Fig. 1
From: Oxidative stress in acute pulmonary embolism: emerging roles and therapeutic implications
Metabolic pathways of ROS and how NO affects smooth muscle cells. Oxidative stress is the imbalance of the oxidative system and antioxidant system. Several enzymes and pathways form the antioxidant system. The ROS-producting system contains enzymes such as enzyme in mitochondria, XO, eNOS, and NOX. Under the catalysis of enzymes, O2− and H2O2 are produced. ROS can either be transformed into water or strong oxidizing substances. NO relaxes smooth muscle by signal transduction. By inhibiting the cGMP pathway specifically, sildenafil promotes the relaxation of smooth muscle cells (ADP: adenosine diphosphate; ATP: adenosine triphosphate; CAT: catalase; cGMP: cyclic guanosine monophosphate; Cl−: chloride ion; eNOS: endothelial nitric oxide synthase; Fe2+: ferrous iron; GC: guanylate cyclase; GTP: guanosine triphosphate; H2O2: hydrogen peroxide; L-Arg: L-arginine; MPO: myeloperoxidase; NO: nitric oxide;NOX: nicotinamide adenine dinucleotide phosphate oxidase; NADPH: nicotinamide adenine dinucleotide phosphate-reduced; NADP+: nicotinamide adenine dinucleotide phosphate; NO: nitric oxide; O2: oxygen; ·O2−: superoxide;·ONOO−: peroxynitrite anion;·OH: hydroxyl radical; OCl−: hypochlorite ion; PDE-5: 5-phosphodiesterase; PKG: protein kinase G SOD: superoxide dismutase; XO: xanthine oxidase)