Protein reactivity of 3,4-dihydroxyphenylacetaldehyde, an endogenous, potential neurotoxin relevant to Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by two pathological hallmarks, selective loss of dopaminergic neurons and intraneuronal protein aggregation. The presence of an endogenous neurotoxin has been implicated in the pathogenesis of the disease, to explain the observed neurodegeneration. Dopamine (DA) has been indicated to be an endogenous neurotoxin as DA readily undergoes auto-oxidation to an o-quinone capable of protein modification. However, DA is metabolized by monoamine oxidase to form the intermediate 3,4-dihydroxyphenylacetaldehyde (DOPAL) and several studies have demonstrated DOPAL to be orders of magnitude more toxic than DA. An accumulation of DOPAL may cause dopaminergic cell death via the formation of free radicals, inhibition of the mitochondrial transition pore or protein modification. The hypothesis of this work is that DOPAL, a potential endogenous neurotoxin relevant to PD, is capable of protein modification and protein cross-linking through reactivity with amine and thiol nucleophiles. Results demonstrate that elevated DOPAL concentrations in striatal synaptosomes will yield considerable protein modification. In addition, DOPAL was demonstrated to be highly reactive towards amine nucleophiles in comparison to thiol nucleophiles. However, DOPAL was demonstrated to mediate protein cross-linking through reactivity with protein thiols subsequent to modification of amines, indicating DOPAL to be a bifunctional electrophile. Furthermore, a novel isolation procedure was developed, and through a proteomics-based approach, twelve proteins were identified to be relevant to PD and susceptible to DOPAL modification. This research demonstrates increased concentrations of DOPAL lead to significant cellular consequences (i.e. protein modification) and implicate DOPAL as a potential neurotoxin relevant to the pathogenesis of PD.