ApoD (apolipoprotein D) is up-regulated in AD (Alzheimer's disease) and upon oxidative stress. ApoD inhibits brain lipid peroxidation in vivo, but the mechanism is unknown. Specific methionine residues may inhibit lipid peroxidation by reducing radical-propagating L-OOHs (lipid hydroperoxides) to non-reactive hydroxides via a reaction that generates MetSO (methionine sulfoxide). Since apoD has three conserved methionine residues (Met(49), Met(93) and Met(157)), we generated recombinant proteins with either one or all methionine residues replaced by alanine and assessed their capacity to reduce HpETEs (hydroperoxyeicosatetraenoic acids) to their HETE (hydroxyeicosatetraenoic acid) derivatives. ApoD, apoD(M49-A) and apoD(M157-A) all catalysed the reduction of HpETEs to their corresponding HETEs. Amino acid analysis of HpETE-treated apoD revealed a loss of one third of the methionine residues accompanied by the formation of MetSO. Additional studies using apoD(M93-A) indicated that Met(93) was required for HpETE reduction. We also assessed the impact that apoD MetSO formation has on protein aggregation by Western blotting of HpETE-treated apoD and human brain samples. ApoD methionine oxidation was associated with formation of apoD aggregates that were also detected in the hippocampus of AD patients. In conclusion, conversion of HpETE into HETE is mediated by apoD Met(93), a process that may contribute to apoD antioxidant function.