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Biomedical subjects

S Izawa

Publications and source records attributed to S Izawa.

At least 55 records · Page 3Linked to original sources
PubMed1977

Reversible Effects of Toxin from Helminthosporium maydis Race T on Oxidative Phosphorylation by Mitochondria from Maize.

Host-selective toxin from Helminthosporium maydis race T inhibited oxidative phosphorylation (AT(32)P formation) and stimulated ATPase activity by mitochondria from male-sterile (T) but not from normal (N) cytoplasm maize (Zea mays L.). Toxin increased the rate of NADH oxidation, but succinate oxidation was slightly, and malate-pyruvate oxidation was strongly inhibited as the associated ATP formation was abolished. There was a 1-minute lag before toxin gave maximal stimulation of NADH oxidation; the responses to 2,4-dinitrophenol and valinomycin were immediate. There was also a delay in the effect of toxin on ATP formation. T mitochondria were more sensitive than were N mitochondria to uncoupling by nigericin plus K(+); there was no evidence, however, that the action of toxin is related to that of nigericin or other ionophores. With NADH as the substrate, the degree of uncoupling increased with increases in toxin concentration up to a saturating level; kinetics of the response suggested reversibility. T mitochondria exposed to toxin for 5 minutes regained normal rates of respiration and of ATP formation when they were washed with toxin-free medium, showing that the uncoupling effect is reversible. Evidently HM-T toxin does not bind firmly to its site(s) of action, in contrast to reports for another hostselective toxin.

Journal Article↗
PubMed1976

Concentration-dependent effects of salicylaldoxime on chloroplast reactions.

Salicylaldoxime has been found to have a variety of concentration-dependent effects on chloroplast activities. At low concentrations (less than 10 mM), salicyladoxime reversibly inhibits all reactions which involve Photosystem II. Since the DCMU-insensitive silicomolybdate Hill reaction is also inhibited, one site of inhibition is definitely located before the DCMU-sensitive site, possibly before the photoact. The inhibition kinetics and the response of chloroplast fluorescence may indicate another site in the DCMU-sensitive region. At almost exactly the same concentrations (less than 10 mM), salicylaldoxime uncouples phosphorylation reversibly, whether it is supported by Photosystem II or by Photosystem I. At higher concentrations (approx. 20 mM) salicylaldoxime inhibits Photosystem II irreversibly, uncouples irreversibly, and begins to cause changes in chloroplast light scattering which could be manifestations of membrane damage. At very high concentrations (approx. 45 mM) salicylaldoxime irreversibly inhibits Photosystem I activity in the region of plastocyanin. This is indicated by the ability of salicylaldoxime to inhibit the photooxidation of cytochrome f but not the photooxidation of P-700.

Chloroplasts↗
PubMed1974

Light-dependent redistribution of ions in suspensions of chloroplast thylakoid membranes.

Ion movements associated with the pH rise that is observed upon illumination of thylakoid suspensions at low pH have been studied by a multiparameter technique. Light-dependent, dark-reversible fluxes of H(+), Cl(-), Na(+), K(+) and divalent cations were monitored, together with simultaneous changes in the optical density of the suspension. Extensive uptake of Cl(-) and efflux of Mg(2+) accompany the apparent inward movement of H(+) in the light. Only minor efflux of K(+) is seen and Na(+) appears immobile. The Cl(-) and Mg(2+) fluxes together compensate for most of the charge transferred as H(+), contributing respectively about 49% and 43% on an equivalent basis. The ratio of Cl(-) influx to Mg(2+) efflux is variable, but usually >1.0. The Mg(2+) flux can be supplanted by (1) K(+) flux, if the K(+)/Mg(2+) activity ratio in the suspension is high, and (2) Ca(2+) flux, if the thylakoids are equilibrated with suspending media containing Ca(2+). The affinity of the divalentcation-binding sites, or carrier mechanism, is greater for Ca(2+) than for Mg(2+). Schemes can be drawn up to account for the observed ion movements on the basis of either a chemical or a chemiosmotic mechanism for energy transduction in chloroplasts. In intact chloroplasts, light-dependent control of Mg(2+) distribution between thylakoid and stroma could serve to regulate enzyme activities in the carbon fixation pathway, and hence photosynthesis.

Binding Sites↗
PubMed1974

Studies on the Energy-coupling Sites of Photophosphorylation: V. Phosphorylation Efficiencies (P/e(2)) Associated with Aerobic Photooxidation of Artificial Electron Donors.

The rate of Hill reaction can be measured accurately as O(2) uptake (the Mehler reaction) if a rapidly autoxidizable electron acceptor (e.g., methylviologen) is used. However, when an artificial electron donor-ascorbate couple (or ascorbate alone) replaces the natural donor, water, the rate of O(2) consumption is no longer a reliable measure of the electron flux, because superoxide radical reactions contribute to O(2) uptake. Such radical reactions, however, can be suppressed by adding enough superoxide dismutase to the reaction mixture. Indeed in all of the photosystem I- and photosystem II-donor reactions tested (except with benzidine which was tested without ascorbate added), the O(2) uptake was inhibited by 30 to 50% by the addition of superoxide dismutase. The rate of phosphorylation was totally unaffected by the enzyme. The reasessment of the phosphorylation efficiencies thus made by the use of superoxide dismutase led us to the following conclusions. The phosphorylation efficiency associated with the transfer of electrons from a donor to methlylviologen (than to O(2)) through both photosystems II and I is practically independent of the donor used-catechol, benzidine, p-aminophenol, dicyanohydroquinone, or water. The P/e(2) ratio is 1.0 +/- 0.1. Only ascorbate gives a slightly lower value (P/e(2) = 0.9). (NH(2)OH-treated, non-water-splitting chloroplasts were used for reactions with these artificial donors.) The phosphorylation efficiency associated with DCMU-insensitive, photosystem I-mediated transfer of electrons from a donor to methylviologen (then to O(2)) is again largely independent of the donor used, such as diaminodurene, diaminotoluene, and reduced 2,6-dichlorphenol-indophenol. The P/e(2) ratio is 0.6 +/- 0.08.

Journal Article↗
PubMed1973

Studies on the Energy-coupling Sites of Photophosphorylation: II. Treatment of Chloroplasts with NH(2)OH Plus Ethylenediaminetetraacetate to Inhibit Water Oxidation while Maintaining Energy-coupling Efficiencies.

Artificial electron donors to photosystem II provide an important means for characterizing the newly discovered site of energy coupling near photosystem II. However, water oxidation must be completely abolished, without harming the phosphorylation mechanism, for these donor reactions and the associated phosphorylation to withstand rigorous quantitative analysis. In this paper we have demonstrated that treatment of chloroplasts with hydroxylamine plus EDTA at pH 7.5 in the presence of Mg(2+) followed by washing to remove the amine is a highly reliable technique for this purpose. The decline of the Hill reaction and the coupled phosphorylation during the treatment were carefully followed. No change in the efficiency of phosphorylation (P/e(2) 1.0-1.1) was observed until the reactions became immeasurable. Photosystem I-dependent reactions, such as the transfer of electrons from diaminodurene or reduced 2,6-dichlorophenolindophenol to methylviologen, and the associated phosphorylation were totally unaffected. It is clear that the hydroxylamine treatment is highly specific, with no adverse effect on the mechanism of phosphorylation itself. Benzidine photooxidation via both photosystems II and I in hydroxylamine-treated chloroplasts (electron acceptor, methylviologen; assayed as O(2) uptake) supports phosphorylation with the same efficiency as that observed for the normal Hill reaction (P/e(2) = 1.1). An apparent P/e(2) ratio of 0.6 was computed for the photooxidation of ascorbate.

Journal Article↗
PubMed1972

Use of uncoupling acridine dyes as stoichiometric energy probes in chloroplasts.

In a suspension of spinach chloroplasts the fluorescence of atebrin and other uncoupling acridine dyes is quenched upon energization which is associated with a proportional binding of the dyes to the organelles. There is a stoichiometric relation between the amount of dye bound and the actual steady state level of energy. When the concentration of atebrin is increased in energized chloroplasts the fluorescence is completely quenched until a certain concentration is attained above which the response sharply declines. Such titrations with atebrin were carried out under conditions of partial electron transport governed by photosystems I and II, in the presence of 3-(3,4-dichlorophenyl)-1, 1-dimethylurea and cyanide, respectively, and of complete electron transport governed by the two photo-systems. The sum of the saturating amounts of atebrin obtained in these partial electron flow systems equals that obtained in the complete system. This lends strong support to the view that two sites of energy conservation are coupled to the linear photosynthetic electron transport.When ATP was the energy donor the saturating amounts of atebrin were the same in both control and cyanide-treated chloroplasts, indicating that the energy-conserving mechanism was unimpaired in the latter.Removal of the chloroplast-coupling factor by ethylenedia-minetetraacetate treatment leads to inhibition of the probe responses, which can be restored again in recoupled chloroplasts.

Journal Article↗