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PubMed1999

Consideration of individual susceptibility in adverse pesticide effects.

The modern environmental awareness leads to the realisation that the human metabolism is stressed by a huge number of chemical substances. Generally, these background exposures, consisting predominantly from natural and partly from industrial as well as life style sources, are tolerated without any adverse effects. Pesticides are chemicals intentionally introduced to the environment and have become necessities in modern agriculture as well as in indoor pest control. Their residues, therefore, is attracting more and more concern. For the majority of pesticides neither occupational nor environmental medical risk evaluations are so far available. Therefore, at the moment the occupational as well as the environmental supported preventive concept may only be achieved, if binding instructions upon experience and guide values are developed for the assessment of the individual risk of handling pesticides. In the occupational and environmental pesticide prophylaxis the ubiquitous background exposure levels in consideration with individual susceptibility factors should be recommended as provisional biological tolerance guide values. The suitability of this guide values concept for pesticides is demonstrated by determining the background exposure and the biomarkers of susceptibility of 250 unexposed persons as well as of more than 1200 occupationally exposed persons. As a result, a significant dependence of their health fidelity from the background exposure profile impressed on the individual polymorphism of the key enzymes was observed. Especially, the cumulative adducts of electrophilic substances and their metabolites with macromolecules like HSA and Hb turned out to be sensitive markers for the capacity of the individual metabolic rate. For alkylating and arylating pesticides the observed interindividual susceptibility to their adverse effects depends on the variability of the individual 'toxifying' and 'detoxifying' metabolic rates. Until scientific evaluation of official biological tolerance values for pesticides is carried out, it is advisable for risk prophylaxis to orientate the assessment of any individual tolerable stress and strain from pesticides to the synergism between background exposure, life style factors and biomarkers of specific susceptibility. They may be examined by a monitoring of conjugates and polymorphism marked by the individual metabolic rate. The monitoring and surveillance of pesticide exposures is mainly introduced by the recommendation of tolerable biological values from the reference value concept. This concept is an essential contribution to an objective risk discussion with regard to individual stress and strain profiles in environmental exposure scenarios.

Diuron↗
PubMed2003

Rapid and specific detection of herbicides using a self-assembled photosynthetic reaction center from purple bacterium on an SPR chip.

In this study, a direct detection system for herbicides inhibiting photosynthetic electron transfer was developed using the photosynthetic reaction center (RC) from the purple bacterium, Rhodobacter sphaeroides, and surface plasmon resonance (SPR) apparatus. The heavy-subunit-histidine-tagged RCs (HHisRCs) were immobilized on an SPR sensor chip via nickel chelation chemistry as a binder for one of the triazine herbicides, atrazine. Immediately after injection of atrazine solution on the HHisRCs-immobilized chip, the SPR responses increased and reached plateaus within 1 min. The SPR signals were proportional to the sample concentrations of atrazine in the range 1-100 microg/ml. To evaluate the binding specificity to atrazine, chlorinated aromatic herbicides, DCMU and MCPP, were investigated using the HHisRCs-immobilized chip. An RC inhibitor, DCMU, could also be detected with a higher detection limit of 20 microg/ml than atrazine (1 microg/ml). MCPP showed no signals because its inhibition mechanism against plants is different from that of atrazine and DCMU. These results indicated that the sensor chip immobilized RCs could be used for the specific detection of photosynthetic inhibitors.

2-Methyl-4-chlorophenoxyacetic Acid↗
PubMed1999

An ISFET-algal (Chlamydomonas) hybrid provides a system for eco-toxicological tests.

A cellular sensoring system was designed in which metabolism-dedicated pH-ISFETs and the unicellular green alga Chlamydomonas reinhardtii as a biological component, were combined. The system permits on-line detection of pH changes caused by the metabolic and photosynthetic activities of the cells. Photosynthetic activity results in a basification of the medium caused by uptake of CO2. In darkness, an acidification of the medium, resulting from the production of CO2 by degradation of starch was observed. Both, acidification and basification, are sensitive indicators for the physiological activity of the alga. Experiments using inhibitors of energy metabolism or photosynthesis illustrate the utility of this system for an on-line monitoring of substances of eco-toxicological importance.

Animals↗
PubMed2003

Different kinetics of photoinactivation of photosystem I-mediated electron transport and P700 in isolated thylakoid membranes.

Photoinactivation kinetics of photosystem I (PSI)-mediated electron transport rate was compared to that of P700 content at room (22 degrees C) and low (4 degrees C) temperatures in isolated spinach thylakoid membranes. The high light treatment was carried out under aerobic and anaerobic conditions. At 22 degrees C the decrease of electron transport rate showed first order exponential kinetics. The amount of P700 decreased linearly, being less affected in the first hours of illumination. During photoinhibition at 4 degrees C in the presence of oxygen, the kinetics of inactivation of PSI photochemical activity and the content of P700 were different. It was found that 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) had different protective effect on the electron transport rate and on P700 content at both temperatures. Treatment with high light intensity under N(2) atmosphere had no effect on the electron transport rate or P700 content. The possible degradation of PSI reaction centre proteins was determined using immunoblot methods. In the presence of linear electron transport at 22 degrees C correlation between formation of toxic hydroxyl radicals and inhibition of oxygen uptake was observed.

Anaerobiosis↗
PubMed1997

Photoacoustically monitored thermal energy dissipation and xanthophyll cycle carotenoids in higher plant leaves.

When barley leaves were suddenly exposed to strong white light, the rate of thermal de-excitation of the absorbed light energy, measured with a photoacoustic device, increased by around 12% within a few minutes. This phenomenon was paralleled by a quenching of chlorophyll fluorescence. Simultaneous measurements of the heat emission increase and chlorophyll fluorescence quenching allowed the absolute yield phi F of chlorophyll fluorescence to be determined in vivo; phi F varied from around 2% (at Fo) to around 15% (at Fm) in a variety of plant species. No correlation was found between the time course of heat emission increase and the time course of violaxanthin-to-zeaxanthin conversion in barley leaves exposed to various light and temperature treatments, indicating that the zeaxanthin pool built up in the light was not directly involved in the increased heat emission. However, the operation of the violaxanthin cycle accelerated the photoinduced rise in thermal energy dissipation. Photoacoustic measurements on leaves of a zeaxanthin-accumulating Arabidopsis thaliana mutant lacking the violaxanthin cycle indicated that this acceleration could be ascribed to the disappearance of violaxanthin rather than to the formation of zeaxanthin.

Acoustics↗
PubMed2003

Cyclic, pseudocyclic and noncyclic photophosphorylation: new links in the chain.

Photosynthetic electron transport is coupled to ATP synthesis. This process - photosynthetic phosphorylation - proceeds by several alternative electron-transport pathways in isolated chloroplasts. The question: 'Which of these works in real life?' has long occupied students of photosynthesis. Recent results from structural biology and genomics suggest that the answer is 'All of them'. The interplay between the pathways might explain the flexibility of photosynthesis in meeting different metabolic demands for ATP.

Adenosine Triphosphate↗
PubMed1993

Mutation of histidine residues in CP47 leads to destabilization of the photosystem II complex and to impairment of light energy transfer.

Site-directed mutagenesis has been used to change conserved histidine residues in hydrophobic regions of the photosystem II chlorophyll-binding protein CP47 in the cyanobacterium Synechocystis sp. PCC 6803. Nine mutants with one, four mutants with two, and four mutants with three His mutations in CP47 have been generated and characterized. Mutation of any one of seven different His residues to Tyr leads to slower photoautotrophic growth and apparent destabilization of the PS II complex. Mutations introduced into multiple His residues in one mutant exhibited a cumulative effect. Replacing His by Asn leads to a much smaller effect than observed upon mutation to Tyr. This is consistent with the hypothesis that the mutated His residues are chlorophyll ligands: Asn can substitute as chlorophyll ligand, whereas Tyr cannot. Further evidence supporting a role of the mutated His residues in chlorophyll binding comes from measurements of the light intensity needed to half-saturate oxygen evolution. All His mutants with impaired PS II function needed higher light intensities for half-saturation than wild type. A possible explanation for this decrease in antenna efficiency in the mutants is a loss of the Mg in the chlorophyll due to a loss of the fifth ligand, and thus the formation of a pheophytin molecule in the antenna. We conclude that conserved His residues in hydrophobic regions of CP47 indeed are chlorophyll ligands and that these ligands are important for PS II stability as well as efficient antenna function.

Binding Sites↗
PubMed1991

Proton release during successive oxidation steps of the photosynthetic water oxidation process: stoichiometries and pH dependence.

Flash-induced absorption changes of pH-indicating dyes were investigated in photosystem II enriched membrane fragments, in order to retrieve the individual contributions to proton release of the successive transitions of the Kok cycle. These stoichiometric coefficients were found to be, in general, noninteger and to vary as a function of pH. Proton release on the S0----S1 step decreases from 1.75 at pH 5.5 to 1 at pH 8, while, on S1----S2 the stoichiometry increases from 0 to 0.5 in the same pH range and remains close to 1 for S2----S3. These findings are analyzed in terms of pK shifts of neighboring amino acid residues caused by electrostatic interactions with the redox centers involved in the two first transitions. The electrochromic shift of a chlorophyll, associated with the S transitions, responding to local electrostatic effects was investigated under similar conditions. The pH dependence of this signal upon the successive transitions was found correlated with the titration of the proton release stoichiometries, expressing the electrostatic balance between the oxidation and deprotonation processes.

Chlorophyll↗
PubMed1992

Investigation of the origin of the "S3" EPR signal from the oxygen-evolving complex of photosystem 2: the role of tyrosine Z.

The origin of the "S3" EPR signal from calcium-depleted photosystem 2 samples has been investigated. This signal is observed after freezing samples under illumination and has been assigned to an interaction between the manganese cluster and an oxidized histidine radical [Boussac et al. (1990) Nature 347; 303-306]. In calcium-depleted samples prepared by three different methods, we observed the trapping of the tyrosine radical YZ+ under conditions which also formed the "S3" signal. An "S3"-type signal and YZ+ were also formed in PS2 samples treated with the water analogue ammonia. Following illumination at 277 K, the "S3" and YZ+ signals decayed at the same rate at 273 K in the dark. Both the YZ+ and "S3" signals decayed on storage at 77 K and could be subsequently regenerated by illumination at 8-77 K. No evidence to support histidine oxidation was found. The effects of DCMU, chelators, and alkaline pH on the dark-stable multiline S2 and the "S3" signals from calcium-depleted samples were determined. Both signals required the presence of EGTA or citrate for maximum yield. The addition of DCMU caused a reduction in the yield of "S3" generated by freezing under illumination. Incubation at pH 7.5 resulted in the loss of both signals. We propose that a variety of treatments which affect calcium and chloride binding cause a stabilization of the S2 state and slow the reduction of YZ+. This allows the trapping of YZ+, the interaction with the manganese cluster (probably in the S2 state) resulting in the "S3" signal. The data allow the position of the manganese cluster to be estimated as within 10 A of tyrosine Z (D1-161).

Ammonia↗
PubMed1992

Evidence from directed mutagenesis that aspartate 170 of the D1 polypeptide influences the assembly and/or stability of the manganese cluster in the photosynthetic water-splitting complex.

To identify amino acid residues that influence the assembly or stability of the manganese cluster in photosystem II, we have generated site-directed mutations in the D1 polypeptide of the cyanobacterium, Synechocystis sp. PCC 6803. Indirect evidence has suggested that the D1 polypeptide provides some of the ligands that are required for metal binding. Mutations at position 170 of D1 were selected for characterization, since an aspartate to asparagine mutation (DN170D1) at this position completely abolishes photoautotrophic growth, while retention of a carboxylic acid at this position (aspartate to glutamate, DE170D1) supports photoautotrophic growth. Photosystem II particles were purified from control, DE170D1, and DN170D1 cells by a procedure that retains high rates of oxygen evolution activity in control particles [Noren, G.H., Boerner, R.J., & Barry, B.A. (1991) Biochemistry 30, 3943-3950]. Spectroscopic analysis shows that the tyrosine radical, Z+, which normally oxidizes the manganese cluster, is rapidly reduced in the DE170D1 mutant, but not in the DN170D1 mutant. A possible explanation of this block or dramatic decrease in the rate of electron transfer between the manganese cluster and tyrosine Z is an alteration in the properties of the metal center. Quantitation of manganese in these particles is consistent with aspartate 170 influencing the stability or assembly of the manganese cluster, since the aspartate to asparagine mutation results in a decrease in the manganese content per reaction center. Photosystem II particles from DN170D1 show a 60% decrease in the amount of specifically bound manganese per reaction center, when compared to control particles. Also, we observe a 70% decrease in the amount of specifically bound manganese per reaction center in partially purified DN170D1 particles and at least an 80% decrease in the amount of hydroxylamine-reducible manganese in DN170D1 thylakoid membranes. Single-turnover fluorescence assays and steady-state EPR measurements demonstrate that the remaining, endogenous manganese does not rapidly reduce tyrosine Z+ in the DN170D1 mutant. Additional evidence that aspartate 170 influences the assembly or stability of the metal site comes from analysis of the DE170D1 mutant. Although this mutant assembles a functional manganese cluster, as assessed by oxygen evolution and spectroscopic assays, the properties of the manganese site are perturbed.

Amino Acid Sequence↗
PubMed1992

Oxygen yield and thermoluminescence characteristics of a cyanobacterium lacking the manganese-stabilizing protein of photosystem II.

Previous experiments have shown that a Synechocystis sp. PCC 6803 mutant (delta psbO) lacking the extrinsic manganese-stabilizing protein (MSP) exhibits impaired, but significant levels of H2O-splitting activity [Burnap, R., & Sherman, L.A. (1991) Biochemistry 30, 440-446]. [14C]DCMU-binding experiments now show that the number and affinity of DCMU-binding sites (normalized to chlorophyll) are equivalent in delta psbO and the wild type, suggesting equal concentrations of assembled reaction centers. A similar conclusion is reached on the basis of measurements of PSII electron transport (DPC-supported DCPIP reduction) by mutant and wild-type thylakoids. The pattern of flash O2 yield by delta psbO cells measured with a bare platinum electrode exhibits a period four oscillation (with a maximum on the third flash), indicating that the H2O-splitting enzyme in delta psbO retains the basic mechanistic features found in normal cells. However, the amplitude of these signals is smaller and more highly damped than those obtained from wild-type cells, suggesting the absence of MSP results in a higher miss probability and/or a reduction in the number of centers competent in oxygen evolution. Analysis of the rise kinetics of the ampermeric signal on the bare platinum electrode indicates that the S3-[S4]-S0 transition is retarded by at least a factor of 5 in the mutant. Thermoluminescence emission peak temperatures indicate that the S2QA-, S2QB-, and S3QB-charge pairs are significantly more stable with respect to recombination in the mutant. The intensities of the thermoluminescence emissions are also significantly reduced in the mutant.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding Sites↗
PubMed1994

Influence of DCMU and ferricyanide on photodamage in photosystem II.

The effect of strong illumination of thylakoid membranes was studied under a range of conditions. Under anaerobic conditions, the relatively small quenching of the maximum fluorescence (Fmax) is accompanied by a large increase of the initial fluorescence (F0), which is partially reversible. Changes in the extent of the QA-Fe2+ and chlorophyll triplet EPR signal during anaerobic photoinhibition were consistent with double reduction of QA [as reported by Vass et al. ((1992) Proc. Natl. Acad. Sci. U.S.A. 89, 1408-1412)]. When illumination was done in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) or ferricyanide, no change occurred in F0 while Fmax was quenched. The quenching of Fmax occurred more rapidly than the loss of oxygen evolution, and they were both irreversible. In the presence of ferricyanide, the percentage of inhibition of oxygen evolution was larger than the decrease in the extent of the QA-Fe2+ signal, indicating that damage of the donor side occurred. In the presence of DCMU, a decrease of the QA-Fe2+ EPR signal occurred which corresponded to the inhibition of oxygen evolution and to an increase of the triplet EPR signal, indicating a possible overreduction of QA. However, these changes were less marked in the DCMU-treated samples than in the sample without additions and occurred despite the quenching of Fmax. These results suggest that strong illumination of thylakoids, in the presence of DCMU, results in a slower formation of stable forms of reduced QA, thereby allowing the occurrence of side-path reactions leading to Fmax quenching.(ABSTRACT TRUNCATED AT 250 WORDS)

Aerobiosis↗
PubMed1994

Site-directed photosystem II mutants with perturbed oxygen-evolving properties. 1. Instability or inefficient assembly of the manganese cluster in vivo.

Several site-directed photosystem II mutants with substitutions at Asp-170 of the D1 polypeptide were characterized by noninvasive methods in vivo. In several mutants, including some that evolve oxygen, a significant fraction of photosystem II reaction centers are shown to lack photooxidizable Mn ions. In this fraction of reaction centers, either the high-affinity site from which Mn ions rapidly reduce the oxidized secondary electron donor, YZ+, is devoid of Mn ions or the Mn ion(s) bound at this site are unable to reduce YZ+. It is concluded that the Mn clusters in these mutants are unstable or are assembled inefficiently in vivo. Mutants were constructed in the unicellular cyanobacterium Synechocystis sp. PCC 6803. The in vivo characterization procedures employed in this study involved measuring changes in the yield of variable chlorophyll a fluorescence following a saturating flash or brief illumination given in the presence of the electron transfer inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea, or following each of a series of saturating flashes given in the absence of this inhibitor. These procedures are easily applied to mutants that evolve little or no oxygen, facilitate the characterization of mutants with labile oxygen-evolving complexes, permit photosystem II isolation efforts to be concentrated on mutants having the stablest Mn clusters, and guide systematic spectroscopic studies of isolated photosystem II particles to mutants of particular interest.

Aspartic Acid↗
PubMed1990

Light-dependent degradation of the D1 protein in photosystem II is accelerated after inhibition of the water splitting reaction.

Strong illumination of oxygen-evolving organisms inhibits the electron transport through photosystem II (photoinhibition). In addition the illumination leads to a rapid turnover of the D1 protein in the reaction center of photosystem II. In this study the light-dependent degradation of the D1 reaction center protein and the light-dependent inhibition of electron-transport reactions have been studied in thylakoid membranes in which the oxygen evolution has been reversibly inhibited by Cl- depletion. The results show that Cl(-)-depleted thylakoid membranes are very vulnerable to damage induced by illumination. Both the D1 protein and the inhibition of the oxygen evolution are 15-20 times more sensitive to illumination than in control thylakoid membranes. The presence, during the illumination, of the herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) prevented both the light-dependent degradation of the D1 protein and the inhibition of the electron transport. The protection exerted by DCMU is seen only in Cl(-)-depleted thylakoid membranes. These observations lead to the proposal that continuous illumination of Cl(-)-depleted thylakoid membranes generates anomalously long-lived, highly oxidizing radicals on the oxidizing side of photosystem II, which are responsible for the light-induced protein damage and inhibition. The presence of DCMU during the illumination prevents the formation of these radicals, which explains the protective effects of the herbicide. It is also observed that in Cl(-)-depleted thylakoid membranes, oxygen evolution (measured after the readdition of Cl-) is inhibited before electron transfer from diphenylcarbazide to dichlorophenolindophenol.(ABSTRACT TRUNCATED AT 250 WORDS)

Chlorides↗
PubMed1976

The mechanism of quinonediimine acceptor activity in photosynthetic electron transport.

The rates of electron flow catalyzed by a variety of unsubstituted and C- or N-methylated quinonediimine electron acceptors in a reaction requiring photosystem II in KCN-inhibited chloroplasts vary according to the structure of acceptor used. Quinonediimine, but not quinone, electron acceptor activities are inhibited by a variety of uncouplers. Kinetic analysis of this inhibition shows that it is competitive. Low concentrations of aniline also inhibit the activity of C-methylated quinonediimines, but this appears to be due to a chemical reaction between the acceptor and aniline at low pH inside the chloroplast. Light-induced uptake of a quinonediimine, p-phenylenediimine, was shown to occur in a DCMU-sensitive reaction. Methylamine uncoupling inhibits this uptake to the same extent as it inhibits electron flow. Experiments with a lipophobic acceptor, N,N,N',N'-tetramethyl-p-phenylenediimine, indicate that it catalyzes electron flow by the same mechanism as other quinonediimines. A model is proposed to account for quinonediimine-catalyzed electron flow.

Biological Transport↗
PubMed1975

The possible relationship between a membrane conformational change and photosystem II dependent hydrogen ion accumulation and adenosine 5'-triphosphate synthesis.

Data are presented which suggest that photosystem II dependent hydrogen ion accumulation and ATP synthesis can occur only after the lamellar membranes have undergone a conformational change. This membrane conformational change is detected by the electron transport dependent incorporation of diazonium benzene[35S]sulfonate into membrane components. Previously it was established that electron flux from the photosystem II primary acceptor to plastoquinone is a necessary event for the occurrence of the diazonium-detected conformational change. These studies indicate that the release of hydrogen ions during photosystem II oxidation of the primary reductant is also a necessary event for the diazonium-detected conformational change. When iodide were substituted for water (or other proton-releasing donors) as the primary reductant of system II the conformational change did not occur even though a substantial rate of electron flow from the primary acceptor to plastoquinone occurred.

Adenosine Triphosphate↗