Extensive research has linked the amyloid-beta (Aβ) peptide to neurological dysfunction in Alzheimer's disease (AD). Insoluble Aβ plaques in the AD patient brain contain high concentrations of advanced glycation end-products (AGEs) as well as transition metal ions. This research elucidated the roles of Aβ, sugars, and Cu in the oxidative stress mechanism of AD at the molecular level. Mass spectral (MS) analysis of the reactions of Aβ with two representative sugars, ribose-5-phosphate (R5P) and methylglyoxal (MG), revealed Lys-16 and Arg-5 as the primary glycation sites. Quantitative analysis of superoxide [Formula: see text] production by a cyt c assay showed that Lys-16 generated four times as much [Formula: see text] as Arg-5. Lys-16 and Arg-5 in Aβ are both adjacent to histidine residues, which are suggested to catalyze glycation. Additionally, Lys-16 is close to the central hydrophobic core (Leu-17-Ala-21) and to His-13, both of which are known to lower the pKa of the residue, leading to increased deprotonation of the amine and an enhanced glycation reactivity compared to Arg-5. Gel electrophoresis results indicated that all three components of AD plaques-Aβ, sugars, and Cu-are necessary for DNA damage. It is concluded that the glycation of Aβ with sugars generates significant amounts of [Formula: see text], owing to the rapid glycation of Lys-16 and Arg-5. In the presence of Cu, [Formula: see text] converts to hydroxyl radical (HO·), the source of oxidative stress in AD.

We previously reported that a Pt(IV) complex, [Pt(IV)(dach)Cl4] [trans-d,l-1,2-diaminocyclohexanetetrachloroplatinum(IV)] binds to the N7 of 5'-dGMP (deoxyguanosine-5'-monophosphate) at a relatively fast rate and oxidizes it to 8-oxo-5'-dGMP. Here, we further studied the kinetics of the oxidation of 5'-dGMP by the Pt(IV) complex. The electron transfer rate constants between 5'-dGMP and Pt(IV) in [H8-5'-dGMP-Pt(IV)] and [D8-5'-dGMP-Pt(IV)] were similar, giving a small value of the kinetic isotope effect (KIE: 1.2 ± 0.2). This small KIE indicates that the deprotonation of H8 in [H8-5'-dGMP-Pt(IV)] is not involved in the rate-determining step in the electron transfer between guanine (G) and Pt(IV). We also studied the reaction of 5'-dGDP (deoxyguanosine-5'-diphosphate) and 5'-dGTP (deoxyguanosine-5'-triphosphate) with the Pt(IV) complex. Our results showed that [Pt(IV)(dach)Cl4] oxidized 5'-dGDP and 5'-dGTP to 8-oxo-5'-dGDP and 8-oxo-5'-dGTP, respectively, by the same mechanism and kinetics as for 5'-dGMP. The Pt(IV) complex binds to N7 followed by a two-electron inner sphere electron transfer from G to Pt(IV). The reaction was catalyzed by Pt(II) and occurred faster at higher pH. The electron transfer was initiated by either an intramolecular nucleophilic attack by any of the phosphate groups or an intermolecular nucleophilic attack by free OH(-) in the solution. The rates of reactions for the three nucleotides followed the order: 5'-dGMP > 5'-dGDP > 5'-dGTP, indicating that the bulkier the phosphate groups are, the slower the reaction is, due to the larger steric hindrance and rotational barrier of the phosphate groups.