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"...earlier study, published January 2013 in Proceedings of the National Academy of Sciences, the UCLA team discovered a link between Parkinson's and the pesticide benomyl, a fungicide that has been banned by the U.S. Environmental Protection Agency. That study found that benomyl prevents the enzyme aldehyde dehydrogenase (ALDH) from converting aldehydes — organic compounds that are highly toxic to dopamine cells in the brain — into less toxic agents, thereby contributing to the risk of Parkinson's.

For the current study, UCLA researchers...tested a number of additional pesticides and found 11 that also inhibit ALDH and increase the risk of Parkinson's — and at levels much lower than they are currently being used, said the study's lead author, Jeff Bronstein, a professor of neurology and director of the movement disorders program at UCLA.

... team also found that people with a common genetic variant of the ALDH2 gene are particularly sensitive to the effects of ALDH-inhibiting pesticides and are two to six times more likely to develop Parkinson's when exposed to these pesticides than those without the variant...

...Exposure to pesticides starts a cascade of cellular events, preventing ALDH from keeping a lid on the aldehyde DOPAL, a toxin that naturally occurs in the brain. When ALDH does not detoxify DOPAL sufficiently, it accumulates, damages neurons and increases an individual's risk of developing Parkinson's.

"ALDH inhibition appears to be an important mechanism by which these environmental toxins contribute to Parkinson's pathogenesis, especially in genetically vulnerable individuals," said study author Beate Ritz, a professor of epidemiology at UCLA's Fielding School of Public Health. "This suggests several potential interventions to reduce Parkinson's occurrence or to slow its progression."

In the current study, the research team developed a lab test to determine which pesticides inhibited ALDH..

Researchers ID more pesticides linked to Parkinson's, gene that increases risk | UCLA

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(Gene referred to: mitochondrial ALDH2 gene)

All of the metal-coordinating dithiocarbamates tested (e.g., maneb, ziram), 2 imidazoles (benomyl, triflumizole), 2 dicarboxymides (captan, folpet), and 1 organochlorine (dieldrin) inhibited ALDH activity, potentially via metabolic byproducts (e.g., carbon disulfide, thiophosgene). Fifteen screened pesticides did not inhibit ALDH. Exposures to ALDH-inhibiting pesticides were associated with 2- to 6-fold increases in PD risk; genetic variation in ALDH2 exacerbated PD risk in subjects exposed to ALDH-inhibiting pesticides.
CONCLUSION:
ALDH inhibition appears to be an important mechanism through which environmental toxicants contribute to PD pathogenesis, especially in genetically vulnerable individuals, suggesting several potential interventions to reduce PD occurrence or slow or reverse its progression

Aldehyde dehydrogenase variation enhances effect of pesticides associated with Parkinson disease. - PubMed - NCBI