The parkin protein functions as a RING-type ubiquitin protein ligase. Considering the possibility that impaired ubiquitin-proteosomal system activity may impair antioxidant defenses and enhance oxidative stress, we have investigated the activity of mitochondrial respiratory enzymes in patients with parkin gene mutations. A significant decrease in the leukocyte complex I activity was found both in patients with parkin mutations (62.5%) and idiopathic PD (64.5%) compared with age-matched controls (P < 0.001). Complex IV activity was also decreased significantly in idiopathic PD patients (60%), but no difference was detected between controls and patients with parkin mutations. (C) 2003 Movement Disorder Society.