Interaction of α-tocopherol quinone, α-tocopherol and other prenyllipids with photosystem II

Interaction of α-tocopherol quinone, α-tocopherol and otherprenyllipids with photosystem II

Jerzy Kruk a, Georg H. Schmidb, Kazimierz Strzałkaa*

a Department of Plant Physiology and Biochemistry, The Jan Zurzycki Institute of Molecular Biology, Jagiellonian University,Al. Mickiewicza 3, 31-120 Kraków, Polandb Lehrstuhl Zellphysiologie, Fakultät für Biologie, Universität Bielefeld, Postfach 10 01 31, 33501 Bielefeld, Germany

* Author to whom correspondence should be addressed (fax +48 12 633 6907); e-mail [email protected])

(Received 4 August 1999; accepted 25 November 1999)

Abstract – We have found that plastoquinone-A (PQ-A) andα-tocopherol (α-Toc) increased the reduction level of thehigh-potential form of cytochrome b-559 (cyt. b-559 HP) andα-tocopherol quinone (α-TQ) decreased the level of thiscytochrome form inScenedesmus obliquus wild-type, while the investigated prenyllipids were not active in the restoration ofthe cyt. b-559 HP form inScenedesmus PS28 mutant andSynechococcus 6301 (Anacystis nidulans) where the cyt. b-559 HPform is naturally not present. Among the tested prenyllipids,α-TQ quenched fluorescence in thylakoids ofS. obliquus wild-type,the PS28 mutant and tobacco to the highest extent, while PQ-A was less effective in this respect.α-Tocopherol showed theopposite effect toα-TQ and it was rather small. The fluorescence quenching measurements of thylakoids in the presence ofDCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) showed that theα-Toc and FCCP (carbonylcyanide-p-trifluoromethoxy-phenyl-hydrazone) did not quench non-photochemically chlorophyll fluorescence while PQ-9 andα-TQ were effectivefluorescence quenchers at higher concentrations (> 15 µM). However, at the lowerα-TQ concentrations where its effectivefluorescence quenching was found in DCMU-free samples, there was nearly no quenching effect byα-TQ observed inDCMU-treated thylakoids. This suggested a specific, not non-photochemical, DCMU sensitive, fluorescence quenching ofphotosystem II (PSII) at lowα-TQ concentrations which is probably connected with the cyclic electron transport around PSIIand might have a function of excess light energy dissipation. The effects ofα-TQ on PSII resembled those of FCCP under manyrespects which might suggest similar mechanism of action of these compounds on PSII, i.e. the catalytic deprotonation and/orredox changes of some components of PSII such as the water splitting system, tyrosine D, Chlz or cytochrome b-559. © 2000Éditions scientifiques et médicales Elsevier SAS

Cytochrome b-559 / fluorescence quenching / photosystem II / plastoquinone /α-tocopherol / α-tocopherol quinone /Scenedesmus

cyt. b-559, cytochrome b-559 / DCMU, 3-(3,4-dichlorophenyl)-1,1-dimethylurea / FCCP, carbonylcyanide-p-trifluoromethoxy-phenyl-hydrazone / HP, high-potential / PQ, plastoquinone / PS II, photosystem II / QA and QB, theprimary and secondary quinone electron acceptor of photosystem II /α-Toc, α-tocopherol / α-TQ, α-tocopherol quinone

1. INTRODUCTION

α-Tocopherol quinone (α-TQ) and α-tocopherol(α-Toc) are natural components of the photosyntheticmembranes occurring at about 10 and 25–30 % of theamount of plastoquinone-A (PQ-A, PQ-9), respec-tively [19, 23]. The function ofα-Toc as an antioxidantis well recognized [24]. Moreover, there are someindications of its additional function in electron trans-port reactions [3, 26]. There are some data suggestingparticipation ofα-TQ in the photosynthetic electrontransport [19] but its site of action has not been

identified. It was found [5, 21, 22, 27] that in modelsystems,α-TQ in its reduced state (α-TQH2) showsantioxidant properties similar to those ofα-Toc. Arecent report [20] indicated thatα-TQ decreasedoxygen evolution of tobacco thylakoids, while theeffect ofα-Toc was opposite in this respect. It was alsoshown [20] thatα-TQ was not active as an electronacceptor of photosystem II (PSII). The results of flashlight experiments on PSII preparations showed [20]that bothα-TQ andα-Toc changed the fitting param-eters of the mathematical simulation of the oxygen

Plant Physiol. Biochem., 2000,38 (4), 271−277 / © 2000 Éditions scientifiques et médicales Elsevier SAS. All rights reservedS0981942800007476/FLA

Plant Physiol. Biochem., 0981-9428/00/4/© 2000 E´ ditions scientifiques et médicales Elsevier SAS. All rights reserved

evolution pattern, however differently from PQ-A.These results suggest some specific interaction of bothprenyllipids with PSII. One such site could be cyto-chrome b-559 (cyt. b-559) where decyl-PQ and PQ-9were found to stimulate photoreduction of the low-potential form (LP) of the cytochrome b-559 [8, 12].Moreover, PQ-9 showed restoration of the high-potential (HP) form of this cytochrome [9]. Theinteraction of cyt. b-559 HP form with α-TQ andα-Toc could be suggested by the existence of twoalgae in which α-TQ and α-Toc are absent, namely theScenedesmus obliquus PS28 mutant and Synechococ-cus 6301 (Anacystis nidulans). At the same time, thesetwo species are also unique in not having the cyt.b-559 HP form [7, 29]. These facts may indicate thatthe presence of α-TQ and/or α-Toc could be necessaryfor the stabilization of the HP form of this cytochrome.

A method which could provide new information onthe interaction of prenyllipids with the components ofPSII is the analysis of fluorescence induction kineticsof photosynthetic membranes. In our study, we havemeasured the influence of different prenyllipids onfluorescence yield of thylakoids isolated from the S.obliquus PS28 mutant, the corresponding wild-typeand the tobacco used in our previous experiments [20].We also measured the influence of different prenyllip-ids on the content of different forms of cytochromeb-559 in thylakoids from Scenedesmus strains and thecyanobacterium Synechococcus 6301 to verify thehypothesis on stabilization of cyt. b-559 HP form byprenyllipids.

2. RESULTS

2.1. Cytochrome b-559 measurements

The difference redox absorption spectra in theα-band (520–600 nm) region of cytochromes of theScenedesmus PS28 mutant and of the cyanobacteriumSynechococcus 6301 (figure 1) thylakoids confirmedthat both species practically did not contain any cyt.b-559 in its high-potential form but only in its low-potential form. Addition of any of the investigatedprenyllipids did not recover the HP form (data notshown). Thylakoids of Scenedesmus wild-type straincontained the HP form at about 55 % of the total cyt.b-559 (figure 2 and table I) and the reduction level ofthe HP form was about 16 %. Among the investigatedprenyllipids PQ-A and α-Toc were the most effectivein increasing the amount of the reduced HP form(table I and figure 2), whereas α-TQ caused a slightdecrease in the level of the reduced HP form and FCCP

(carbonylcyanide-p-trifluoromethoxy-phenyl-hydra-zone) causes disappearance of this cyt. b-559 form.None of the compounds tested, changed the amount ofthe total HP form to a significant degree (table I).

2.2. Fluorescence induction measurements

The analysis of fluorescence induction curves forScenedesmus wild-type in the presence of α-TQ,α-Toc and PQ-A shows (figure 3; table II) that α-TQquenched the variable fluorescence (Fv/Fm ratio) to thehighest extent, whereas the effect of α-Toc was oppo-site, although rather small. PQ-A also quenched fluo-rescence, however, not as much as α-TQ. The differ-ences are better seen if evaluated as Fv/Fo parameter orFm/Fmc ratio (Fmc is Fm of the control sample). Theabove effects were more pronounced for higher pre-nyllipid contents (prenyllipid:chlorophyll ratio of 1:2.5,mol·mol–1). Moreover, the kinetics for α-TQ andPQ-A were different (figure 3). In the case of α-TQ,the kinetics resembled that of the control sample,

Figure 1. Absorption spectra of cyt. b-559 in the α band region ofthylakoids isolated from the S. obliquus PS28 mutant (top) andSynechococcus 6301 (bottom); chlorophyll concentration100 µg·mL–1. Scenedesmus thylakoids were suspended in 50 mMHepes buffer (pH 7.0), 30 mM KCl and 5 mM MgCl2, Synechococcusthylakoids in 50 mM Hepes buffer (pH 7.0) and 50 mM CaCl2.

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while for PQ-A, the Fm value was reached moreslowly. The effect of PQ-A is typical for an electronacceptor. In the presence of FCCP, the effects of theprenyllipids were preserved (table II). The reduced

form of α-TQ, α-TQH2, decreased the Fv/Fo parameterto an even higher extent than α-TQ (table II) but thekinetics in the presence of α-TQH2 resembled that ofDCMU inhibited chloroplasts. On the other hand,PQH2-A did not show such an effect.

The PS 28 mutant showed a high Fo value and lowvariable fluorescence upon illumination (figure 4), nev-ertheless the effect of prenyllipids on the fluorescenceyield was similar to the wild-type’s (table III). The

Figure 2. Absorption spectra of cyt. b-559 in the α band region ofthylakoids isolated from S. obliquus wild-type (top) and S. obliquuswild-type thylakoids with α-Toc (bottom). α-Toc:chlorophyll molarratio was 1:2.5. Other conditions are as in table I.

Table I. Influence of different prenyllipids on the reduction level ofcyt. b-559 HP (HPreduced/HPtotal) and the ratio of the cyt. b-559 HPtotal

to the total cyt. b-559 (HPtotal + LP) in thylakoids of S. obliquuswild-type. The prenyllipid:chlorophyll molar ratio was 1:2.5, chloro-phyll concentration 100 µg·mL–1. Scenedesmus thylakoids were sus-pended in 50 mM Hepes buffer (pH 7.0), 30 mM KCl and 5 mMMgCl2. α-TQ, α-Tocopherol quinone; α-Toc, α-tocopherol; PQ,plastoquinone.

Prenyllipid cyt. b-559

HPreduced/HPtotal (%) HPtotal/(HPtotal + LP) (%)

Control 16 ± 2 55 ± 3α-TQ 12 ± 1 45 ± 4α-Toc 53 ± 5 50 ± 7PQ-A 75 ± 5 48 ± 4α-TQH2 20 ± 3 45 ± 51 µM FCCP ∼ 0 50 ± 5

Figure 3. Fluorescence induction curves of thylakoids isolated fromScenedesmus obliquus wild-type strain under the influence of differentprenyllipids at a prenyllipid:chlorophyll molar ratio of 1:2.5 (↑ , lighton). The Fm level was determined when the kinetics reached a plateau.Other conditions are as in table II. Abbreviations as in table I.

Table II. Fluorescence yield [Fv/Fm = (Fm – Fo)/Fm], Fv/Fo valuesand Fm/Fmc ratios (Fmc is Fm of the control sample) of thylakoidsisolated from S. obliquus wild-type strain under influence of differentprenyllipids. The values in brackets refer to the prenyllipid:chloro-phyll molar ratio. Thylakoids were suspended in 50 mM Hepes buffer(pH 7.0), 30 mM KCl and 5 mM MgCl2. Chlorophyll concentrationwas 25 µg·mL–1. The uncertainty in Fv/Fm, Fv/Fo, and Fm/Fmc is± 0.003, ± 0.03 and 5 %, respectively. Abbreviations as in table I.

Sample Fv/Fm Fv/Fo Fm/Fmc (%)

Control 0.650 1.85 100α-TQ (1:5) 0.623 1.65 85α-TQH2 (1:5) 0.599 1.49 87α-Toc (1:5) 0.653 1.89 97PQ-A (1:5) 0.633 1.72 922 µM FCCP 0.571 1.33 832 µM FCCP + TQ (1:5) 0.522 1.09 652 µM FCCP + Toc (1:5) 0.577 1.36 812 µM FCCP + PQ-A (1:5) 0.542 1.19 77α-TQ (1:2.5) 0.591 1.44 70α-TQH2(1:2.5) 0.587 1.42 86α-Toc (1:2.5) 0.651 1.86 99PQ-A (1:2.5) 0.615 1.60 77PQH2-A (1:2.5) 0.632 1.72 79

Interaction of prenyllipids with photosystem II 273

vol. 38 (4) 2000

sensitivity of the PS28 mutant to FCCP was evenhigher than that of wild-type. At the same FCCPconcentration (2 µM) the Fv/Fo ratio dropped by 70 %for the mutant, while this change was only 30 % forwild-type (table II). Fluorescence quenching by FCCPwas explained by stimulating the oxidation of the cyt.b-559 HP form by its deprotonation and driving thecyclic electron transport around PSII [25]. The PS28mutant lacks the cytochrome b-559 HP form, thusfluorescence quenching by FCCP suggests that themechanism of action of this protonophore is not onlyconnected with the oxidation of the cyt. b-559 HPform.

The influence of prenyllipids on the fluorescenceyield of tobacco thylakoids was even stronger than for

Scenedesmus strains (table IV). Especially, the effectof α-Toc, manifesting itself in the increase of the Fv/Fo

ratio, was more pronounced. We have found that theeffect of α-Toc decreased with the sample’s age.

2.3. Fluorescence quenching measurements

Since it is known that the oxidized quinones quenchchlorophyll fluorescence non-photochemically [1, 32,34], i.e. are not acting as electron acceptors from PSII,to explain the above observed effects it was necessaryto measure the quenching activities of the investigatedcompounds on the fluorescence of DCMU poisonedthylakoids, where their action as electron acceptorsfrom PSII at the QB site (DCMU binding site) isabolished. The fluorescence intensity of tobacco thy-lakoids at Fm as a function of added α-Toc, α-TQ andPQ-9 concentration in the presence of 10 µM DCMUis shown in figure 5 A. Both α-Toc and FCCP (data notshown) did not quench fluorescence while α-TQ andPQ-9 effectively quenched fluorescence at higherquinone concentrations. The corresponding Stern-Volmer plots (Io/I = 1 + Ksv[Q]) for α-TQ and PQ-9are shown in figure 5 B. The quenching at low quinoneconcentrations (< 10 µM) is negligible and increaseswith the increase in α-TQ and PQ-9 concentrations.The Stern-Volmer quenching constants, Ksv, deter-mined for α-TQ and PQ-9 concentrations above 25 µMis about 2·104 M–1 which is in the order of otherquenching constants for other synthetic quinones [17].In the case of PQ-9, its 5-µM concentration (Q:chl =1:5 molar ratio) quenched fluorescence by 2 % and its10-µM concentration (Q:chl = 1:2.5) by 10 % (figure 5A). The corresponding values in the absence of DCMU

Figure 4. Fluorescence induction curves of thylakoids isolated fromthe S. obliquus PS 28 mutant under the influence of differentprenyllipids at a prenyllipid:chlorophyll molar ratio of 1:2.5 (↑ , lighton). The Fm level was determined when the kinetics reached a plateau.Other conditions are as in table II. Abbreviations as in table I.

Table III. Fluorescence yield (Fv/Fm), Fv/Fo values and Fm/Fmc ratiosof thylakoids isolated from the S. obliquus PS 28 mutant underinfluence of different prenyllipids. Other conditions are as in table II.The uncertainty in Fv/Fm, Fv/Fo, and Fm/Fmc is ± 0.002, ± 0.02 and5 %, respectively. Abbreviations as in table I.


Control (1:5) 0.303 0.43 100α-TQ (1:5) 0.255 0.34 79α-Toc (1:5) 0.300 0.43 99PQ-A (1:5) 0.293 0.41 911 µM FCCP 0.190 0.23 892 µM FCCP 0.123 0.14 805 µM FCCP 0.056 0.06 74α-TQ (1:2.5) 0.235 0.30 66α-TQH2 (1:2.5) 0.204 0.25 66α-Toc (1:2.5) 0.306 0.44 98PQ-A (1:2.5) 0.272 0.37 83

Table IV. Fluorescence yield (Fv/Fm), Fv/Fo values and Fm/Fmc ratiosof tobacco thylakoids under influence of different prenyllipids. Thethylakoids were suspended in 50 mM Hepes buffer (pH 7.0), 10 mMKCl and 5 mM MgCl2. Other conditions are as in figure 1. Theuncertainty in Fv/Fm, Fv/Fo, and Fm/Fmc is ± 0.002, ± 0.04 and 6 %,respectively. Abbreviations as in table I.


Control 0.791 3.78 100α-TQ (1:5) 0.783 3.60 84α-TQH2 (1:5) 0.755 3.09 90α-Toc (1:5) 0.803 4.09 100PQ-A (1:5) 0.779 3.74 92α-TQ (1:2.5) 0.723 2.60 69α-TQH2 (1:2.5) 0.732 2.70 83α-Toc (1:2.5) 0.806 4.14 98PQ-A (1:2.5) 0.760 3.17 86PQH2-A (1:2.5) 0.786 3.67 97

274 J. Kruk et al.


were 8 and 15 %, respectively (tables II, IV). Whenthe fluorescence induction kinetics is saturated (Fm

value is reached), the PQ-pool should be in thereduced state, and the formed PQH2 should not quenchthe Fm value (table IV), but probably not all addedPQ-9 is incorporated into the PQ-pool and the non-incorporated PQ-9 can partially give rise to non-photochemical quenching [16, 35]. In DCMU treatedthylakoids, α-TQ quenches no fluorescence at 5 µM(Q:chl = 1:5) and only 2 % at 10 µM (Q:chl = 1:2.5).The corresponding values in the absence of DCMUwere 15 and 30 %, respectively (tables II, IV). Itseems that the non-photochemical quenching by α-TQis negligible for the concentrations used in the fluo-rescence induction kinetics measurements, but takinginto account the fact that α-TQ cannot act as anelectron acceptor at the QB site, its quenching is alsonot of typical photochemical type. Its quenchingstrongly resembles the mechanism of quenching byFCCP which is DCMU sensitive.

3. DISCUSSION

Inactivity of both α-TQ and α-Toc in the restorationof the HP form of cyt. b-559 of the Scenedesmus PS28

mutant and Synechococcus 6301 indicates that, althoughthese species are unique organisms in not having eitherthe HP form of cyt. b-559 or α-TQ and α-Toc, theabsence of both prenyllipids in these organisms is notthe only reason for the lack of the cyt. b-559 HP form.In the case of the Scenedesmus PS28 strain, the reasoncould be connected with the fact that it is also a phytolmutant [15] having geranylgeraniol instead of phytolin its chlorophyll molecules.

The action of α-TQ on PSII resembles that of FCCPunder many respects, i.e. both compounds quencheffectively PSII fluorescence at low concentrations, thequenching is prevented by DCMU, both compoundsdecrease the level of the reduced HP form of cyt. b-559and are not electron acceptors at the QB site. Themechanism of action of FCCP on PSII is not fullyexplained. FCCP is known as an uncoupler of theproton gradient across the membranes, which is con-nected with its reversible binding of protons [28]. Itbelongs to a class of compounds which acceleratedeactivation reactions of the water-splitting system,the ADRY reagents [11]. The ADRY agents weresuggested to catalyse the S2 and S3 reduction via thetyrosine D residue [13] or Chlz molecule [14] which issupposed to be the direct electron donor to P680 in thecyclic electron transport around PSII and the oxidantof the HP form of cyt. b-559 [33]. It is also known thatFCCP stimulates oxidation of the HP form of cyt.b-559 by its deprotonation [2]. The cytochrome is thenreduced by the PQH2-pool which enables reoxidationof the QA and quenching of fluorescence [25]. Such anmechanism of action of FCCP explains DCMU sensi-tivity of its fluorescence quenching. It seems that thereare probably multiple sites of FCCP action in PSII.The observation that both FCCP and α-TQ quenchfluorescence at least as effectively in ScenedesmusPS28 mutant which lacks HP form of cyt. b-559 as inthe wild-type, suggests that there exist also a differentmechanism of action of α-TQ and FCCP than thatconnected with the oxidation of the cyt. b-559 HPform, and hence with the cyclic electron transportaround PSII. Similar conclusions may be drawn fromthe observation on the inhibitory and stimulatoryeffects of α-TQ and α-Toc, respectively, on the oxygenevolution of PSII particles [20]. In that case, the actionof these compounds was probably connected with theirdirect or indirect influence on the water-splittingcomplex, which was also manifested in the change ofits S-state distribution and deactivation rates [20].Inhibitory action on oxygen evolution and deactivationof S-states is also known as an effect of FCCP. Another

Figure 5. Chlorophyll fluorescence quenching by prenyllipids intobacco thylakoids in the presence of DCMU. A, Fluorescenceintensity at Fm as a function of added α-Toc, α-TQ and PQ-9concentration in the presence of 10 µM DCMU. B, Stern-Volmer plotsfor α-TQ and PQ-9. Io and I are the fluorescence intensities in theabsence and presence of a prenyllipid, respectively. Abbreviations asin table I.


vol. 38 (4) 2000

possible site of action of α-TQ and FCCP could be theLP form of cyt. b-559.

In the case of α-Toc, its site of action could be theHP form of the cyt. b-559 since α-Toc was not activein increasing the fluorescence yield in the Scenedes-mus PS28 mutant lacking the cyt. b-559 HP form, onlyslightly active in the wild-type and most active intobacco thylakoids which showed a higher content ofthe cyt. b-559 HP form than the wild-type of Scene-desmus (data not shown). Moreover, it increased thereduction level of cyt. b-559 HP in contrast to α-TQ(table I). PQ-A and α-Toc cannot reduce directly theHP form, however, both prenyllipids may stabilise thereduced HP form, preventing it from oxidation. It wasalready shown that PQ-A restores the HP form [9] andthat the reduced form of cyt. b-559 HP is more slowlyconverted to the LP form than the oxidised HP form[30].

The fluorescence induction kinetics of α-TQH2

treated samples, which resembles those in the presenceof DCMU (data not shown), could suggest that α-TQH2

acts as an inhibitor of the linear electron transport.However, we have found that α-TQH2 inhibited theoxygen evolution activity when measured with theClark type electrode only by 10–15 % at theα-TQH2:chlorophyll molar ratio of 1:2.5 and by 30 %at 1:1 ratio (data not shown), so the interaction ofα-TQH2 with PSII is certainly based not only on itsaction as the inhibitor at the QB site.

Our results indicate that α-TQ may play a specificfunction in PSII, probably in excess light energydissipation which manifests in quenching of PSIIfluorescence and the mechanism of α-TQ action isprobably similar to FCCP, i.e. to catalyse deprotona-tion and/or redox changes of some components of PSIIsuch as the water splitting complex, tyrosine D, Chlzor cytochrome b-559. Closer identification of the siteof α-TQ action requires further studies.

4. METHODS

Scenedesmus obliquus wild-type and PS28 mutantwere grown as described by Bishop [6] and theirthylakoids were isolated according to Berzborn andBishop [4]. Synechococcus 6301 (Anacystis nidulans)thylakoids were prepared as described by Engels et al.[10] and tobacco thylakoids (Nicotiana tabacum var.John William’s Broadleaf) according to the methoddescribed by Robinson and Yocum [31]. PQ-A, α-TQand their reduced forms were obtained as described byKruk [18]. α-Toc was obtained from Merck, FCCP

from Serva. The prenyllipids were added to thylakoidsuspensions as ethanol solutions. After the prenyllipidaddition, the samples were dark-adapted for 3 minbefore the fluorescence measurements. Fluorescenceinduction kinetics were measured on a home-builtfluorimeter with the ms time resolution and excitedwith blue light (BG12 filters) and recorded with theuse of 630 nm cut-off filters. Steady-state fluorescencemeasurements (DCMU poisoned samples) were per-formed on a Perkin-Elmer LS-50 fluorimeter on preil-luminated samples at chlorophyll concentration of25 µg·mL–1 in 5 × 5 mm cuvettes, using 480 nm exci-tation and 685 nm emission wavelength. The thylakoidcytochromes were determined spectrophotometricallywith a SLM Aminco instrument from difference redoxabsorption spectra as follows: cyt. b-559 HP(reduced) =control – ferricyanide (FeCy), cyt. b-559 HP (total) =hydroquinone (HQ) – FeCy, cyt. b-559 LP = ascorbate(Asc) – HQ.

Acknowledgments

We are indebted to Professor Norman I. Bishop fora kind gift of Scenedesmus strains. This work wassupported by the Deutsche Forschungsgemeinschaft(DFG) within the frame of a co-operative projectbetween the DFG and the Polish Academy of Sciences(No. 436 POL-113/57/0) and a grant No. 6 PO4A009 10 from the Committee for Scientific Research ofPoland (KBN).

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vol. 38 (4) 2000

Documents

Interaction of α-tocopherol quinone, α-tocopherol and other prenyllipids with photosystem II