Modulation effect of acidulated human serum albumin on Cu2+-mediated amyloid β-protein aggregation and cytotoxicity under a mildly acidic condition.

Aggregation of amyloid β-proteins (Aβ) induced by Cu2+ is a crucial element in the pathogenesis of Alzheimer's disease (AD), and cerebral acidosis is a common complication of AD. Under mildly acidic conditions, Cu2+-Aβ species have higher tendency to generate neurotoxic aggregates. Hence it is of significance to develop potent agents that inhibit Cu2+-mediated Aβ aggregation under a mildly acidic condition. Herein we synthesized acidulated human serum albumin (A-HSA) to mitigate Cu2+-mediated Aβ42 aggregation and cytotoxicity at pH6.6. Extensive experiments showed that A-HSA altered the pathway of Cu2+-mediated Aβ42 aggregation and protected SH-SY5Y cells from cytotoxicity and oxidative damage induced by Cu2+-Aβ42 species. Equimolar A-HSA increased cell viability from 52% to 91% as compared to Cu2+-Aβ42-treated group. Stopped-flow fluorescence analysis revealed that A-HSA changed the Cu2+-Aβ42 coordination mode from component I to II on the second timescale at pH6.6, which avoided the formation of aggregation-prone Cu2+-Aβ42 aggregates. The findings revealed that the more negative charges on A-HSA surface could stabilize the protonated form of the adjacent histidine residues of Aβ42. Hence, component I, which is necessary to form toxic aggregates, became unstable in the presence of A-HSA. On the other hand, hydrophobic binding and electrostatic repulsion could work simultaneously on the bound Aβ42 on A-HSA surface. The two opposite forces stretched Aβ42 conformations, which inhibited the formation of toxic Cu2+-Aβ42 aggregates. Thus, A-HSA worked as a bifunctional inhibitor against Cu2+-mediated Aβ42 aggregation and cytotoxicity under a mildly acidic condition.