ИСТИНА

Oxygen photoreduction (Mehler reaction) was studied in photosystem 2 (PS2) preparations of BBY-type exposed to light of high intensities. Oxygen uptake was low during 10-15 min of illumination by white light (850W/m2) and composed 8-9% of the oxygen evolving activity. This value of oxygen consumption under photoinhibitory conditions was close to ones obtained previously under normal light conditions. This fact points to the registered oxygen uptake by BBY-particles reflect oxygen reduction in Mehler-type reaction and there are no active oxygen species overproduction under photoinhibitory light. Upon addition of artificial electron acceptor, potassium ferricyanide, to the suspensions of BBY-particles we found that oxygen evolution capacity under photoinhibitory light was near to ones registered at moderate light (200W/m2). After 10 min of illumination with high light in the presence of ferricyanide we revealed the oxygen uptake to be 3 fold higher than that without acceptor. Addition of catalase brought about recovery of the oxygen evolution to the initial intensity and kinetics. The obtained data suggest that electron flow to oxygen under photoinhibitory conditions is regulated by ferricyanide by means of structural modifications of PS2 proteins. Photoreduction of oxygen to superoxide in the PS2 preparations was studied with spin trapping EPR spectroscopy in the presence of Tiron. Oxygen evolution by PS2 particles reached saturation at light intensities about 200 W/m2. At the same time superoxide production saturated about 800 W/m2. Thus one may conclude that the oxygen evolution and oxygen photoreduction in Mehler-type reaction have different points of light saturation. The presented data demonstrate that electron transport to oxygen is more effective under high light intensities. So it could serve as effective regulatory mechanism for dissipation of excessive of absorbed energy under photoinhibitory conditions.