In vitro peroxidase oxidation induces stable dimers of β-amyloid (1-42) through dityrosine bridge formation

Abstract

β-amyloid (Aβ) is a normal soluble peptide found in the cerebrospinal fluid (CSF) and other biological fluids. Aβ fibrils are associated with Alzheimer's disease (AD) senile plaques. We have used purified soluble Aβ (1-42) and Aβ (12-28) peptides in order to determine the oxidative modification induced in these peptides by exposure to peroxidase and hydrogen peroxide. We have demonstrated that under these in vitro conditions, dimeric forms of Aβ (1-42) can be detected by high-resolution polyacrylamide SDS-PAGE electrophoresis. Further experiments performed by reverse-phase high performance liquid chromatography (RP-HPLC), and monitored by fluorescence detection, showed that the dimeric Aβ (1-42) forms induced by the peroxidase reaction are the outcomes of dityrosine bridge formation. This cross-link results from the enzyme catalyzed oxidation. During this reaction, phenolic coupling of tyrosine residues of two Aβ (1-42) peptides occurs. No detectable peroxidative modifications were observed with the Aβ (12-28) peptide which lacks a tyrosine residue. Since oxidative stress is thought to be associated with AD, the experimental model described here can help in understanding the early events leading to chemical, structural and conformational modifications before the conversion of sAβ to amyloidfibrils and eventually the formation of senile plaques in AD.