have been chosen for study that represent protein targets in three important protein classes, i.e., the DNA binding domain of glucocorticoid receptor (a hormone-mediated transcription factor), the heme domain of cytochrome P450BM-3 (a xenobiotic-metabolizing enzyme) and sub-domains of XPA (a DNA repair protein). The specific hypothesis to be tested is that these toxic metals bind selectively to one or more critical Cys residues of a target protein and thereby alter the protein's structure, or initiate reactions involving the thiols, either of which could compromise the function of the protein. Thus the goal will be to determine the molecular basis for the effects of these toxic metals on the functional properties of these model proteins. Understanding the molecular mechanisms by which arsenic, chromium and nickel act as human carcinogens will be important in evaluating their overall health effects in exposed populations. In addition, these studies may provide evidence of specific protein adducts that could serve as potential biomarkers both for toxic metal exposure and for biological effects, including unique metal-dependent protein structures, metal-specific modified protein residues, and stable long-lived metal- protein complexes, each of may elicit a specific antibody response.
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