A soluble-form of pro-oxidant lumazine isolated from cyanobacterial cells generates superoxide anion under near-UV irradiation

Abstract

By monitoring the generation of superoxide anion radical (O 2 − ), a new and water-soluble near-UV photosensitizable compound was isolated from filamentous cyanobacterium Phormidium lapideum. This compound, having an absorption peak at 325 nm at pH 7.0, was identified as lumazine (2,4(1 H,3 H)-pteridinedione) on the basis of absorption, fluorescence, MS and NMR spectra. Lumazine was also detected in the extract from unicellular cyanobacterium, Synechococcus PCC 7942. In a lumazine-sensitized reaction, NO 2 − accumulation from the reaction of NH 2OH with 0 2 −, proceeded linearly, with the time of irradiation. The accumulation was inhibited by superoxide dismutase. O 2-Consumption in the presence of NH 2OH was accompanied by a formation of half-molar N0 2 −. Moreover added cytochrome c apparently abolished the observed oxygen consumption, indicating that the excited lumazine generated O 2 − from O 2 stoichiometrically. These results indicate that near-UV photons reaching into the cyanobacterial cells cause oxidative stress by producing active species of oxygen via the sensitized reaction of pro-oxidant(s) such as lumazine.