NOS3 and cancer: Cancer cells are characterized by increased aerobic glycolysis and high levels of oxidative stress.199 This oxidative stress is exerted by reactive oxygen species (ROS) that accumulate as a result of an imbalance between ROS generation and elimination.200 ROS are constantly produced by both enzymatic and non-enzymatic reactions.200 Enzyme-catalyzed reactions that generate ROS include those involving NADPH oxidase, xanthine oxidase, uncoupled endothelial nitric oxide synthase (eNOS), arachidonic acid and metabolic enzymes such as the cytochrome P450 enzymes, lipoxygenase and cyclooxygenase.