Copper-dependent inhibition of cytochrome c oxidase by Aβ1−42 requires reduced methionine at residue 35 of the Aβ peptide.

By altering key amino acid residues of the Alzheimer's disease- associated amyloid-β peptide, we investigated the mechanism through which amyloid-β inhibits cytochrome c oxidase (EC 1.9.3.1). Native amyloid-β inhibited cytochrome oxidase by up to 65%, and the level of inhibition was determined by the period of amyloid-β ageing before the cytochrome oxidase assay. Substituting tyrosine-10 with alanine did not affect maximal enzyme inhibition, but the altered peptide required a longer period of ageing. By contrast, oxidizing the sulfur of methionine-35 to a sulfoxide, or substituting methionine-35 with valine, completely abrogated the peptide's inhibitory potential towards cytochrome oxidase. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed that the loss of inhibitory potential towards cytochrome oxidase with the methionine-35-altered peptides did not correlate with a substantially different distribution of amyloid-β oligomeric species. Although the amyloid-βmediated inhibition of cytochrome oxidase was completely dependent on the presence of divalent Cu2+, it was not supported by monovalent Cu+, and experiments with catalase and H2O2 indicated that the mechanism of cytochrome oxidase inhibition does not involve amyloid-β-mediated H2O2 production. We propose that amyloid-β-mediated inhibition of cytochrome oxidase is dependent on the peptide's capacity to bind, then reduce Cu2+, and that it may involve the formation of a redox active amyloid-β-methionine radical. [ABSTRACT FROM AUTHOR]