Cysteinyl-glycine reduction as marker for levodopa-induced oxidative stress in Parkinson's disease patients.

Oxidative stress is influenced by the thiol homeostasis, which determines the redox milieu. One of its components is Cysteinyl-glycine (Cys-Gly) generation, as its metabolic precursor is the free radicals scavenging glutathione. Levodopa is under suspicion to promote oxidative stress via the turnover of its metabolite dopamine in abundant mitochondria. Objective was to investigate the impact of levodopa on Cys-Gly plasma metabolism. Fifteen patients with Parkinson's disease orally took one 200-mg levodopa/50-mg carbidopa (CD) containing tablet. Levodopa, its derivative 3-O-methyldopa (3-OMD), and free Cys-Gly were measured at baseline, 60 and 120 min following levodopa/CD administration. Cys-gly concentrations decreased, levodopa and 3-OMD levels increased. Inverse relationships appeared between computed differences of Cys-gly and 3-OMD bioavailability. We conclude that Cys-Gly decline is related to levodopa metabolism to 3-OMD. Cys-Gly decay may result from the alternative transformation of glutathione to its oxidized form glutathione dissulfide as consequence of free radical scavenging.