Photoconsumption of Oxygen in Photosystem II Preparations under Impairment of the Water-Oxidizing Complex

S. A. Khorobrykh, A. A. Khorobrykh, V. V. Klimov, and B. N. Ivanov^*

Institute of Basic Biological Problems, Russian Academy of Sciences, Pushchino, Moscow Region, 142290 Russia; fax: (0967) 790-532; E-mail: ivabor@issp.serpukhov.su

^* To whom correspondence should be addressed.

Received April 18, 2001; Revision received August 2, 2001

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Oxygen consumption in photosystem II (PSII) preparations in the light was 2 µmol O₂/h per mg Chl at weakly acidic and at neutral pH values. It increased fourfold to fivefold at pH 8.5-9.0. The addition of either artificial electron donors for PSII such as MnCl₂ or diphenylcarbazide, or diuron as an inhibitor of electron transfer from Q_A, the primary bound quinone acceptor, to Q_B, the secondary bound quinone acceptor of PSII, resulted in a decrease in oxygen consumption rate at basic pH to value close to ones measured at pH 6.5. Such additions did not affect oxygen consumption at lower pH values. The induction of variable chlorophyll fluorescence yield in the light differed greatly at pH 6.5 and 8.5. While at pH 6.5 the fluorescence yield, after an initial fast rise almost to F_max, only slightly decreased, at pH 8.5 after such a rise it dropped promptly to a low value. The additions of the artificial electron donors at pH 8.5 resulted in the induction kinetics close to that observed at pH 6.5. These data indicate impairment of electron donation to P680⁺ that could be caused by damage to the water oxidation system at basic pH values. In experiments with PSII preparations treated with Tris to destroy the water-oxidizing complex, photoconsumption of oxygen in the entire pH region was close to the values in untreated preparations at basic pH. In untreated preparations the rate of light-induced oxygen consumption decreased in the presence of catalase, which decomposes H₂O₂, as well as in the presence of electron acceptor potassium ferricyanide. From these data it is suggested that the light-induced oxygen consumption in PSII is caused by two processes, by an interaction of O₂ with organic radicals, which were formed due to oxidation of components of the donor side of this photosystem (proteins, lipids, pigments) by cation-radical P680⁺, as well as by oxygen reduction by still unidentified components of PSII.

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KEY WORDS: oxygen, hydrogen peroxide, superoxide anion-radical, photosystem II

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