SAM is part of a complex methylation cycle. It normally gives up its methyl group (provided by glycine or TMG) in the body.

My Drug-Nutrient Depletion handbook lists for Levodopa and the combo Levodopa/Carbidopa that SAM and potassium are depleted with long term use.

When SAM gives up that methyl group for whatever reason, it is recycled back to homocysteine. Hence for people taking extra SAM in the form of SAMe supplement it is recommended to take
B12, B6 and folate as insurance to prevent SAM from raising homocysteine. (in all people using it)

This is the methylation map I found once and keep in my favorites:
http://www.heartfixer.com/AMRI-Outco...yl%20Cycle.htm

Of course, B6 is not recommended for people using levodopa, and that has been a long term recommendation. So I don't know where that leaves homocysteine regeneration issues?

And carbidopa itself , reacts with 5-HTP to cause a scleroderma like condition of the skin... 5-HTP is made in the body from tryptophan and B6 and magnesium. So some sources list this combo as serious and to be avoided.

So this is why PD and some supplements become a murky situation IMO. For those not using levodopa at all, the question is moot.