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Y Shahak

Publications and source records attributed to Y Shahak.

24 records · Page 2Linked to original sources
PubMed1982

Activation and Deactivation of H-ATPase in Intact Chloroplasts.

The light activation mechanism of the latent H(+)-ATPase was investigated in intact spinach (Spinacia oleracea, Hybrid 424) chloroplasts. The following observations were made. (a) Photosystem I electron acceptors such as methyl viologen, nitrite, oxaloacetate, etc., inhibit the light activation of the enzyme. (b) The electron transfer inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) fully inhibits the process. (c) Ascorbate plus diaminodurene or dithionite can restore light activation in DCMU-poisoned chloroplasts. (d) The activated state of the enzyme decays rather slowly (within a few minutes) after illumination of the intact chloroplasts. (e) The rate of dark decay is accelerated by oxidants (H(2)O(2) or ferricyanide) and slowed down by dithiothreitol.It is suggested that the physiological mechanism for regulation of the H(+)-ATPase involves oxidation and reduction reactions in a manner which resembles the regulation of the light-activated carbon cycle enzymes.

Journal Article↗
PubMed1981

The involvement of ferredoxin-NADP+ reductase in cyclic electron transport in chloroplasts.

The sites of action, in spinach thylakoid, of known inhibitors of electron transport at the reducing end of photosystem I have been more accurately located by parallel investigation of effects on three partial reactions: photo-reduction (from water) of added NADP+, photoreduction of added cytochrome c, and dark reduction of cyto-chrome c by added NADPH. Comparison with inhibitory effects on cyclic electron flow (registered by the slow phase of the electrochromic response during repetitive flash excitation) permitted assessment of the role of ferredoxin and ferredoxin-NADP+ reductase (ferredoxin: NADP+ oxidoreductase, EC 1.18.1.3) in the cyclic electron transport pathway around photosystem I. Disulfodisalicylidenepropane-1,1-diamine inhibited all the above partial reactions except the ferredoxin-dependent photoreduction of cytochrome C. thereby indicating its interference with the reductase or the complexation between reductase and ferredoxin. Studies with purified ferredoxin-NADP+ reductase established it as the sensitive component. Cyclic flow is also sensitive to the above inhibitor and thus presumably involves the reductase. Supporting evidence for this came from studies of inhibition by substituted maleimides, which are inhibitors of electron transfer through the isolated reductase; these also inhibited the slow phase of the electrochromic response and all partial reactions except the photoreduction of cytochrome c. In contrast, an antiserum against the reductase affected only reactions involving NADP. The conclusion is drawn that the pathway of cyclic electron transport includes both ferredoxin and ferredoxin-NADP+ reductase, but not the NADP-binding site on the reductase.

Chloroplasts↗
PubMed1980

Quercetin interaction with the chloroplast ATPase complex.

1. Quercetin, a flavonoid which acts as an energy transfer inhibitor in photophosphorylation is shown to inhibit the P-ATP exchange activity of membrane-bound CF1 and the ATPase activity of isolated CF1. Quercetin, affects also the proton uptake in chloroplasts in a manner similar to that of dicyclohexylcarbodiimide. 2. The light-dependent proton uptake in EDTA-treated chloroplasts is stimulated by quercetin. In untreated chloroplasts quercetin has a dual effect: it enhances at pH above 7.5 while at lower pH values it decreases the extent of H+ uptake. Similar effects were obtained with dicyclohexylcarbodiimide. 3. Like quercetin, dicyclohexylcarbodiimide was also found to inhibit the ATPase activity of isolated CF1. 4. Quercetin inhibits uncoupled electron transport induced by either EDTA-treatment of chloroplasts or by addition of uncouplers. Quercetin restores H+ uptake in both types of uncoupled chloroplasts. 5. The mode of action of quercetin and dicyclohexylcarbodiimide in photophosphorylation is discussed, and interaction with both CF1 and F0 is suggested.

Adenosine Triphosphatases↗
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↗