To activate a G protein-coupled receptor permanently with cell surface photodynamic action in the gastrointestinal tract

Figure 1 Reactive oxygen species and their average lifetime. The nicotinamide adenine dinucleotide phosphate oxidase of phagocytes catalyzes the generation and release of superoxide (O₂^-.) after transfer of an electron to molecular oxygen (O₂), O₂^-. is transformed into hydrogen peroxide (H₂O₂) by superoxide dismutase. H₂O₂ reacts with chloride anion, a major anion in the extracellular fluid, catalyzed by myeloperoxidase, to produce hypochlorous acid (HOCl), a bactericidal chemical. Nitric oxide (NO) produced by NO synthase reacts with O₂^-. to generate peroxynitrite anion. HOCl reacts with H₂O₂ to generate delta singlet oxygen (¹O₂). Ferrous iron reacts with H₂O₂ in Fenton reaction to produce ferric ion and hydroxyl radical (OH^.). Note that H₂O₂, NO, HOCl are relatively stable, but peroxynitrite anion, O₂^-., ¹O₂, OH^., has decreasing lifetimes of 10 ms, ms, μs, and ns respectively. ¹O₂ shows specificity in oxidative reactions (than OH^-, for example) and has a diffusion distance from 10 nm to 150 nm, depending on the site of generation and methods of measurement and detection. NE: Neutrophil elastase; NADPH: Nicotinamide adenine dinucleotide phosphate; NAD: Nicotinamide adenine dinucleotide; NOS: Nitric oxide synthase; NO: Nitric oxide; H₂O₂: Hydrogen peroxide; HOCl: Hypochlorous acid; ONOO: Peroxynitrite anion; O₂^-.: Superoxide; ¹O₂: Delta singlet oxygen; OH^.: Hydroxyl radical; DPI: Diphenyleneiodonium chloride; MPO: Myeloperoxidase; NOX: Nicotinamide adenine dinucleotide phosphate oxidase; Cl^-: Chloride anion; SOD: Superoxide dismutase; H⁺: Proton; Fe²⁺: Ferrous iron; Fe³⁺: Ferric ion.