MPO and cancer: There were significant and dose-dependent increases in inducible nitric oxide synthase (iNOS), nitrate/nitrite, and myeloperoxidase (MPO), and decreases in the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), and glutathione reductase (GSR) in all cells, but responses were stronger in the cancer cells.