Measurement of Interpeptidic CuII Exchange Rate Constants of CuII-Amyloid-β Complexes to Small Peptide Motifs by Tryptophan Fluorescence Quenching.

Abstract

The interpeptidic CuII exchange rate constants were measured for two Cu amyloid-β complexes, Cu(Aβ1-16) and Cu(Aβ1-28), to fluorescent peptides GHW and DAHW using a quantitative tryptophan fluorescence quenching methodology. The second-order rate constants were determined at three pH values (6.8, 7.4, and 8.7) important to the two Cu(Aβ) coordination complexes, components Cu(Aβ)I and Cu(Aβ)II. The interpeptidic CuII exchange rate constant is approximately 104 M-1 s-1 but varies in magnitude depending on many variables. These include pH, length of the Aβ peptide, location of the anchoring histidine ligand in the fluorescent peptide, number of amide deprotonations required in the tryptophan peptide to coordinate CuII, and interconversion between Cu(Aβ)I and Cu(Aβ)II. We also present EPR data probing the CuII exchange between peptides and the formation of ternary species between Cu(Aβ) and GHW. As the nonfluorescent GHK and DAHK peptides are important motifs found in the blood and serum, their ability to sequester CuII ions from Cu(Aβ) complexes may be relevant for the metal homeostasis and its implication in Alzheimer's disease. Thus, their kinetic CuII interpeptidic exchange rate constants are important chemical rate constants that can help elucidate the complex CuII trafficking puzzle in the synaptic cleft.