The Oxidation-reduction Potential of the Luciferin-oxyluciferin System

The oxidation-reduction potential of the Cypridina luciferin-oxyluciferin system determined by a method of "bracketing" lies somewhere between that of anthraquinone 2-6-di Na sulfonate (E(o) (') at pH of 7.7 = -.22) which reduces luciferin, and quinhydrone (E(o) (') at pH of 7.7 = +.24), which oxidizes luciferin. Systems having an E(o) (') value between -.22 and +.24 volt neither reduce oxyluciferin nor oxidize luciferin. If the luciferin-oxyluciferin system were truly reversible considerable reduction and oxidation should occur between -.22 and +.24. The system appears to be an irreversible one, with both "apparent oxidation" and "apparent reduction potentials" in Conant's sense. Hydrosulfites, sulfides, CrCl(2), TiCl(3), and nascent hydrogen reduce oxyluciferin readily in absence of oxygen but without luminescence. Luminescence only appears in water solution if luciferin is oxidized by dissolved oxygen in presence of luciferase. Rapid oxidation of luciferin by oxygen without luciferase or oxidation by K(3)Fe(CN)(6) in presence of luciferase but without oxygen never gives luminescence.