Extracellular superoxide (O2-.) was detected in cultures of monkey arterial smooth muscle cells as measured by the superoxide dismutase-inhibitable reduction of cytochrome c and acetylated cytochrome c. Reduction of cytochrome c by these cells required L-cystine in the incubation medium. A variety of other sulfur-containing amino acids, including D-cystine, L-cystathionine, L-methionine, and djenkolic acid did not support O2-. generation when present at concentrations equimolar to L-cystine. At millimolar concentrations, the chelators EDTA and diethylene triamine penta-acetic acid inhibited O2-. production by smooth muscle cells. This effect was maximal when the chelator was present at the same concentration as the sum of the Ca2+ and Mg2+ in the medium, suggesting a role for these cations in O2-. generation by cells. Modification of low density lipoprotein (LDL) by arterial smooth muscle cells, as assessed by changes in lipid peroxide content, mobility on agarose gel electrophoresis, and apoprotein B fragmentation, was also L-cystine-dependent. LDL modification also required micromolar concentrations of the transition metal ion Cu(II) or Fe(III) and was inhibited by superoxide dismutase. LDL modified by smooth muscle cells in the presence of L-cystine and Cu(II) was taken up and degraded less well than native LDL by human skin fibroblasts, suggesting that recognition by the LDL receptor was lost. In contrast, LDL modified by smooth muscle cells was taken up and degraded to a greater degree than native LDL by mouse peritoneal macrophages, consistent with recognition by the scavenger receptor. These results indicate that monkey arterial smooth muscle cells produce O2-. and modify LDL by an L-cystine-dependent process. This may involve reduction of cystine to a thiol, possibly cysteine or a cysteine-containing peptide such as glutathione. Sulfur-containing amino acids may play a role in atherogenesis by supporting cell-mediated generation of reactive oxygen species and modification of lipoprotein to a form recognized by the scavenger receptor.