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Biomedical subjects

M Gibbs

Publications and source records attributed to M Gibbs.

At least 145 records · Page 8Linked to original sources
PubMed1969

Enzyme activities of the carbon reduction cycle in some photosynthetic organisms.

Profile analyses of the enzymes comprising the photosynthetic carbon reduction cycle have been performed in extracts of dark grown and greening Euglena gracilis var. bacillaris. Chlorella pyrenoidosa grown photoautotrophically, in the light with glucose or in the dark with glucose, Tolypothrix tenuis, Chromatium and leaves of spinach. Amounts of activity are compared with the level of photosynthetic CO(2) fixation. Only in Chromatium were all enzyme activities sufficient to support the in vivo rate of CO(2) fixation. In organisms other than Chromatium, some enzymes and particularly fructose 1,6-phosphatase and ribulose 1.5-diphosphate carboxylase appeared to be present in insufficient amounts to support the photosynthetic rate of the intact cell. Developmental studies with Euglena and growth studies with Chlorella led to the conclusion that these enzymes were associated with the cycle. Suppression of CO(2) fixation in heterotrophically grown Chlorella was accompanied by a striking decrease in the same enzymes whose activities increased in greening Euglena.

Journal Article↗
PubMed1969

Level of photosynthetic intermediates in isolated spinach chloroplasts.

The level of intermediates of the photosynthetic carbon cycle was measured in intact spinach chloroplasts in an attempt to determine the cause of the induction lag in CO(2) assimilation. In addition, transient changes in the level of the intermediates were determined as affected by a light-dark period and by the addition of an excess amount of bicarbonate during a period of steady photosynthesis. Assayed enzymically were: ribulose 1,5-diphosphate, pentose monophosphates (mixture of ribose 5-phosphate, ribulose 5-phosphate and xylulose 5-phosphate, hexose monophosphates (mixture of glucose 6-phosphate, glucose 1-phosphate, and fructose 6-phosphate), glyceraldehyde 3-phosphate, dihydroxyacetone phosphate, glycerate acid 3-phosphate, a mixture of fructose 1,6-diphosphate and sedoheptulose 1,7-diphosphate, adenosine triphosphate (ATP), adenosine diphosphate (ADP), and adenosine monophosphate (AMP).The lag in CO(2) fixation appeared to be the result of low levels of pentose monophosphates. The level of ribulose 1,5-diphosphate was roughly equal in chloroplasts showing immediate linear kinetics with respect to CO(2) fixation and chloroplasts which exhibited an initial lag.Following a light-dark transition, CO(2) fixation ceased immediately but the level of glycerate 3-phosphate increased while ribulose 1,5-diphosphate was only slightly effected. The increase in level of glycerate 3-phosphate was correlated with a decrease in triose phosphate. Within 3 to 5 min in the light, ATP reached a maximum concentration while in darkness, all was utilized in 30 to 60 sec. The rapid loss of ATP was ascribed to an ATPase rather than to its utilization in kinase reactions.A rapid increase in CO(2) concentration enhanced the level of triose phosphate, but the level of glycerate 3-phosphate showed only a small overshoot and was considered as evidence that reducing power was not a rate limiting factor. Data were obtained indicating that triose phosphates similar to pentose monophosphates and in contrast to fructose 6-phosphate, glucose 6-phosphate and glucose 1-phosphate could be transported between chloroplast and suspending medium. Differential import and export of phosphorylated compounds may serve as routes alternative to starch and sucrose for the flow of carbon into biosynthetic pathways.

Journal Article↗
PubMed1969

Changes in Succinyl CoA Synthetase Activity in Etiolated Bean Leaves Caused by Illumination.

The illumination of etiolated bean leaves (Phaseolus vulgaris) causes an increase in the activity of succinyl coenzyme A synthetase. Continuous white light or short periods of red or blue light followed by darkness will induce an increase with the highest activity at about 6 hr after the onset of illumination. Thereafter the activity decreases so that at 12 hr it is the same as the initial dark activity. Treatment with cycloheximide before illumination prevents the increase in activity. A number of other enzymes have been studied in an attempt to determine the significance of the transient nature of the changes in succinyl CoA synthetase activity.

Journal Article↗
PubMed1969

Inhibition of photosynthesis by oxygen in isolated spinach chloroplasts.

The inhibition of photosynthetic CO(2) fixation by O(2), commonly referred to as the Warburg effect, was examined in isolated intact spinach (Spinacia oleracea) chloroplasts. The major characteristics of this effect in isolated chloroplasts are rapid reversibility when O(2) is replaced by N(2), an increased inhibition by O(2) at low concentrations of CO(2) and a decreased effect of O(2) with increased concentrations of CO(2).Both the DPN- and TPN-linked glyceraldehyde 3-phosphate dehydrogenases but not aldolase were inhibited by O(2). The photoreduction of TPN measured in fragmented chloroplast preparations was similar in N(2) and O(2) down to a concentration of 5 micromolar TPN. The effect of 100% O(2) on (14)CO(2) assimilation was overcome completely by fructose 1,6-diphosphate and by ribose 5-phosphate but not by ascorbate, cysteine, dithiothreitol and reduced lipoate. Glycolate became the major photosynthetic product at high partial pressures of O(2) or at low CO(2) concentrations. It is concluded that O(2) depresses photosynthesis primarily by causing a shift of a major portion of the total carbon into glycolate and impairing the functioning of the photosynthetic carbon reduction cycle. The mechanism whereby O(2) alters the flow of carbon into glycolate remains unknown.

Journal Article↗
PubMed1968

D-glyceraldehyde 3-phosphate dehydrogenases of higher plants.

The d-glyceraldehyde 3-P dehydrogenases of spinach leaf, pea seed, and pea shoot were purified. The NADP and NAD-linked enzymes of either spinach leaves and pea shoots could not be separated. Changes in the ratio of NADP- to NAD-linked activity of the spinach leaf and pea shoot enzymes were observed during both purification and storage of crude extracts. The spinach leaf, pea shoot, and pea seed enzymes differ electrophoretically from each other and from the rabbit muscle enzyme. The pea seed and shoot enzymes contain bound nucleotide cofactor, resist proteolytic attack, have similar Michaelis-Menton kinetic constants and are competitively inhibited by d-sedoheptulose-7-phosphate and d-sedoheptulose 1,7-diphosphate. Charcoal removes the bound nucleotide from the pea seed enzyme but not from the pea shoot enzymes. NADP and NADPH were found to inhibit the reductive but not oxidative reaction catalyzed by the charcoal treated seed enzyme. The function of the pea shoot NADP and NAD-linked enzymes in chloroplast metabolism is discussed in regard to their location and catalytic properties. Although the NADP-linked activity can be assigned a primary, if not exclusive function in photosynthesis, the assignment of a distinct metabolic function to the NAD-linked activity cannot be made at present.

Glyceraldehyde-3-Phosphate Dehydrogenases↗
PubMed1968

Evidence for the participation of the reductive pentose phosphate cycle in photoreduction and the oxyhydrogen reaction.

The assimilation of (14)C-sodium bicarbonate has been measured in Scenedesmus obliquus as 1) photosynthesis, 2) photoreduction (light dependent incorporation of carbon dioxide by hydrogen adapted cells under conditions where photosynthesis is inoperative), and 3) the oxyhydrogen reaction (dark assimilation of carbon dioxide by hydrogen adapted cells in an atmosphere of hydrogen and 1% oxygen). Degradation of the glucose formed in each of these reactions using the Leuconostoc technique establishes the participation of the reductive pentose phosphate cycle.

Journal Article↗
PubMed1968

Role of Aldolase in Photosynthesis. II Demonstration of Aldolase Types in Photosynthetic Organisms.

Spinach leaves and photoautotrophically grown Euglena and Chlorella possess fructose 1,6-diphosphate aldolases inhibited by p-chloromercuribenzoate but insensitive to K(+) or ethylenediamine tetraacetate (Type I). Dark grown Euglena and Chlorella have aldolases inhibited by p-chloromercuribenzoate and ethylenediamine tetraacetate but stimulated by K(+) (Type II). The red alga, Chondrus, and the golden-brown alga, Ochromonas, appear to possess both types. Bean, pea, and spinach seeds and the leaves and cotyledons of etiolated bean seedlings contain a p-chloromercuribenzoate insensitive, apparently non-sulfhydryl variant of Type I. Sensitivity of leaf aldolase to p-chloromercuribenzoate occurs in etiolated bean seedlings only after an extended period of illumination. Type II aldolase activity in cell-free extracts of 4 blue-green algae has been demonstrated.

Journal Article↗
PubMed1967

Sucrose as a product of photosynthesis in isolated spinach chloroplasts.

Sucrose has been detected as a seasonal photosynthetic product in spinach chloroplast preparations. Sucrose when present accounted for up to 30% of the CO(2) fixed. Experiments in which sucrose was formed have been compared with experiments in which it was not formed, and a possible control mechanism for sucrose synthesis is discussed.

Journal Article↗
PubMed1967

Enhanced Dark CO(2) Fixation by Preilluminated Chlorella pyrenoidosa and Anacystis nidulans.

The products of short time photosynthesis and of enhanced dark (14)CO(2) fixation (illumination in helium prior to addition of (14)CO(2) in dark) by Chlorella pyrenoidosa and Anacystis nidulans were compared. Glycerate 3-phosphate, phosphoenolpyruvate, alanine, and aspartate accounted for the bulk of the (14)C assimilated during enhanced dark fixation while hexose and pentose phosphates accounted for the largest fraction of isotope assimilated during photosynthesis. During the enhanced dark fixation period, glycerate 3-phosphate is carboxyl labeled and glucose 6-phosphate is predominantly labeled in carbon atom 4 with lesser amounts in the upper half of the C(6) chain and traces in carbon atoms 5 and 6. Tracer spread throughout all the carbon atoms of photosynthetically synthesized glycerate 3-phosphate and glucose 6-phosphate. During the enhanced dark fixation period, there was a slow formation of sugar phosphates which subsequently continued at 5 times the initial rate long after the cessation of (14)CO(2) uptake. To explain the kinetics of changes in the labelling patterns and in the limited formation of the sugar phosphates during enhanced dark CO(2) fixation, the suggestion is made that most of the reductant mediating these effects did not have its origin in the preillumination phase.It is concluded that a complete photosynthetic carbon reduction cycle operates to a limited extent, if at all, in the dark period subsequent to preillumination.

Journal Article↗
PubMed1966

Dark and photometabolism of sugars by a blue green alga: Tolypothrix tenuis.

The carbohydrate metabolism of the autotrophically grown blue-green alga, Tolypothrix tenuis, was studied. The alga respires glucose, fructose, galactose, and ribose. About 60% of the glucose consumed is converted by starved cells into a glucose polysaccharide. Glucose uptake and O(2) consumption are not inhibited by 0.01 m arsenite or by 0.005 m iodoacetamide. The distribution of (14)C in the polysaccharide glucose was established after feeding of glucose-1-(14)C, -2-(14)C, -6-(14)C, ribose-1-(14)C, and fructose-6-(14)C. Randomization of isotope between the 2 halves of the glucose from polysaccharide is limited when the experiments are carried out in the dark. After an extended incubation glucose-2-(14)C yields a glucose molecule with isotope labeled approximately equal in C-1, C-2 and C-3. When the labeled glucoses were fed at a light intensity of compensation point, and in the presence of carbon dioxide, a greater degree of randomization of isotope occurred. The enhanced randomization of isotope is thought to result from an additional supply of triose phosphates as a result of photosynthesis which creates an environment favorable to the reversal of the glycolytic reactions.To account for the labeling patterns and the resistance of respiration to the inhibitors, it is proposed that the oxidative pentose phosphate cycle is the major pathway of carbohydrate breakdown in this alga.

Carbohydrate Metabolism↗
PubMed1966

Regulation of Photosynthetic Capacity in Chlamydomonas mundana.

A regulatory system has been described in the obligately phototrophic green alga Chlamydomonas mundana. Cells grown in acetate media are unable to fix carbon dioxide in the light but carry out a photoassimilation of acetate to carbohydrate: cells cultured with carbon dioxide as the sole source of cellular carbon carry out typical green plant photosynthesis. The control appears to take place at the level of the reductive pentose phosphate cycle. The presence of sodium acetate in the medium strongly inhibits formation of ribulose-1.5-diphosphate carboxylase, ribulose-5-phosphate kinase, and one of the 2 fructose-1,6-diphosphate aldolase activities of the cell. Ribose-5-phosphate isomerase is present in higher activity in autotrophic cells. Changes in the levels of triose phosphate dehydrogenase were also noted. The total pigment content of the cell and the photosynthetic electron transport reactions are not altered under different conditions of growth.

Journal Article↗