[Comparative study of the oxygen reduction in photosystem II of the cyanobacteria, Anacystis nidulans and Anabaena variabilis].

Oxygen reduction in the photosystem II (PS II) of thylakoid membranes from the cyanobacteria Anacystis nidulans and Anabaena variabilis was studied in the system SiMo + DCMU in whose presence competitive electron transport to SiMo and O2 in possible. The reagents of the Mehler reaction were used; these reagents activate oxygen uptake by interacting with the reduced forms of oxygen (malonate and oxalate with O2; glyoxylate and catalase + ethanol with H2O2). The use of the reagents as shown that there was a considerable electron transport to O2 which decreased the rate of SiMo reduction and visible oxygen evolution. Oxygen was reduced to O2 and H2O2 in the photosystem II of A. nidulans whereas practically no generation of H2O2 was observed in the photosystem II of A. variabilis. Malate was found to be capable of activating oxygen uptake in the PS II of the cyanobacteria by interacting with O2. Oxalate was capable of this interaction only in the membranes of A. variabilis, but not in the membranes of A. nidulans where it competed with water as an electron donor. A decrease in SiMo reduction and oxygen evolution in A. nidulans in the presence of glycolate was due to oxidation of glyoxylate which was formed in the course of enzyme-catalyzed oxidation of glycolate by the membranes possessing the activity of glycolate oxidase, and the subsequent interaction of glyoxylate with H2O2. SiMo reduction by the cyanobacterial membranes and by the spheroplasts of A. variabilis, in contrast to the chloroplasts of higher plants, did not decrease in the presence of DCMU, but went on at a linear rate until the substrate was completely exhausted. The role of reduced O2 forms in metabolism of a phototrophic cell is discussed.