Scavenging of superoxide and hydrogen peroxide in blue-green algae (cyanobacteria)

Blue-green algae (cyanobacteria) have evolved as the most primitive, oxygen ic, plant-type photosynthetic organisms. Within a single prokaryotic cell, they have uniquely accommodated both oxygenic photosynthesis and aerobic re spiration, which are known to produce superoxide and hydrogen peroxide as i nevitable byproducts. Two types of superoxide dismutase have been character ized in both N-2-fixing and non-N-2-fixing cyanobacteria, namely cytosolic iron-containing superoxide dismutase and thylakoid-bound manganese-containi ng superoxide dismutase. No qualitative differences between various cell ty pes (vegetative cells, heterocysts) were found. In contrast to chloroplasts , most of the cyanobacterial species show catalatic activity. From two spec ies the corresponding enzymes have been characterized as typical prokaryoti c (bifunctional) catalase-peroxidases with homologies to cytochrome c perox idases and ascorbate peroxidases. In addition to catalatic activity, some s trains exhibit ascorbate peroxidase activity, but to dale there are no repo rts detailing purification and characterization. Cyanobacteria were found to contain low intracellular ascorbate concentrati ons (30-100 mu M) and 2-5 mM glutathione. Both monodehydroascorbate and glu tathione reductase activities were detected in most species examined, where as dehydroascorbate reductase activity was absent. The question as to wheth er a glutathione-ascorbate cycle exists in cyanobacteria cannot be answered at present.