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Biomedical subjects

M Avron

Publications and source records attributed to M Avron.

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

Quantum yields for oxygenic and anoxygenic photosynthesis in the cyanobacterium Oscillatoria limnetica.

A comparison of the quantum yield spectra of the oxygenic (H(2)O as the electron donor) with the anoxygenic (H(2)S as the electron donor) photosynthesis of the cyanobacterium, Oscillatoria limnetica reveals that anoxygenic photosynthesis is driven by photosystem I only. The highest quantum yields of the latter (maximum; 0.059 CO(2) molecules/quantum of absorbed light) were obtained with wavelengths which preferentially excite photosystem I (<550, >650) in which chlorophyll a and carotenoids are the major pigments. The addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea had no effect on anoxygenic photosynthesis, and no enhancement in quantum efficiency was observed by a superimposition of light preferentially exciting photosystem II.Oxygenic photosynthesis efficiently utilizes only a narrow range of the absorption spectrum (550-650 nm) where light is provided in excess to photosystem II via phycocyanin. The quantum yield (0.033 CO(2) molecules/quantum of absorbed light) is lower than the theoretical yield by a factor of 3, possibly due to inefficient light transfer from photosystem II to I. Thus, 3-fold enhancement of oxygenic photosynthesis by superimposition of photosystem I light, and low quantum yields for anoxygenic photosynthesis, were obtained in this region. These results are consonant with the suggestion that such a cyanobacterium represents an intermediate stage in phototrophic evolution.

Journal Article↗
PubMed1977

Dihydroxyacetone Kinase Activity in Dunaliella parva.

An enzyme catalyzing the phosphorylation of dihydroxyacetone has been identified in the halophilic alga, Dunaliella parva. Since glycerol and glyceraldehyde are not substrates, the enzyme is referred to as dihydroxyacetone kinase. Dihydroxyacetone kinase was purified 9-fold by ammonium sulfate fractionation followed by DEAE-cellulose chromatography.

Journal Article↗
PubMed1976

A method for measuring the internal pH in illuminated chloroplasts based on the stimulation of proton uptake by amines.

A simple method for measuring the internal pH of chloroplasts during steady-state illuminations based on the stimulation of proton uptake by monofunctional amines was developed. Predictions of a mathematical derivation concerning the dependence of the stimulation on the amine concentration, the internal volume, the pK of the amine and the external pH have been verified experimentally. To circumvent uncoupling and swelling due to large internal accumulation of amines extrapolation of the stimulation to low amine concentrations was suggested and shown to lead to valid values. Alternatively, swelling could be largely reduced in a medium containing potassium aspartate and valinomycin.

Amines↗
PubMed1976

Photosynthetic activities of membrane preparations of the blue-green alga Phormidium luridum.

Techniques have been developed for the isolation of whole spheroplaste and membrane preparations from the blue-green alga Phormidium luridum which are active in photosystem-II-sensitized and photosystem-I-sensitized electron-transport reactions and in photophosphorylation. Attention is called to the necessity of supplying magnesium and phosphate throughout the preparative and assay procedures for obtaining active membranes. Selective washing of a proper membrane preparation is shown to remove a new protein factor, termed protein factor 1, which restores the ability of the preparation ot catalyze photosystem-I-sensitized reactions.

Calcium↗
PubMed1976

Measurement of transmembrane potentials in Rhodospirillum rubrum chromatophores with an oxacarbocyanine dye.

The fluorescent dye 3,3-dipentyloxacarbocyanine (OCC) can be used as a fluorescence probe to measure transmembrane potentials across Rhodospirillum rubrum chromatophore membranes. A reversible fluorescence increase is observed in the light which is sensitive to inhibitors, permeable ions and uncouplers. Partial interchangeability between the electrical potential and the proton concentration gradient has been demonstrated by measurement of the fluorescence increase with OCC and the fluorescence quenching with 9-aminoacridine. OCC fluorescence changes can be induced also in the dark by injection of permeable salts and by rapid pH changes presumably indicating diffusion potentials. Using salt-induced diffusion potentials for calibrating the light signals and with several assumptions, the light-induced potentials were estimated as 170 mV for the maximal signal and 90-110 mV at the steady state. OCC has been shown to apparently increase the electrical conductivity of the chromatophore membrane, a fact which may be relevant to the mechanism of action of this probe. A red shift in the OCC absorption spectrum occurs when mixed with chromatophores, with a difference spectrum maximum at 495 nm. The absorption changes at 495 nm taking place in the light are similar in kinetics to the fluorescence changes. The absorbance spectrum of OCC in organic solvents is red shifted and the extent of the shift depends on the hydrophobicity of the medium. The difference spectrum compared to water in sec-butyl acetate/n-hexane (3 : 1, v/v) with a dipole moment of 5 was nearly identical to that of chromatophore-associated dye. The uncoupling properties of OCC at high concentrations and some difficulties in calibration limit the usefulness of this probe for quantitative measurements of transmembrane potentials.

Bacterial Chromatophores↗
PubMed1976

Light Modulation of Enzyme Activity in Chloroplasts: Generation of Membrane-bound Vicinal-Dithiol Groups by Photosynthetic Electron Transport.

Inhibitor experiments indicate that photosynthetic electron transport is required for light activation of the pea (Pisum sativum) leaf chloroplast enzymes NADP-linked glyceraldehyde-3-phosphate dehydrogenase, NADP-linked malic dehydrogenase, ribulose-5-phosphate kinase and sedoheptulose-1,7-diphosphate phosphatase, and for inactivation of glucose-6-phosphate dehydrogenase. Modulation of the activity of the dehydrogenases and kinase apparently involves a component preceding ferredoxin in the photosynthetic electron transport chain; activation of the phosphatase involves an electron transport component at the level of ferredoxin. Modulation of enzyme activity can be obtained in a broken chloroplast system consisting of membrane fragments and stromal extract. The capacity for light regulation in this system is reduced or eliminated when the membrane fraction is exposed to arsenite in the light or to sulfite in light or dark. Light-generated vicinal-dithiols seem therefore to be involved in modulation of the activity of the enzymes included in this study.

Journal Article↗
PubMed1976

Evidence for a Block between Plastoquinone and Cytochrome f in a Photosynthetic Mutant of Lemna with Abnormal Flowering Behavior.

Mutant strain 1073 of Lemna perpusilla is concluded to be blocked between plastoquinone and cytochrome f in the photosynthetic electron transport system. The location of the block is based on the following observations of activities in chloroplasts isolated from the mutant and wild-type plants. (a) Relative to wild type, electron flow rates from water to ferricyanide, 2,6-dichlorophenol indophenol or NADP were very low in the mutant, but rates of photosystem I-dependent electron flow and cyclic phosphorylation were high. (b) Chlorophyll a fluorescence induction curves for mutant and wild type were similar. (c) Silicomolybdate and lipophilic acceptors in the mutant were photoreduced at rates comparable to wild type. (d) Cytochrome f of the mutant chloroplasts was not reduced by red light, but was oxidized by red or far red light. (e) Reduction of the primary electron acceptor of photosystem II (Q) by ATP-driven reverse electron flow was not observed in the mutant.

Journal Article↗
PubMed1975

Properties of Photosynthetic Mutants Isolated from Euglena gracilis.

Four different photosynthetic mutants of Euglena gracilis were characterized as to their lesions in photosynthetic electron transport. Two were defective around photosystem II: one, in electron transport on the oxidizing side of photosystem II, and the second lacked cytochrome 558. The location of the defect in the third mutant was concluded to be in the carbon fixation cycle, since it could catalyse both photosynthetic electron transport and photophosphorylation. The fourth mutant had a defect in its mechanism of photophosphorylation.

Journal Article↗
PubMed1975

A Method for Producing, Selecting, and Isolating Photosynthetic Mutants of Euglena gracilis.

A method was developed for the isolation of photosynthetic mutants of Euglena gracilis. It consists of the following steps. (a) Incubation of the cells under phototrophic conditions in the presence of 3 (3,4-dichlorophenyl)-1, 1-dimethylurea for 1 week. This step caused a drastic reduction in the number of chloroplasts per cell; (b) mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine; (c) phototrophic growth for a few days to allow for phenotype expression; (d) selection by incubation in the presence of arsenate under phototrophic conditions for 2 days; (e) plating and growth under photoorganotrophic conditions; (f) assay of green colonies for ability to evolve oxygen. About 10% of the green colonies were found to be deficient in their ability to evolve oxygen. In principle the method may prove suitable for the isolation of other types of mutants of Euglena.

Journal Article↗
PubMed1975

Site of action of inhibitors of carbon dioxide assimilation by whole lettuce chloroplasts.

The sites of action of several compounds, reported to inhibit CO(2) fixation by chloroplast preparations were located by developing assays in lettuce chloroplasts to test their effect on partial reactions of the carbon cycle and on carbonic anhydrase. The results indicated that: d, l-glyceral-dehyde and 5'-AMP inhibit phosphoribulose kinase or isomerase. 3-Phosphoglyceric acid and 6-phosphogluconate inhibit ribulose diphosphate carboxylase. Azide, Mg(2+), and nitrite inhibit the activity of carbonic anhydrase of lettuce chloroplasts and light-dependent CO(2) fixation by intact chloroplasts with similar sensitivities. None of these inhibited CO(2) fixation in ruptured chloroplasts. It is suggested that the inhibition by azide, nitrite, and magnesium ions of CO(2) fixation by intact chloroplasts is due to their inhibition of the activity of carbonic anhydrase.

Journal Article↗