Document Detail

Superoxide contributes to the rapid inactivation of specific secondary donors of the photosystem II reaction center during photodamage of manganese-depleted photosystem II membranes.
MedLine Citation:
PMID:  7857943     Owner:  NLM     Status:  MEDLINE    
The role of superoxide in the mechanism of photoinactivation of the secondary donors of the reaction center of photosystem II membranes depleted of Mn by extraction with NH2OH plus EDTA (NH2OH/EDTA-PSII) was assessed. EPR analyses (g = 2 region) in continuous light, optical kinetic spectrophotometric analyses of P680+ and Car+, and AT-band emission measurements were made after various durations of weak and strong light treatment of NH2OH/EDTA-PSII in the presence and absence of superoxide dismutase, or of PSII electron acceptors to suppress superoxide formation. Additionally, flash-induced variable fluorescence of chlorophyll a and the capabilities of the membranes of photooxidize Mn2+ (in the presence of H2O2) via a high-affinity site (Km approximately 180 nM) and to carry out the photoactivation of the Mn-cluster were determined. In the absence of any additions to the NH2OH/EDTA-PSII membranes which were highly depleted of Mn, weak light treatment caused rapid (t1/2 approximately 20 s) and parallel losses of (a) the approximately 10 microseconds phase of P680+ reduction, which reflects the TyrZ-->P680+ reaction, (b) the amplitude of chlorophyll a variable fluorescence, (c) the capability to accumulate the TyrZ(+)-radical in continuous light, and (d) the capability to photooxidize Mn2+/H2O2 in continuous light. As reported previously [Blubaugh et al. (1991) Biochemistry 30, 7586-7597], a dark-stable 12-G-wide featureless EPR signal centered at g = 2.004 was formed rapidly during illumination. This signal previously was tentatively identified as a Car+ radical and was suggested to contribute to the quenching of chlorophyll a variable fluorescence and to the slowing of the TyrZ-->P680+ reaction. However, we failed to detect Car+ formation by sensitive optical spectrophotometry and obtained no definable evidence for either a quencher of fluorescence other than P680+ itself or a slowing of the TyrZ-->P680+ reaction. Addition of a saturating concentration (96 units/mL) of superoxide dismutase diminished the rate of photodamage(s) by approximately 30-fold, but did not abolish it completely. Superoxide dismutase similarly suppressed strong light-induced photodamages, causing the loss of capability to photooxidize Mn2+/H2O2, to carry out photoactivation, and to generate the AT-band emission as well as TyrZ+ EPR signal. In contrast to others, we found no evidence that the initial target(s) of photodamage is (are) different in weak versus strong light treatment. The totality of the results suggests that the initial event in either weak light or strong light photodamage of NH2OH/EDTA-PSII is a decoupling of the redox activity of TyrZ from P680.(ABSTRACT TRUNCATED AT 400 WORDS)
G X Chen; D J Blubaugh; P H Homann; J H Golbeck; G M Cheniae
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Publication Detail:
Type:  In Vitro; Journal Article; Research Support, U.S. Gov't, Non-P.H.S.    
Journal Detail:
Title:  Biochemistry     Volume:  34     ISSN:  0006-2960     ISO Abbreviation:  Biochemistry     Publication Date:  1995 Feb 
Date Detail:
Created Date:  1995-03-22     Completed Date:  1995-03-22     Revised Date:  2006-11-15    
Medline Journal Info:
Nlm Unique ID:  0370623     Medline TA:  Biochemistry     Country:  UNITED STATES    
Other Details:
Languages:  eng     Pagination:  2317-32     Citation Subset:  IM    
University of Kentucky, Lexington 40546-0091.
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MeSH Terms
Chlorophyll / metabolism
Electron Spin Resonance Spectroscopy
Intracellular Membranes / drug effects,  radiation effects
Light-Harvesting Protein Complexes
Manganese / physiology
Photosynthetic Reaction Center Complex Proteins / drug effects,  metabolism*,  radiation effects
Photosystem II Protein Complex
Spectrometry, Fluorescence
Spinacia oleracea
Superoxide Dismutase / metabolism
Superoxides / pharmacology*
Tyrosine / chemistry
Reg. No./Substance:
0/Light-Harvesting Protein Complexes; 0/Photosynthetic Reaction Center Complex Proteins; 0/Photosystem II Protein Complex; 11062-77-4/Superoxides; 1406-65-1/Chlorophyll; 53808-91-6/P-680; 55520-40-6/Tyrosine; 7439-96-5/Manganese; EC Dismutase

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