| Summary: | Amyloid fibrils are associated with a large number of diseases in which proteins and peptides abnormally assemble to form insoluble amyloid that deposit in the tissues. However, oxidative stress has been implicated in the pathogenesis of a number of neurodegenerative diseases and is believed to play an important role in the amyloid deposition through protein cross-linkings. Under oxidative stress conditions, tyrosyl radicals can be formed and coupled to form dityrosine cross-linkage. The formation of dityrosine cross-linked oligomers is one of the oxidative modifications that may mediate the toxicity of amyloid β (Aβ) and α-synuclein (α-syn) in Alzheimer's disease (AD) and Parkinson's disease (PD) respectively. In this thesis, I explored the oxidative modification of two short peptides, HYFNIF and VIYKI, using a Cu2+/H2O2 oxidation system, and studied the morphological and conformational changes of these amyloid fibrils during the oxidation process. These peptides were selected as simple amyloid model systems that have been previously structurally characterised, to better understand the dityrosine formation at a structural level and to optimise the oxidation conditions. Oxidative stress has been implicated in AD. Here, I have explored the formation of dityrosine cross-linked Aβ42 in vitro. We have shown that dityrosine is generated in internalised Aβ in cell cultures. Results also revealed the prevalence of dityrosine crosslinks in amyloid plaques in brain tissue and cerebrospinal fluid from AD patients, indicating that dityrosine could be used as a biomarker of oxidative stress in AD. The ability of the Cu2+ ion to promote the formation of in vitro dityrosine cross-linked α-syn was also explored and the effect on α-syn fibrillogenesis and conformation induced by Cu2+ was investigated. The results revealed the possibility of involvement the dityrosine cross-linked α-syn dimer as a nucleus to initiate the polymerisation process of α-syn to form amyloid fibrils. Dityrosine cross-linkages can be generated in vitro using oxidation system of Cu2+/H2O2, and might play an important role in the solubility and assembly of amyloidogenic peptides and proteins that are associated in the pathogenesis of many neurodegenerative disease including AD and PD. Dityrosine cross-linkages can lend a further stability to the already stable amyloid fibrils, and this may explain their protease resistance. Dityrosine cross-links formation represents one of the possible pathways by which oligomers can be formed. Dityrosine cross-linked oligomers represent a good bio-index of oxidatively coupled tyrosine-contained proteins due to their high stability.
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