Rapid detection of DNA damage could serve as a basis for in vitro genotoxicity screening for new organic compounds. Ultrathin films (20-40 nm) containing myoglobin or cytochrome P450(cam) and DNA grown layer-by-layer on electrodes were activated by hydrogen peroxide, and the enzyme in the film generated metabolite styrene oxide from styrene. This styrene oxide reacted with double stranded (ds)DNA in the same film, mimicking metabolism and DNA damage in human liver. DNA damage was detected by square wave voltammetry (SWV) by using catalytic oxidation with Ru(bPY)(3)(2+) (bpy = 2,2'-bipyridine) and by monitoring the binding of Co(bpy)(3)(3+). Damaged DNA reacts more rapidly than intact ds-DNA with Ru(bpy)(3)(3+), giving SWV peaks at similar to1 V versus SCE that grow larger with reaction time. Co(bpy)(3)(3+) binds more strongly to intact ds-DNA, and its SWV peaks at 0.04 V decreased as DNA was damaged. Little change in SWV signals was found for incubations of DNA/enzyme films with unreactive organic controls or hydrogen peroxide. Capillary electrophoresis and HPLC-MS suggested the formation of styrene oxide adducts of DNA bases under similar reaction conditions in thin films and in solution. The catalytic SWV method was more sensitive than the Co(bpy)(3)(3+) binding assay, providing multiple measurements over a 5 min reaction time.