PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “PENTOSEPHOSPHATES”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 595 records · Page 33Linked to original sources
PubMed1975

Regulation of glucose-6-phosphate dehydrogenase in spinach chloroplasts by ribulose 1,5-diphosphate and NADPH/NADP+ ratios.

The activity of glucose-6-phosphate dehydrogenase (EC 1.1.1.49) FROM SPINACH CHLOROPLASTS IS STRONGLY REGULATED BY THE RATIO OF NADPH/NADP+, with the extent of this regulation controlled by the concentration of ribulose 1,5-diphosphate. Other metabolites of the reductive pentose phosphate cycle are far less effective in mediating the regulation of the enzyme activity by NADPH/NADP+ ratio. With a ratio of NADPH/NADP+ of 2, and a concentration of ribulose 1,5-diphosphate of 0.6 mM, the activity of the enzyme is completely inhibited. This level of ribulose 1,5-diphosphate is well within the concentration range which has been reported for unicellular green algae photosynthesizing in vivo. Ratios of NADPH/NADP+ of 2.0 have been measured for isolated spinach chloroplasts in the light and under physiological conditions. Since ribulose 1,5-diphosphate is a metabolite unique to the reductive pentose phosphate cycle and inhibits glucose-6-phosphate dehydrogenase in the presence of NADPH/NADP+ ratios found in chloroplasts in the light, it is proposed that regulation of the oxidative pentose phosphate cycle is accomplished in vivo by the levels of ribulose 1,5-diphosphate, NADPH, and NADP+. It already has been shown that several key reactions of the reductive pentose phosphate cycle in chloroplasts are regulated by levels of NADPH/NADP+ or other electron-carrying cofactors, and at least one key-regulated step, the carboxylation reaction is strongly affected by 6-phosphogluconate, the metabolic unique to the oxidative pentose phosphate cycle. Thus there is an interesting inverse regulation system in chloroplasts, in which reduced/oxidized coenzymes provide a general regulatory mechanism. The reductive cycle is activated at high NADPH/NADP+ ratios where the oxidative cycle is inhibited, and ribulose 1,5-diphosphate and 6-phosphogluconate provide further control of the cycles, each regulating the cycle in which it is not a metabolite.

Adenosine Triphosphate↗
PubMed1976

The effect of hydrogen peroxide on CO2 fixation of isolated intact chloroplasts.

Low concentration of hydrogen peroxide strongly inhibit CO2 fixation of isolated intact chloroplasts (50% inhibition at 10(-5) M hydrogen peroxide). Addition of catalase to a suspension of intact chloroplasts stimulates CO2 fixation 2--6 fold, indicating that this process is partially inhibited by endogenous hydrogen peroxide formed in a Mehler reaction. The rate of CO2 fixation is strongly increased by addition of Calvin cycle intermediates if the catalase activity of the preparation is low. However, at high catalase activity addition of Calvin cycle intermediates remains without effect. Obviously the hydrogen peroxide formed at low catalase activity leads to a loss of Calvin cycle substrates which reduces the rate of CO2 fixation. 3-Phosphoglycerate-dependent O2-evolution is not influenced by hydrogen peroxide at a concentration (5x10(-4) M) which inhibits CO2 fixation almost completely. Therefore the inhibition site of hydrogen peroxide cannot be at the step of 3-phosphoglycerate reduction. Dark CO2 fixation of lysed chloroplasts in a hypotonic medium is not or only slightly inhibited by hydrogen peroxide (2,5x10(-4) M), if ribulose-1,5-diphosphate, ribose 5-phosphate or xylulose 5-phosphate were added as substrates. However, there is a strong inhibition of CO2 fixation by hydrogen peroxide, if fructose 6-phosphate together with triose phosphate are used as substrates. This indicates that hydrogen peroxide interrupts the Calvin cycle at the transketolase step, leading to a reduced supply of the CO2-acceptor ribulose 1,5-diphosphate.

Carbon Dioxide↗
PubMed1976

Transient inhibition by ribose 5-phosphate of photosynthetic O2 evolution in a reconstituted chloroplast system.

Photosynthetic oxygen evolution by a reconstituted chloroplast system utilising sn-phospho-3-glycerol (3-phosphoglycerate) ceases upon the addition of ribose 5-phosphate even though the presence of this metabolite permits a rapid and immediate CO2 fixation. The period of cessation is appreciable at 0.1 mM ribose 5-phosphate. It is lengthened as the amount of added ribose 5-phosphate is increased and by the addition of dithiothreitol, a known activator of ribulose-5-phosphate kinase. Ribulose 1,5-bisphosphate is without effect. A similar interruption of O2 evolution may also be brought about by the addition of ADP or by ADP-generating systems such as glucose plus hexokinase. Spectrophotometric experiments indicate that the reoxidation of NADPH in the presence of sn-phospho-3-glycerol is similarly affected. The transient inhibition by ribose 5-phosphate is not observed in the presence of an active ATP-generating system or in the presence of sufficient DL-glyceraldehyde to inhibit ribulose-5-phosphate kinase activity. It is concluded that ribose 5-phosphate inhibits photosynthetic O2 evolution by adversely affecting the steady-state ATP/ADP ratio and consequently the reduction of sn-phospho-3-glycerol to glyceraldehyde 3-phosphate. The results are discussed in their relation to ADP regulation of photosynthetic carbon assimilation and metabolite transport.

Carbon Dioxide↗
PubMed1977

The distribution of metabolites between spinach chloroplasts and medium during photosynthesis in vitro.

1. The formation of metabolites in the stroma compartment of isolated chloroplasts during carbon fixation, and their export to the medium, have been investigated using improved techniques. 2. Rapid separation of photosynthesising chloroplasts from the medium, accompanied by simultaneous quenching of metabolism was achieved by using silicone oil layer filtering centrifugation under illumination. Metabolites were separated by microscale ion-exchange chromatography. Quantitative determination of each metabolite was based on labelling with 32P. 3. It was found that fixed carbon was exported from the chloroplasts only as triose phosphate and phosphoglycerate, and to a minor extent, as pentose monophosphate. The main compounds accumulating in the stroma were hexose and heptose monophosphates and phosphoglycerate. A marked decrease in the concentration of inorganic phosphate in the stroma during the first 5 min of illumination was accompanied by a complementary increase in organic phosphate so that the total amount of phosphate within the chloroplasts remained constant. 4. The concentration difference for phosphoglycerate between the stroma and the medium was much higher than for triose phosphate or inorganic phosphate, although all three compounds are transported across the inner membrane of the chloroplast envelope by the same carrier. It was concluded that the efflux of phosphoglycerate was restricted.

Adenosine Triphosphate↗
PubMed1978

Modes of reduction of nitrogen in heterocysts isolated from Anabaena species.

N2 fixation (acetylene reduction) has been studied with heterocysts isolated from Anabaena cylindrica and Anabaena 7120. In the presence of ATP and at very low concentrations of sodium dithionite, reducing equivalents for activity of nitrogenase in these cells can be derived from several compounds. In the dark, D-glucose 6-phosphate, 6-phosphogluconate and DL-isocitrate support acetylene reduction via NADPH. In the light, reductant can be generated by Photosystem I.

Acetylene↗
PubMed1981

A 31P-NMR study of mono- and dimagnesium complexes of adenosine 5'-triphosphate and model systems.

31P-NMR chemical shifts and spin-lattice relaxation times of ATP (adenosine 5'-triphosphate), ribose 5'-triphosphate and tripolyphosphate show closely similar behaviour in aqueous solution at pH 7.5 on titration with Mg2+. The results are interpreted in terms of formation of 1 : 1 and 2 : 1 (dimagnesium) complexes with Mg2+ bound exclusively to the triphosphate chain. Stability constants for these complexes are reported. It is suggested that the predominant form of the 1 : 1 complexes has Mg2+ bound in tridentate manner (via non-bridging oxygen) to the alpha, beta and gamma phosphorus atoms; whilst that of the 2 : 1 complexes has each Mg2+ bound in bidentate manner, one to the alpha and beta, and the other to the beta and gamma, phosphorus positions.

Adenosine Triphosphate↗