The presence of the cholesterol ozonolysis products, 3beta-hydroxy-5-oxo-5,6-secocholestan-6-al (atheronal-A) and its aldolization product 3beta-hydroxy-5beta-hydroxy-B-norcholestane-6beta-carboxaldehyde (atheronal-B) in human atherosclerotic tissues was recently reported as evidence for the generation of ozone by activated human neutrophils. However, the mechanism for the formation of atheronals in atherosclerotic tissues is unknown. In this study, we found that atheronals were formed by the reaction of cholesterol with human myeloperoxidase (MPO) in the presence of its substrates H(2)O(2) and Cl(-). The omission of either H(2)O(2) or Cl(-) from the MPO-H(2)O(2)-Cl(-) system resulted in a significant reduction in yields. The formation of atheronals by the MPO-H(2)O(2)-Cl(-) system was inhibited by an inhibitor of MPO and scavengers of reactive oxygen species such as sodium azide, methionine, beta-carotene, and vinylbenzoic acid. Our results suggest that MPO produces atheronals at least partly through an ozone-free mechanism, via the reaction of cholesterol with singlet oxygen generated from HOCl and H(2)O(2).