Psoralens, together with ultraviolet light A (PUVA), are used in the treatment of epidermal proliferative disorders. Although these compounds can enter cells and photo cross-link DNA, lipids and proteins, including a specific membrane receptor, are also potential targets for the psoralens. To better elucidate the site of action of the psoralens, we have synthesized a family of 5'-mercurio-substituted derivatives of 4',5'-dihydropsoralen. These compounds are identified by their heavy metal content and can be used as a model to deliver thiol reactive psoralen derivatives into keratinocytes. The 5'-mercuriopsoralen derivatives were found to be effective inhibitors of keratinocyte growth without photoactivation. The most active compound, 4,8-dimethyl-5'-iodomercuriomethyl-4',5'-dihydropsoralen (IC50=10 microM), was also a potent photosensitizer (IC50=0.3 microM). Depletion of keratinocyte GSH with buthionine sulfoximine markedly increased their sensitivity to this analog, both with and without UVA light. In contrast, N-acetyl-L-cysteine partially protected the cells from growth inhibition, indicating that a sulfhydryl-sensitive site is growth limiting and that this target can be photoactivated. Iodomercurio-4',5'-dihydropsoralen was found to form adducts with GSH and cysteine, which were not active without UVA light. Thus, these adducts may also contribute to the photosensitization reactions of the parent compound. Using plasmid DNA unwinding assays, iodomercurio-4',5'-dihydropsoralen was also found to modify DNA, an activity that increased following UVA light treatment. This suggests that DNA damage may contribute to the actions of these psoralens. Taken together, our data demonstrate that there are multiple sites of action for mercuriopsoralens. These compounds may prove useful for understanding the mechanisms of psoralen-induced growth inhibition in the skin.