MPO and chronic obstructive pulmonary disease: Airway epithelial cells from COPD patients produce intracellular ROS mainly from leaky abnormal mitochondria [28], but also from membrane-bound reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOX), from the xanthine/xanthine oxidase system and also from neutrophil-derived myeloperoxidase (MPO) [20].