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Biomedical subjects

M Avron

Publications and source records attributed to M Avron.

At least 19 recordsLinked to original sources
PubMed1991

Polyphosphate metabolism in the alga Dunaliella salina studied by 31P-NMR.

Polyphosphate synthesis and the state of the intracellular polyphosphates in the unicellular green alga Dunaliella salina were studied using in vivo 31P-NMR spectroscopy. By perfusing phosphate-depleted algal cells trapped inside agarose beads with orthophosphate (Pi) containing medium, we were able to follow the process of polyphosphate synthesis in whole, living cells. The results suggest that, in Dunaliella, low molecular weight, probably cyclic, polyphosphate intermediates are synthesized from Pi, and are then condensed to high molecular weight polymers. Studies of the intracellular organization of the polyphosphates by electron microscopy and solid-state NMR techniques indicate that most of these polymers are stored in the cell in a soluble form, and not in solid-like structures.

Chlorophyta↗
PubMed1991

Proton flow through the ATP synthase in chloroplasts regulates the distribution of light energy between PS I and PS II.

The involvement of ATP synthase in the imbalance between the photoactivities of PS I and PS II under light-limiting conditions, was examined in broken lettuce chloroplasts using modulated fluorimetry. The imbalance, in favor of PS II, was minimal and roughly constant between pH 6.5-7.3 (ratio of PS II/PS I activities about 1.1), and maximal at pH 8.5 (ratio of PS II/PS I activities about 1.4). This increase was strongly inhibited by a treatment of the chloroplasts with the CF0 ATP synthase inhibitor DCCD, but unaffected by the CF1 ATPase inhibitor, tentoxin. However, tentoxin plus ADP-P1 did inhibit the high pH-induced increased imbalance. These results, when considered with the previous results on the effect of high pH on proton flux through the ATP synthase, suggest that the rate of such proton flow controls the imbalance between the two photo-systems. It is possible that there is an in vivo fine-tuning regulating mechanism of the photosystems imbalance via the opening and closing of proton gradient dissipation through the ATP synthase. This mechanism may help alleviate photoinhibitory damage.

Adenosine Diphosphate↗
PubMed1990

Metabolic studies with NMR spectroscopy of the alga Dunaliella salina trapped within agarose beads.

A technique for the entrapment of the unicellular algae Dunaliella salina in agarose beads and their perfusion during NMR measurements is presented. The trapped cells maintained their ability to proliferate under normal growth conditions, and remained viable and stable under steady-state conditions for long periods during NMR measurements. Following osmotic shock in the dark, prominent changes were observed in the intracellular level of ATP and polyphosphates, but little to no changes in the intracellular pH or orthoposphate content. When cells were subjected to hyperosmotic shock, the ATP level decreased. The content of NMR-visible polyphosphates decreased as well, presumably due to the production of longer, NMR-invisible structures. Following hypoosmotic shock, the ATP content increased and longer polyphosphates were broken down to shorter, more mobile polymers.

Adenosine Triphosphate↗
PubMed1990

The role of intracellular orthophosphate in triggering osmoregulation in the alga Dunaliella salina.

A new hypothesis is presented for the mechanism of metabolic response during osmoregulation in the alga Dunaliella salina. We propose that the osmotic response is initiated by differential volume changes of the cytoplasm and the chloroplast (observed using the electron microscope) which alter the cytoplasmic orthophosphate concentration. This triggers a flow through the Pi/triose-phosphate shuttle, activating chloroplast enzymes in the direction of either starch or glycerol synthesis. The Pi-dependent response was investigated in vivo using NMR. The rates of glycerol synthesis or elimination following osmotic shocks were modulated by the intracellular Pi level as predicted by the hypothesis.

Chemical Phenomena↗
PubMed1990

Accumulation in chick livers of 9-cis versus all-trans beta-carotene.

Previous results indicated that beta-carotene, as present within the unicellular alga Dunaliella, accumulated in the liver of chicks to a much higher degree than did synthetic all-trans beta-carotene. Because the Dunaliella beta-carotene is composed of an approximately equal mixture of two stereoisomers, 9-cis and all-trans, we tested whether a similar mixture of purified beta-carotene isomers would yield comparable results. Indeed, analysis of liver following 2 wk of feeding show a 10-fold higher accumulation of beta-carotene in the chicks fed a mixture of purified 9-cis and all-trans beta-carotene as compared with those fed only all-trans beta-carotene. The ratio of 9-cis to all-trans beta-carotene in the liver was about three times higher than that present in the beta-carotene fed to the chicks. Retinol and retinyl esters accumulated to a similar degree in the livers of chicks given all treatments.

Animals↗
PubMed1989

Characterization and reconstitution of the Na+/H+ antiporter from the plasma membrane of the halotolerant alga Dunaliella.

Na+/H+ exchange activity in plasma membrane preparations isolated from the unicellular halotolerant alga, Dunaliella salina, is shown to be competitively inhibited by amiloride or Li+, with Ki values of 25 and 30 microM, respectively. The activity can be followed by either the sodium-dependent change in transvesicular delta pH, as monitored by absorbance changes of Acridine orange, or by the delta pH-dependent uptake of 22Na+ into the intravesicular space. The activity was solubilized, by extraction with Triton, and reconstituted into active proteoliposomes. The activity of the reconstituted proteoliposomes was strongly stimulated by the presence of valinomycin and KCl, suggesting that the exchanger is electrogenic, presumably exchanging more than one proton for each Na+ ion. Partial purification of the Triton-extracted exchanger was obtained by fractionation on a DEAE-cellulose column.

Amiloride↗
PubMed1989

Bioavailability of a natural isomer mixture as compared with synthetic all-trans-beta-carotene in rats and chicks.

The unicellular halotolerant alga Dunaliella bardawil was previously shown to contain high concentrations of beta-carotene composed of about equal amounts of the all-trans and 9-cis isomers. One-d-old chicks and 7-wk-old male rats were fed diets supplemented with synthetic all-trans-beta-carotene or dry D. bardawil at equivalent levels of beta-carotene. The chicks were fed diets containing up to 0.025% beta-carotene for 2 mo, and the rats up to 0.1% beta-carotene for 2 wk. Liver analyses at the end of these periods indicated that both species showed at least a tenfold higher accumulation of the algal beta-carotene isomer mixture than of the synthetic all-trans-beta-carotene. The ratio of 9-cis-beta-carotene to the all-trans isomer in the livers of the algae-fed rats and chicks was similar to or higher, respectively, than that present in the algae. Retinol plus retinyl ester accumulated to a similar extent in the rats and chicks fed diets supplemented with synthetic all-trans or the natural isomer mixture of beta-carotene. The preferable accumulation of the natural isomer mixture of beta-carotene suggests that attention should be paid to the different sources of beta-carotene when testing their efficacy in effects other than providing retinol, such as in their possible role in the prevention of some types of cancer.

Administration, Oral↗
PubMed1988

The beta-carotene-rich alga Dunaliella bardawil as a source of retinol in a rat diet.

1. Dunaliella bardawil, a beta-carotene-accumulating alga, has been tested as a source of retinol and beta-carotene in a diet given to rats. The beta-carotene in this alga is composed of about equal amounts of the 9-cis and all-trans isomers. Male weanling rats were fed on a retinol-deficient diet for 60 d. Thereafter, the rats were divided into groups and fed on a diet deficient in retinol or supplemented with retinol, synthetic beta-carotene, dry alga or an algal oil-extract. Following further growth for 7 d, samples were taken for liver analyses of retinol, retinol isomers and beta-carotene. 2. Liver analyses revealed a comparable content of retinol and normal conversion rates in the rats grown on the diets supplemented with synthetic or natural beta-carotene. Rats fed on the alga and the algal-oil-supplemented diets accumulated 9-cis retinol in addition to the all-trans isomer. Rats fed on synthetic beta-carotene, alga and algal oil had a liver retinol: beta-carotene value of about 3:1. 3. These studies demonstrate the possibility of using dried D. bardawil or an oil extract of the alga as a dietary natural beta-carotene supplement which satisfies the total requirement of retinol in rats. 4. Rats fed on alga or on algal oil, accumulated in the liver 9-cis beta-carotene and all-trans beta-carotene in a ratio similar to that present in the alga.

Animals↗
PubMed1979

Properties of ATP-driven reverse electron flow in chloroplasts.

1. The reverse reactions induced by coupled ATP hydrolysis were studied in spinach chloroplasts by measurements of the ATP-induced increase in chlorophyll fluorescence reflecting reverse electron flow, and of the ATP-induced decrease in 9-aminoacridine fluorescence, representing formation of the transthylakoidal proton gradient (deltapH). ATP-induced reverse electron flow was kinetically analysed into three phases, of which only the second and third one were paralleled by corresponding phases in deltapH formation. The rapid first phase and formation of a deltapH occur also in the absence of the electron transfer mediator phenazine methosulfate. 2. The rate and extent of the reverse reactions were measured at temperatures in the range from 0 to 30 degrees C. The rate of formation of delta pH and of reverse electron flow were faster at high temperatures, but the maximal extent of delta pH and chlorophyll fluorescence increase were observed at the lowest temperature. Considering rate and extent of the ATP-stimulated reactions, a temperature optimum around 15 degrees C was found. Light activation of the ATPase occurred throughout the range studied. At 0 degrees C and in the presence of inorganic phosphate the activated state for ATPase was maintained for more than 10 min. 3. The ATP-induced rise in chlorophyll fluorescence yield was found to be of similar magnitude as the rise induced by 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea (DCMU), when both were measured with an extremely weak measuring beam. It is concluded, that both effects, although derived via distinctly different pathways, are limited by the same electron donating or electron accepting pool.

Adenosine Triphosphate↗
PubMed1979

Properties of ATP-induced chlorophyll luminescence in chloroplasts.

1. The recently described reaction of ATP-induced luminescence is analyzed for its relation to other ATP-induced reactions such as ATP-driven transmembrane proton gradient formation and ATP-driven reverse electron flow. 2. In the absence of phenazine methosulfate ATP-induced luminescence is optimal while the main phase of ATP-driven reverse electron flow is eliminated. 3. DCMU which by itself causes a much smaller luminescence, inhibits the ATP-induced luminescence. 4. Nigericin plus valinomycin, but not each by itself, fully inhibit the ATP-induced luminescence. 5. The observations are interpreted as indicating that ATP stimulates luminescence by a 2-fold mechanism: (a) increasing the amount of the reducing primary electron acceptor of Photosystem II, Q, and (b) creating a transmembrane electrochemical potential which serves to decrease the activation energy required for the charge recombination reaction which leads to luminescence.

Adenosine Triphosphatases↗
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↗