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

beta-amyloid (A beta) is a normal soluble peptide found in the cerebrospina l fluid (CSF) and other biological fluids. A beta fibrils are associated wi th Alzheimer's disease (AD) senile plaques. We have used purified soluble A beta (1-42) and A beta (12-28) peptides in order to determine the oxidativ e modification induced in these peptides by exposure to peroxidase and hydr ogen peroxide. We have demonstrated that under these in vitro conditions, d imeric forms of A beta (1-42) can be detected by high-resolution polyacryla mide SDS-PAGE electrophoresis. Further experiments performed by reverse-pha se high performance liquid chromatography (RP-HPLC), and monitored by fluor escence defection, showed that the dimeric A beta (1-42) forms induced by t he peroxidase reaction are the outcomes of dityrosine bridge formation. Thi s cross-link results from the enzyme catalyzed oxidation. During this react ion, phenolic coupling of tyrosine residues of two A beta (1-42) peptides o ccurs. Mo detectable peroxidative modifications were observed with the A be ta(12-28)peptide which lacks a tyrosine residue Since oxidative stress is t hought 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 sA beta to amyloid fi brils and eventually the formation of senile plaques in AD.