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As the amyloidogenic processing of β-amyloid precursor protein (βAPP) proceeds under conditions of oxidative stress, the methionine-596 residue at the β-secretase cleavage point is likely in an oxidized state. In the present work, possible consequences of the oxidation of Met-596 for the generation of the N-terminus of amyloid β protein were modeled using synthetic peptide substrates, matching 587-606 sequence fragment of βAPP and containing either intact methionine or methionine sulfoxide. Patterns and rates for the cleavage of these substrates by purified mast cell chymase, cathepsin G, cathepsin D, matrix metalloproteinase-3 and neutrophil elastase, were compared. Only the three first proteases, all previously suggested as candidate β-secretases, preferentially cleaved the "intact" substrate after Met-596. For chymase and cathepsin G, the specificity of this cleavage increased upon a shift from optimal alkaline pH to acidic pH, which is also more compatible with the plausible intracellular localization of amyloidogenic βAPP processing. The substitution of methionine sulfoxide for methionine in the substrate slowed down the cleavage rate for all the enzymes tested, by a factor of 6-15. This was associated with shifts of cleavage preferences to points of minor importance for the "intact" peptide, suggesting a specific resistance of the peptide bond after MetSO-596 against proteolysis.