Amyloid beta (Aβ) peptide accumulation has been demonstrated to play a central role in Alzheimer's disease (AD). Substantial evidence indicates that protein nitrotyrosination contributes to Aβ-dependent neurotoxicity; however, the molecular mechanism is unknown. Recent research has shown that Aβ complexes with heme to form Aβ-heme, and increases the pseudo-peroxidase activity of heme. We found that Aβ-heme uses H(2)O(2) and NO(2)(-) to cause nitration of enolase and synaptic proteins more effectively than heme. Thus, the increased peroxidase activity of Aβ-heme may be the molecular link between excess Aβ and the widespread protein nitration in AD. Interestingly, the site of enolase nitration that was catalyzed by Aβ-heme is different from that induced by heme. Moreover, the secondary structural perturbations of Aβ-heme-treated and heme-treated enolase are also different. These observations suggest that Aβ-heme targets specific amino acid sequences in enolase. Furthermore, our data show that Aβ-heme peroxidase activity is independent of the aggregation state of Aβ, suggesting an important role of soluble Aβ in addition to Aβ aggregates and oligomers in AD pathogenesis.