See
http://www.sciencedaily.com/releases...1030153020.htm
where it claims dopal (3,4-dihydroxyphenylacetaldehyde) causes PD.

"The SLU researchers discovered that dopamine itself actually plays a role in destroying the cells that produce it.

In the process that leads to Parkinson's disease, dopamine is converted into a highly toxic chemical called DOPAL. Using test-tube, cell-culture and animal models, the researchers found that it is DOPAL that causes alpha-synuclein protein in the brain to clump together, which in turn triggers the death of dopamine-producing cells and leads to Parkinson's.

"This is very exciting," Burke said. "This is the first time that anyone has ever established that it is a naturally occurring byproduct of dopamine that causes alpha-synuclein to aggregate, or clump together. It's actually DOPAL that kicks this whole process off and results in Parkinson's disease."

I am sure this has been proposed before, for example see below.


Li SW, Lin TS, Minteer S, Burke WJ. 2001. 3,4-Dihydroxyphenylacetaldehyde and
hydrogen peroxide generate a hydroxyl radical: possible role in
Parkinson's disease pathogenesis. Molecular Brain Research 93(1):1-7.
Abstract: 3,4-Dihydroxyphenylacetaldehyde (DOPAL) and 3,4-
dihydroxyphenylglycolaldehyde (DOPEGAL), the monoamine oxidase
(MAO) metabolites of dopamine (DA) and norepinephrine (NE),
respectively, are toxic to catecholamine (CA) neurons in vitro and in vivo.
DOPEGAL generates a free radical and activates mitochondrial permeability
transition, a mechanism implicated in neuron death. To determine if DOPAL
and other DA metabolites generate the hydroxyl radical in the presence of
H2O2, we used HPLC-EC to detect salicylate hydroxylation products. To
determine the relative reducing capacity of DOPAL and DOPEGAL we used
cyclic voltammetry to measure their reduction potentials. Results indicate
that DOPAL, but not DOPEGAL, DA or other DA metabolites. generates
hydroxyl radicals. Atomic absorption spectroscopy and heavy metal
screening indicate that this result is not due to contamination of DOPAL with
iron or other heavy metals. DOPAL reduction potential ( 161 mV) is lower
than that of DOPEGAL (235 mV). DOPAL is present in human substantia
nigra. The implications of these findings to CA neuronal death in
degenerative brain diseases are discussed. (C) 2001 Elsevier Science B.V.