Increased plasma bilirubin in Parkinson patients on L-dopa: evidence against the free radical hypothesis?

Oxidative damage by free radicals may contribute to the etiology of Parkinson's disease (PD), and increased oxidative stress in the nigral cells of PD patients may occur following L-dopa treatment, prompting suggestions that L-dopa therapy should be delayed as long as possible. Bilirubin is a potent antioxidant in vitro, even when bound to albumin, suggesting a physiological role as an antioxidant. Calculations indicate that bilirubin can pass the blood-brain barrier in sufficient quantity to exert a significant antioxidant effect in the brain. We have found a highly significant (about 20%) increase in plasma bilirubin in 162 PD patients on chronic L-dopa treatment compared to 93 untreated parkinsonians and 224 non-parkinsonian controls. We propose that L-dopa-induced increase in nigral oxidative stress in PD may be effectively counteracted by increased bilirubin levels. The mechanism by which plasma bilirubin is increased in patients receiving L-dopa is at present unknown.