Singlet oxygen is generated by several cellular, enzymatic, and chemical reactions as well as by exposure to UV or visible light in the presence of a sensitizer. Consequently, this oxidant has been proposed to be a damaging agent many pathologies. Proteins are major targets for singlet oxygen as a result of their abundance and high rate constants for reaction. In this study, we show that illumination of viable rose bengal-loaded THP-1 (human monocyte-like) cells with visible light gives rise to intracellular protein-derived peroxides. The peroxide yield increases with illumination time, requires the presence of rose bengal, is enhanced in D(2)O, and is decreased by azide, consistent with the mediation of singlet oxygen. The concentration of peroxides detected, which is not affected by glucose or ascorbate loading of the cells, corresponds to about 1.5 nmoles peroxide per 10(6) cells, or 10 nmoles/mg cell protein, and account for up to approximately 15% of the O(2) consumed by the cells. Similar peroxides have been detected on isolated cellular proteins exposed to light in the presence of rose bengal and oxygen. After cessation of illumination, cellular protein peroxide levels decrease with t(1/2) about 4 h at 37 degrees C. Decomposition of protein peroxides formed within cells, or on isolated cellular proteins, by metal ions gives rise to radicals as detected by EPR spin trapping. These studies demonstrate that exposure of intact cells to visible light in the presence of a sensitizer leads to novel long-lived, but reactive, intracellular protein peroxides via singlet oxygen-mediated reactions.