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PubMed1992

Purification and characterization of monodehydroascorbate reductase from soybean root nodules.

Soybean (Glycine max (L.) Merr.) root nodules contain the enzymes of the ascorbate-glutathione cycle as an important defense against activated forms of oxygen. A key enzyme in this cycle--monodehydroascorbate reductase (MR)--was purified 646-fold and appeared as a single band on SDS-PAGE with silver or Coomassie blue staining. Purified MR contained 0.7 mol FAD/mol enzyme and had a specific activity of 288 mumol NADH oxidized.min-1.mg protein-1. The enzyme was a single subunit occurring as two isozymes (MR I and MR II) with Mr values of 39,000 and 40,000. Isoelectric focusing revealed that each isozyme consisted of two forms with pl values of 4.6 to 4.7. Ferricyanide and 2,6-dichlorophenol-indophenol were effective as electron acceptors. The purified enzyme did not possess leghemoglobin reductase activity. Inhibition by p-chloromercuribenzoate indicated the involvement of a thiol group in MR activity. The Km values were 5.6, 150, and 7 microM for NADH, NADPH, and monodehydroascorbate, respectively. The pH optimum was 8 to 9. The N-terminal sequence of 10 amino acids of MR II had little homology to known protein sequences.

Amino Acid Sequence↗
PubMed1992

Role of cytosolic NAD(P)H-quinone oxidoreductase and alcohol dehydrogenase in the reduction of p-nitrosophenol following chronic ethanol ingestion.

Rats fed an ethanol-containing diet for 4 weeks showed a 3- to 5-fold increase over isocalorically pair-fed controls with respect to cytosolic NAD(P)H-quinone oxidoreductase (NQOR) (E.C.1.6.99.2) with both menadione and dichlorophenol-indophenol as substrates. Rates of NAD(P)H-dependent p-nitrosophenol (pNSP) reduction catalyzed by rat liver cytosolic fractions were increased 1.5- to 2-fold upon pretreatment of the animal with ethanol. NQOR contributed almost exclusively to the NADPH-dependent C-nitrosoreductase activity in cytosol as judged by the strong inhibition of the reaction by dicoumarol. In contrast, NADH-dependent C-nitrosoreductase activity was inhibited 70-80% by pyrazole and thus may be attributed mainly to alcohol dehydrogenase(s). Highly purified rat liver cytosolic NQOR catalyzed the NADH- and NADPH-dependent reduction of pNSP to p-aminophenol. We therefore suggest that ethanol ingestion enhances the reduction of the C-nitrosoaromatics formed upon cytosolic metabolism of arylamines or nitroarenes by two mechanisms. Increased NADPH-dependent reduction is mediated by the induction of cytosolic NQOR while an NADH-dependent pathway responds to the increased availability of reduced cofactor upon ethanol ingestion and involves mainly the alcohol dehydrogenase-mediated reduction of such compounds.

Alcohol Dehydrogenase↗
PubMed1995

The effect of N-bromosuccinimide on ferredoxin:NADP+ oxidoreductase.

Treatment of spinach leaf ferredoxin:NADP+ oxidoreductase (FNR) with N-bromosuccinimide (NBS), under conditions where approximately one tryptophan residue per enzyme was modified, resulted in a loss of between 80 and 85% of the activity of the enzyme when electron transfer from NADPH to either ferredoxin or 2,6-dichlorophenol-indophenol was measured. Amino acid analysis revealed no detectable modification by NBS of any FNR amino acids other than tryptophan. Complex formation with ferredoxin, but not with NADP+, prevented both the inhibition of activity and the modification of tryptophan caused by the treatment with NBS. Modification of one FNR tryptophan residue had no significant effect on the Km values of the enzyme for either ferredoxin or NADPH or on the binding constants for the FNR complexes with either ferredoxin or NADP+. NBS treatment had only very small effects on the absorbance and circular dichroism spectra of FNR and did not significantly affect either the oxidation-reduction midpoint potential of the FAD prosthetic group of the enzyme or inhibit the reduction of the FAD group by NADPH. These results raise the possibility that a tryptophan residue may play a role in the electron transfer between the FAD of FNR and the enzyme substrate, ferredoxin.

Bromosuccinimide↗
PubMed1976

Steady-state kinetics of low molecular weight (type-II) NADH dehydrogenase.

(1) The steady-state kinetics of the NADH dehydrogenase activity of Type-II (low molecular weight) NADH dehydrogenase with the acceptors ferricyanide, cytochrome c and 2,6-dichloroindophenol are consistent with the simultaneous operation of an ordered and a ping-pong mechanism. Thus, depending on the acceptor concentration, the reduced enzyme is preferentially oxidized before or after NAD+ disociates from it. (2) The acceptors are able to oxidize the reduced enzyme and its NAD+ complex equally well. In contrast to the kinetics of the Type-I (high molecular weight) enzyme, double substrate inhibition is not found, implying that the site of oxidation of the reduced enzyme by acceptors and the NADH-binding site are remote. (3) With the indophenol, in the concentration range measured, the ordered mechanism is mainly operative. At infinite NADH and acceptor concentrations the rate constant of the reduction of enzyme by bound NADH is measured. (4) With ferricyanide and cytochrome c, in the concentration range measured, erroneous conclusions may be drawn from extrapolations owing to the fact that extrapolated lines in double-reciprocal plots of turnover number against acceptor concentration, at different NADH concentrations, intersect in the third quadrant. A method is described that allows the extrapolation of these data to zero acceptor concentrations. (5) The relation between activity and NADH concentration is sigmoidal (h = 2.0) with ferricyanide or cytochrome c as acceptor, but hyperbolic with 2,6-dichloroindophenol. The latter is also an inhibitor, competitive with respect to NADH. It is concluded that this two-electron acceptor, like ubiquinone, acts as an allosteric effector. (6) Type II is isolated from Type I without gross changes in tertiary structure, as judged by the unaltered rate constants of dissociation of NADH (k-1) and NAD+ (k4) and association of NADH (k1). (7) Type II differs from Type I in two respects, (a) The accessibility of the acceptors is greater by at least two orders of magnitude (k3). (b) The redox potential of the prosthetic group FMN is 120 mV less, as judged by a drop in the value of k2 by four orders of magnitude. It is suggested that one or more of the iron-sulphur proteins present in Type-I but lacking in Type-II dehydrogenase functions as an effector, regulating the redox potential of the FMN.

2,6-Dichloroindophenol↗
PubMed1976

Flash photolysis-electron spin resonance studies of photosystem I. A fast reduction of component of P-700+.

A 300 mus decay component of ESR Signal I (P-700+) in chloroplasts is observed following a 10 mus actinic xenon flash. This transient is inhibited by treatments which block electron transfer from Photosystem II to Photosystem I (e.g. 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), KCN and HgCl2). The fast transient reduction of P-700+ can be restored in the case of DCMU or DBMIB inhibition by addition of an electron donor couple (2,6-dichlorophenol indophenol (Cl2Ind)/ascorbate) which supplies electrons to cytochrome f. However, this donor couple is inefficient in restoring electron transport in chloroplasts which have been inhibited with the plastocyanin inactivators, KCN and HgCl2. Oxidation-reduction measurements reveal that the fast P-700+ reduction component reflects electron transfer from a component with Em = 375 +/- 10 mV (pH = 7.5). These data suggest the assignment of the 300-mus decay kinetics to electron transfer from cytochrome f (Fe2+) to P-700+, thus confirming the recent observations of Haehnel et al. (Z. Naturforsch. 26b, 1171-1174 (1971)).

2,6-Dichloroindophenol↗
PubMed1976

Photosynthetic electron transport, ATP synthesis and nitrogenase activity in isolated heterocysts of Anabaena cylindrica.

Isolated heterocysts of the N2-fixing blue-green alga Anabaena cylindrica contain the Photosystem I components P-700, bound and soluble ferredoxins and ferredoxin-NADP reductase. They also show Photosystem I activity being able to photoreduce both methylviologen and NADP when ascorbate + dichlorophenol-indophenol acts as reductant. They photophosphorylate (64 munol ATP produced/mg chlorophyll a/h) and carry out oxidative phosphorylation (8.7 munol ATP produced/mg chlorophyll a/h). Ninety per cent of the total cell-free extract nitrogenase activity is located in the heterocyst fraction of aerobic cultures.

Adenosine Triphosphate↗
PubMed1976

Light-dependent development of photosynthetic competence in Scenedesmus mutant No. 8.

The heterotrophically grown, P-700-free mutant No. 8 of Scenedesmus obliquus is unable to carry out photosynthesis. Yet, chloroplast particles isolated from the alga reduced ferricyanide. They also reduced methyl viologen in the presence of the artificial donor reduced 2,6-dichlorophenol indophenol with a low yield but an appreciable saturation rate. NADP reduction or P-700 turn-over could not be detected. When grown mixotrophically, the mutant showed increasing P-700 activity with a concomitant increase in the rate of photosynthesis. Both activities were lost again when the algae were returned to darkness. Isolated chloroplast particles showed a good P-700 turn-over and reasonable rates of NADP reduction. The data suggest that the mutation occurred at a site preceding the formation of the pigment. The results on the photochemical activities are discussed in the light of reports concerning the involvement of P-700 in linear electron transport.

Chlorophyta↗
PubMed1977

Photooxidase system of Rhodospirillum rubrum. I. Photooxidations catalyzed by chromatophores isolated from a mutant deficient in photooxidase activity.

The aerobic photooxidations of reduced 2,6-dichlorophenolindophenol and of reaction-center bacteriochlorophyll (P-870) have been investigated in membrane vesicles (chromatophores) isolated from a non-phototrophic Rhodospirillum rubrum strain. In aerobic suspensions of wild-type chromatophores, continuous light elicits an increase of the levels of 2,6-dichlorophenolindophenol and of oxidized P-870, which reach steady-state values shortly after the onset of illumination. In contrast, light induces in mutant suspensions a transient increase of the levels of 2,6-dichlorophenol-indophenol and of oxidized P-870, which fall to low steady-state values within a few seconds. These observations suggest that the mutation has altered a redox constituent located on the low-potential side of the photochemical reaction center, between a pool of acceptors and oxygen. Since endogenous cyclic photophosphorylation is catalyzed by mutant chromatophores at normal rates, it appears that the constituent altered by the mutation does not belong to the cyclic electron-transfer chain responsible for photophosphorylation. However, the system which mediates the aerobic photooxidations and the cyclic system are not completely independent: endogenous photophosphorylation is inhibited by oxygen in wild-type chromatophores but not in mutant chromatophores; in addition, the inhibitor of cyclic electron flow, 2-heptyl-4-hydroxyquinoline-N-oxide, enhances the aerobic photooxidation of reduced 2,6-dichlorophenolindophenol by chromatophores from both strains. These results support a tentative branched model for light-driven electron transfer. In that model, the constituent altered in the mutant strain is located in a side electron-transfer chain which connects the cyclic acceptors to oxygen.

2,6-Dichloroindophenol↗
PubMed1978

Studies on the mechanism of Tris-induced inactivation of oxygen evolution.

A study was made of the inactivation by Tris of O2 evolution in chloroplasts and the subsequent reactivation of O2 evolution. We conclude: 1. At concentrations of Tris sufficient to inhibit O2 evolution directly, a slow rate (t 1/2 approximately 20--25 min) of inactivation occurs; 2. Inactivation is accelerated (t 1/2 approximately 2 min) by weak light absorbed by system II and is rate limited by a dark step with a half-time of about 200 s; 3. Minimally one quantum event within System II is sufficient to inactive 50--70% of the O2 evolving centers; 4. This process is 3-(3,4-dichlorophenyl)-1,1-dimethylurea insensitive but is inhibited by reduced dichlorophenol indophenol and phenazine methosulfate, carbonylcyanide-p-trifluoromethoxyphenylhydrazone, 2-(3-chloro-4-trifluoromethyl)-anilino-3,5-dinitrothiophene and tetraphenylboron; 5. Partial reactivation of inactive O2 evolving centers is affected by the use of the same reagents inhibiting the light induced inactivations; 6. The life-time (t 1/2 approximately 1 to 3 h) of the activable state is correlated with diffusion across thylakoids of the larger manganese pool released from binding sites and remaining in thylakoids following inactivation of O2 evolution.

Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone↗
PubMed1979

Two electrogenic mechanisms contributing to the 560 nm absorption changes in intact Bryopsis chloroplasts.

Light -induced absorbance changes at 560 nm in dark-adapted intact chloroplasts of the green alga, Bryopsis maxima were studied in the time range of 200 ms. The initial rise of the 560 nm signals consists of two major components which are both electrochromic absorbance changes of the carotenoids, siponein and/or siphonaxanthin, but different in mechanisms of the field formation. The first component (component S) is related to electron transport since it was sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) and showed at light-intensity dependence similar to that of electron transport in chloroplasts. In the presence of DCMU, component S could be restored on addition of proton-transporting electron donors such as reduced 2.6-dichlorophenol indophenol and phenazine methosulfate, but not on addition of N,N,N',N'-tetramethyl-p-phenylenediamine which does not carry protons with electrons (Trebst, A. (1974) Annu. Rev. Plant Physiol. 25, 423--458). We propose that component S is due to the electric field set up by the proton translocation across the thylakoid membrane. The second component (component R) was resistant to DCMU and DBMIB. The light-intensity dependency of component R was similar to that of cytochrome f photooxidation which showed saturation at a relatively low light intensity. The magnitude of component R was markedly reduced by phenylmercuric acetate, suggesting the participation of ferredoxin and ferredoxin-NADP oxidoreductase in the mechanism of the field formation responsible for this component. In the presence of DCMU and phenylmercuric acetate, time courses of the 560 nm changes paralleled those of cytochrome f changes. These results indicate that component R is due to the electric field formed between oxidized cytochrome f and other intersystem electron carriers located in the inner part of the thylakoid membrane and reduced electron acceptors of Photosystem I situated on the membrane surface. The complex natures of the 560 nm changes, as well as the contributions of Photosystems I and II to the absorbance changes, are explained in terms of the two electrogenic mechanisms.

Carbonyl Cyanide m-Chlorophenyl Hydrazone↗
PubMed1986

Redox reactions of tonoplast and plasma membranes isolated from soybean hypocotyls by free-flow electrophoresis.

Redox reactions were studied in more than 90% pure tonoplast and plasma membranes isolated by free-flow electrophoresis from soybean (Glycine max) hypocotyls. Both types of membrane contained a b-type cytochrome (alpha max = 561 nm) and a noncovalently bound flavin, two possible components of a transmembrane electron-transport chain. Isolated tonoplast and plasma membranes reduced ferricyanide, indophenol and various iron complexes with NADH or NADPH as electron donors. The redox activity was inhibited in tonoplast membranes by about 60% by 10 microM p-chloromercuribenzene sulfonate, 8% by 500 microM lanthanum nitrate and 10% by 100 microM nitrophenyl acetate. In contrast, the redox activity of isolated plasma membranes was inhibited by about 60% by 500 microM lanthanum nitrate or 100 microM nitrophenyl acetate, but only 25% by 10 microM p-chloromercuribenzene sulfonate. The results show that both tonoplast and plasma membranes of soybean contain active electron-transport systems, but that the two systems respond differently to inhibitors.

Cell Fractionation↗
PubMed1987

Phenylglyoxal modification of the Photosystem II reaction center.

Time-resolved spectroscopic techniques, including optical flash photolysis and electron spin resonance (ESR), have been used in conjunction with fluorescence-induction and dye-reduction assays to monitor electron transport in Photosystem II (PS II) subchloroplast particles incubated with the covalent modifier, phenylglyoxal. Phenylglyoxal-modified digitonin (D-10) particles from spinach are characterized by a high initial fluorescence yield (Fi) and an abolition of the variable component of fluorescence (Fv); an inhibition of PS-II-mediated reduction of dichlorophenol indophenol (DPIP) by sym-diphenylcarbazide; an abolition of flash-induced absorption transients (t1/2 greater than 2 microseconds) at 820 nm attributed to the primary electron donor, P-680+; the inhibition of photoreduction of the acceptor Qa; and the elimination of the ESR Signal 2s and Signal 2f. These observations suggest the critical participation of specific arginine residues on both the oxidizing and reducing sides of Photosystem II and also implicate phenylglyoxal as a quinone-binding site inhibitor (Golbeck, J.H. and Warden, J.T. (1984) Biochim. Biophys. Acta 767, 263-271).

2,6-Dichloroindophenol↗
PubMed1976

Subunit structure of bovine milk xanthine oxidase. Effect of limited cleavage by proteolytic enzymes on activity and structure.

Bovine milk xanthine oxidase (xanthine:oxygen oxidoreductase, EC 1.2.3.2) has been purified by a modified method without the use of proteases, and its structure has been analyzed by polyacrylamide gel electrophoresis. Native xanthine oxidase is found to consist of only two polypeptide chains A with molecular weights of 150 000 each. These chains have NH2-terminal methionine. Limited proteolysis with trypsin, chymotrypsin, or subtilisin at pH 8 did not affect molecular weight and activities of the enzyme while each of the A chains was cleaved under these conditions to three fragments C, E, and F with molecular weights of 92 00, 42 000 and 20 000, respectively. These fragments remained bound to each other and were relatively resistant to subsequent proteolysis. The isolation of xanthine oxidase in the presence of pancreatin as described by Hart et al. (1970, Biochem. J. 116, 851) gives partially digested enzyme composed mainly of chains C, E (Mr 35 000) and a small component (Mr approx. 15 0-0). The action of subtilisin on xanthine oxidase at pH 11 resulted in complete digestion of E chains, FAD separation, and total loss of xanthine:oxygen oxidoreductase activity while xanthine:indophenol oxidoreductase activity was relatively little affected. The residual enzyme has a molecular weight of about 200 000, is composed mainly of two C chains (and may probably contain F and/or proteolytic fragments of low molecular weight), contains molybdenum, and does not contain FAD.

Amino Acid Sequence↗
PubMed1985

Oxygenic photoreduction of methyl viologen and nicotinamide adenine dinucleotide phosphate without the involvement of photosystem I during plastid development.

Studies on the appearance of various electron transport functions were followed during greening of etiolated cucumber cotyledons. Appearance of dichlorodimethoxy-p-benzoquinone, dimethyl quinone, tetramethyl-p-phenylenediamine, dichlorophenol indophenol and ferricyanide Hill reactions were observed after 8h of greening. However, photoreduction of methyl viologen (MV) and nicotinamide adenine dinucleotide phosphate (NADP) was observed from 2h of greening. Variable fluorescence, which is a direct indication of water-splitting function, was observed from 2h of greening in cotyledons, thylakoid membranes and photosystem II (PSII) particles. The decrease in variable fluorescence in the presence of MV (due to rapid reoxidation of Q-) observed from early stages of greening confirmed the photoreduction of MV by PSII. The early development of water-splitting function was further confirmed by the abolition of variable fluorescence in thylakoid membranes and PSII particles by heat treatment and concomittant loss of light dependent oxygen uptake in the presence of MV in heat treated chloroplasts. However, the photoreduction of MV and NADP was insensitive to intersystem electron transport inhibitors, dichlorophenyl dimethylurea or dibromomethyl isopropyl-p-benzoquinone till 8h of greening. Though the oxidation of intersystem electron carrier cytochrome f was observed from early stages of greening, the reduction of cytochrome f was not observed till 8h of greening. All these observations confirm that during early stages of greening MV and NADP are photoreduced by PSII without the involvement of intersystem electron carriers or the collaboration of PSI. Since these observations are at variance with the currently prevalent concept (Z-Scheme) of the photosynthetic generation of reducing power, which requires definite collaboration of the two photosystems, an alternate electron flow pathway is proposed.

Chlorophyll↗
PubMed1984

In vitro effect of n-hexane and its metabolites on selected enzymes in glycolysis, pentose phosphate pathway and citric acid cycle.

The effect of n-hexane, 2-hexanol, 5-hydroxy-2-hexanone, 2,5-hexanediol, methyl n-butyl ketone ( MnBK ) and 2,5-hexanedione (2,5-HD) has been studied in vitro on crystalline glyceraldehyde-3-phosphate dehydrogenase (GAPDH), DL-glyceraldehyde-3-phosphate: NAD oxidoreductase (phosphorylating) EC. 1.2.1.12 and phosphofructokinase (PFK) ATP: D-fructose-6-phosphate-1-phosphotransferase; EC. 2.7.1.11 and lactic dehydrogenase (LDH) L-lactate: NAD+ oxidoreductase, EC. 1.1.1.27. MnBK and 2,5-HD both inhibited GAPDH and PFK activities selectively. n-Hexane and 2-hexanol had no effect on GAPDH and PFK activities; 5-hydroxy-2-hexanone and 2,5-hexanediol exhibited a slight inhibitory effect on these enzymes. Neither metabolites of n-hexane have any effect on LDH activity. 2,5-Hexanedione did not inhibit transketolase (D-sedoheptulose-7-phosphate: D-glyceraldehyde-3-phosphate glycolaldehyde transferase, EC. 2.2.1.1) and succinate dehydrogenase (succinate: 2,6-dichlorophenol-indophenol oxidoreductase, EC. 1.3.99.1) activities. The levels of ATP were reduced in 2,5-HD-treated cat sciatic nerves and returned to normal levels by exposing the nerve to sodium pyruvate.

Adenosine Triphosphate↗
PubMed1996

Putative nitric oxide synthase (NOS)-containing cells in the central nervous system of the leech, Hirudo medicinalis: NADPH-diaphorase histochemistry.

The presence and distribution of putative nitric oxide synthase (NOS)-containing cells in whole-mount preparations of the central nervous system of the leech, Hirudo medicinalis, were studied using NADPH-diaphorase (NADPH-d) histochemistry. Specific staining occurred mainly in somata of some central neurones but NADPH-d-reactive branches and terminals were found in peripheral nerves and connectives: neuropile areas were stained weakly or unstained. Intense staining was located in many neurones on the ventral side of the segmental ganglia, including primary sensory neurones, motoneurones and interneurones, and in the anterior root ganglion. The sex ganglia contained some extra NADPH-d-positive cells. Head and tail ganglia and the dorsal side of the segmental ganglia showed less staining. Specific activity was not detected in salivary glands, crop or intestine. Controls using beta-NADPH or nitro blue tetrazolium (NBT) alone or with NBT plus alpha-NADPH, beta-NAD+, beta-NADH or beta-NADP+ did not induce specific staining. A potential NOS inhibitor, 2,6-dichlorophenol-indophenol (DPiP) at 10(-3) M, totally abolished NADPH-d-positive staining. Long-term fixation did not change the pattern of distribution of NADPH-d-positive cells. We conclude that (i) fixative-resistant NADPH-diaphorase is a characteristic marker of 12-15% of neurones in the leech CNS, and (ii) the specific distribution of the putative NOS-containing neurones suggests that NO may be a natural signal molecule in leeches.

Animals↗
PubMed1976

Urinary metabolites of 3,3-dimethyl-1-phenyltriazene.

Urinary metabolites excreted after a subcutaneous injection of 3,3-dimethyl-1[14C] phenyltriazene (DM[1-14C]PT) to rats accounted for 82% of the applied radioactivity. We have isolated aniline (1-2%), 2-hydroxyaniline (5-7%), 3-hydroxyaniline (about 1%) and 4-hydroxyaniline (31-37%) from ethyl acetate extracts of acid-hydrolysed urine, UV spectrometric determination of 4-hydroxyaniline, using the indophenol reaction, showed that the most abundant metabolite accounted for 56 to 61% of the applied dose. We have also demonstrated the excretion of metabolites containing the intact triazene structure (0.9-1.1%) by cold acid cleavage of these compounds, followed by coupling of the released arenediazonium cations with N-ethyl-1-naphthylamine (EN). The coloured derivatives of these metabolites, 4-benzeneazo-N-ethyl-1-naphthylamine (BAEN) (0.6-0.7%), 4-(2-hydroxybenzeneazo)-N-ethyl-1-napthylamine (2-HO-BAEN) (0.02%) and 4-(4-hydroxybenzeneazo)-N-ethyl-1-naphthylamine (4-HO-BAEN) (0.3-0.4%) were isolated. The identification of BAEN as the principal azo derivative of the excreted triazene metabolites is in full agreement with the proposed in vivo activation of 3,3-dimethyl-1-phenyltriazene (DMPT) to a carcinogenic methylating agent. The hydroxylation of the methyl group at N-3 yields the corresponding aminol, some of which is covalently bonded to a water-soluble compound.

Aniline Compounds↗
PubMed1980

Inhibition of electron flow and energy transduction in isolated spinach chloroplasts by the herbicide dinoseb.

The mode of action of dinoseb (2-sec-butyl-4-6-dinitrophenol) on chloroplast reactions was studied. Electron flow from water or from an artificial electron donor, diphenylcarbazide (DPC), to dichlorophenol indophenol (DCPIP) was inhibited at low concentrations of the herbicide (5--10 microM) suggesting a site for dinoseb inhibition at the oxidizing side of photosystem II (PS II). Ferricyanide photoreduction was also inhibited by dinoseb. Cyclic and non-cyclic photophosphorylation and Mg2+-ATPase activity were inhibited by dinoseb, which indicates that this herbicide also acts as an energy transfer inhibitor. Among the above mentioned activities, non-cyclic photophosphorylation was the most sensitive to the inhibition by dinoseb. Ca2+-ATPase activity of solubilized heat activated chloroplast coupling factor 1 (CF1) was stimulated by dinoseb. However, the same activity was inhibited in chloroplasts, which perhaps reflect a difference in the mode of interaction of dinoseb with solubilized and membrane bound coupling factor.

2,4-Dinitrophenol↗