FLAVIN-MEDIATED PHOTOREACTIONS IN ARTIFICIAL SYSTEMS - POSSIBLE MODEL FOR BLUE-LIGHT PHOTORECEPTOR PIGMENT IN LIVING SYSTEMS

The photoreduction of [horse heart] cytochrome c and the photostimulation of O2 uptake were studied in solutions of flavin and cytochrome as a possible model system for similar photoreactions which were observed in vivo. Light causes the photoreduction of the flavin. Under aerobic conditions the photoreduced flavin reacts with O2 to form the superoxide anion which in turn can reduce cytochrome c. Dismutation of the superoxide anions forms H2O2 which mediates the dark oxidation of the photoreduced cytochrome. Superoxide formation and dismutation also account for the light-induced O2 uptake. Action spectra confirm the role of flavin in the photoreduction of cytochrome c and the photostimulation of O2 uptake. Under anaerobic conditions the photoreduced flavin reduces cytochrome c directly. In the presence of an electron donor only catalytic amounts of flavin are required. In the absence of an added electron donor flavin can act as the electron donor if substrate amounts are present. Azide inhibits all of these flavin-mediated photoresponses. Azide also inhibits the photoreduction of cytochrome b which occurs in the mycelium of Neurospora crassa.