Electrochemical detection of Zn(II)-induced amyloid-β aggregation: Insights into aggregation mechanisms

The tyrosine based electrochemical analysis of the kinetics of Zn(II)-induced aggregation of the 42-amino-acid long amyloid-beta peptide (A beta 42) implicated in Alzheimer's disease pathogenesis was carried out by square way, voltammetry on carbon screen printed electrodes. As previously reported, the A beta 42 electrooxidation peal current can serve as an estimate of the fraction of A beta 42 peptide molecules not included in Zn(II)-induced A beta 4: aggregates/oligomers. The current was found to drop in a Zn(II)-dependent manner prior to the first measure ment (in a minute after the addition of Zn(II) ions) but to remain unchanged during the following 30-mil incubation period. The electrochemical analysis was applied in parallel with turbidimetry and dynamic ligh scattering (DIS) to monitor in time the occurrence of A beta 42 aggregates induced by substoichiometric Zn(II concentrations. In contrast to the current, A beta 42 solution turbidity steadily increased with time in a Zn(II) dependent manner during the 30-min incubation period. The turbidity increase was accompanied by the oc currence of large (micron-sized) A beta 42 aggregates detected by DLS. These findings suggest that the Zn(II)-in duced aggregation of A beta 42 molecules proceeds via a two-stage mechanism which includes as the first step a fas formation of A beta 42 oligomers (large enough to effectively suppress the A beta 42 electrooxidation via a tyrosine residue), followed by the relatively slow assembly of these oligomers into bulky (micron-sized) aggregates. Thi Zn(II)-triggered A beta 42 oligomerization and, consequently, the overall Zn(II)-induced A beta 42 aggregation were found to depend on the solution ionic strength. When applied to N-truncated and phosphorylated A beta 42 isoforms A beta(6-42) and pS8-A beta 42, the combined analysis has revealed that their aggregation behavior differs from that o the A beta 42 peptide. While Zn(II)-triggered A beta(6-42) oligomers demonstrated a higher propensity to assemble int4 micron-sized aggregates, compared to the A beta 42 peptide, Zn(II)-triggered pS8-A beta 42 oligomers lack the ability to form large aggregates. Thus, the direct electrochemistry when combined with methods allowing for detection o aggregates may provide deeper mechanistic insights into protein/peptide aggregation.