Photoinactivation of Photosystem II by in situ-Photoproduced Hydroxyurea Radicals

Abstract

Oxygenic photosystem (PS) II complex from spinach photooxidized hydroxyurea (HU) to produce its aminoxy radical, which was identified by its electron spin resonance spectrum. HU was apparently photooxidized by the water-oxidizing enzyme (WOE) since the photooxidation reaction was blocked by carbonyl cyanide m-chlorophenylhydrazone (CCCP). HU radicals photoproduced by the WOE inhibited the electron transfer between the redox-active tyrosine residue (YZ), which is involved in electron transfer from the WOE to the reaction center chlorophyll of PS II, and the secondary quinone electron acceptor. Treatment of PS II complex with Tris resulted in the appearance of a CCCP-insensitive photooxidation site for HU. Photoproduced HU radicals in oxygenic and Tris-treated PS II complex decayed with first-order kinetics, an indication that the radicals reacted primarily with surrounding molecules rather than decayed through spontaneous dismutation or recombination. HU inhibited the diphenylcarbazide-supported photoreduction of 2,6-dichlorophenolindophenol (DCIP) in Tris-treated PS II complex preincubated only under illumination, but this inhibition was suppressed when ascorbate was added to scavenge HU radicals. If examined in darkness, HU radicals could not, however, inhibit subsequent photoreduction of DCIP. Therefore, the photoproduced HU radicals interact with a photogenerated site(s) in the PS II complex. The photoproduction of YZ., a radical of YZ, was suppressed to about 40% in Tris-treated PS II complex by the in situ-photogenerated HU radicals, and the yield of a cation radical of chlorophyll, close to the PS II reaction center, was increased, while the production of a radical of another redox-active tyrosine residue in PS II (YD.) was hardly affected.(ABSTRACT TRUNCATED AT 250 WORDS)

TheArabidopsis thalianaCheckpoint Kinase WEE1 Protects against Premature Vascular Differentiation during Replication Stress

“…The number of genes that each cluster contains is given on the right. terms enriched in this gene set were all associated with light stimuli (Table 1), probably linked to the photoinactivating effects of HU on photosystem II in plants (Kawamoto et al, 1994).…”

The Nitric Oxide Producing Reactions of Hydroxyurea

“…The spinach oxygenic photosystem photooxidizes hydroxyurea to the nitroxide radical (2) as shown by EPR [122]. This work suggests that the water-oxidizing enzyme converts hydroxyurea to 2, which further inhibits electron transport by reacting with redox active tyrosine residues [122]. Inhibition of photosystem II by this pathway appears related to the toxicity of hydroxyurea to phytoplankton [123].…”

“…These authors suggest that 2 acts as a suicide inhibitor of ascrobate peroxidase under these conditions but do not mention the possible involvement of NO [121]. The spinach oxygenic photosystem photooxidizes hydroxyurea to the nitroxide radical (2) as shown by EPR [122]. This work suggests that the water-oxidizing enzyme converts hydroxyurea to 2, which further inhibits electron transport by reacting with redox active tyrosine residues [122].…”

Radical Production and Scavenging in the Chloroplasts

“…Until this primary step dioxygen is not required, but under aerobic conditions (Ananyev et al, 1994;Chen et al, 1992) and (Hideg et al, 1994b) are photoproduced, which promotes further the photoinactivation. The interaction of (Chen et al, 1992), the hydroxyurea radicals (Kawamoto et al, 1994) or the azidyl radicals (Kawamoto et al, 1995) with Tyr modifies the Tyr Z residue and inhibits the electron transport to P680 from the water oxidase complex. In the case of the donor-side photoinhibition also, the proteolytic degradation of D1 protein is triggered by these oxidants (Miyao, 1994).…”