Other major factors responsible for non-physiological ROS production and their importance in PD are transitional metals, including iron [22,23], mitochondrial abnormalities [10,24,25], alpha-synuclein (SNCA) gene expression [26–28], inflammation mediated by microglial activation [29], reduced levels of endogenous antioxidant nutrients (glutathione [GSH] and ascorbic acid), and antioxidant enzymes (catalase [CAT] and GSH peroxidase). Here, CAT is linked to Parkinson disease.