COMT and Parkinson disease: Although seeming paradoxical at first, this increased L-DOPA methylation metabolism in PD can be explained itself by the usually applied external supplementation of L-DOPA: patients treated with L-DOPA show a decrease of SAM [88] and an increase in methylation products and homocysteine [89], while patients treated with dopamine agonists [90,91] or patients additionally treated with COMT inhibitors [92] do not exhibit such changes.