CAT and Alzheimer disease: The levels of Iron are also found to be elevated in the substantia nigra of patients with PD, which could serve as a catalyst for the Fenton's reaction in producing hydroxyl radicals [119–121], whereas the activities of antioxidant defense enzymes such as glutathione peroxidase (GPx) and reductase (GR), superoxide dismutase (SOD) and catalase (CAT) are reduced in the affected brain regions of patients with AD [117, 122, 123].