It was discussed here 3 years ago and looked promising. People cited articles from the 1980s.
healthunlocked.com/parkinso...
A quick google scholar search throws up stuff like:
Methionine sulfoxide reductase A protects dopaminergic cells from Parkinson's disease-related insults
sciencedirect.com/science/a...
and
S‐adenosyl‐methionine improves depression in patients with Parkinson's disease in an open‐label clinical trial (n=13)