SOD2 and pulmonary arterial hypertension: Another characteristic of PAH is the decrease of NO content in VECs, which may be related to the following two aspects: (1) the decrease in antioxidant manganese superoxide dismutase (MnSOD) in mitochondria since MnSOD increases the biological activity of NO by scavenging the superoxide anion (Fijalkowska et al., 2010); and (2) the high expression level of arginase II, which competes with eNOS for the substrate arginine, thus leading to the subsequent decreased production of NO (Xu et al., 2004).