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M J Merrett

Publications and source records attributed to M J Merrett.

26 records · Page 2Linked to original sources
PubMed1972

Influence of Light Intensity on Reductive Pentose Phosphate Cycle Activity during Photoheterotrophic Growth of Rhodospirillum rubrum.

Light intensity during growth affects the proportion of carbon dioxide fixed by the reductive pentose phosphate cycle relative to that incorporated via C(4) acids in acetate phototrophs of Rhodospirillum rubrum. With cells grown at high light intensity (9000 lux) the specific activities of ribulose-1, 5-diphosphate and propionyl CoA carboxylases were increased compared with cells grown at low light intensity (1500 lux), although pyruvate carboxylase activity was unaltered.Kinetic experiments with cells assimilating acetate at high light intensity showed that when the cells had been grown at high light intensity there was a rapid incorporation of (14)CO(2) into phosphate esters compared with cells grown at low light intensity and fixing (14)CO(2) while assimilating acetate at low light intensity. The percentage of the total radioactivity present in phosphate esters plotted against time gave a negative slope for high light conditions compared with a positive slope for low light conditions. High light-grown cells assimilating acetate at high light intensity showed the greatest combined rate of (14)CO(2) fixation via the reductive pentose phosphate cycle and C(4) acids, and this corresponded to the shortest mean generation time. When cells were grown at high light intensity and allowed to assimilate (14)CO(2) at high light intensity but in the stationary phase, the pattern of (14)CO(2) fixation resembled that for low light-grown cells assimilating acetate and fixing (14)CO(2) at low light intensity, showing that both acetate assimilation and high light intensity were necessary for the rapid incorporation of (14)CO(2) via the reductive pentose phosphate cycle.

Journal Article↗
PubMed1971

The intracellular localization of glycollate oxidoreductase in Euglena gracilis.

1. Lowering of the concentration of carbon dioxide in air available to phototrophically growing Euglena cultures from 5% to the normal value (0.03%) resulted in an increased specific activity of glycollate oxidoreductase. 2. The effects of chloramphenicol and cycloheximide suggested that this increase in activity was due to enzyme synthesis de novo on cytoplasmic ribosomes. 3. The K(m) for glycollate oxidation by the enzyme in crude cell extracts was 3.0x10(-3)m. 4. Differential centrifugation established that glycollate oxidoreductase present in phototrophically grown Euglena is a particulate enzyme. The enzyme was partially solubilized by the non-ionic detergent Triton X-100. 5. Sucrose-density-gradient centrifugation achieved the separation of the particulate glycollate oxidoreductase from chloroplasts and mitochondria. 6. Glutamate-glyoxylate aminotransferase was associated with particulate glycollate oxidoreductase.

Alcohol Oxidoreductases↗
PubMed1971

Photosynthetic products of division synchronized cultures of euglena.

Rates and products of photosynthetic (14)CO(2) fixation by division synchronized cultures of Euglena gracilis strain Z were determined over the cycle. Rate of (14)CO(2) fixation doubled in a continuous manner throughout the light phase followed by a slight reduction of photosynthetic capacity in the dark phase. Greater (14)C incorporation into the nucleic acid-polysaccharide fraction occurred with mature cells. Products of (14)CO(2) fixation varied markedly over the cycle: although with mature cells (14)C-labeled sucrose was not detected, with dividing cells this was the main sugar labeled; in young cells (14)C maltose was formed. Cells removed at end of dark phase accumulated (14)C in glycolate, whereas at other stages over the cycle less (14)C was present in glycolate, and this was accompanied by a rapid incorporation of (14)C into glycine and serine. Glycerate was an early and major product of photosynthesis with cells at the mature stage of the cycle.Changes in ribulose 1,5-diphosphate carboxylase activity paralleled changes in photosynthetic rate, but activity was not great enough to account for the observed rates of CO(2) fixation at most stages of the division cycle investigated.

Journal Article↗
PubMed1971

The regulation of glycolate metabolism in division synchronized cultures of euglena.

Phosphoglycolate and phosphoglycerate phosphatases and glycolate dehydrogenase activities were determined in division synchronized Euglena gracilis strain Z cultures. Phosphoglycolate phosphatase activity remained nearly constant in the light but doubled in the dark, whereas phosphoglycerate phosphatase activity decreased by half in the light and increased 4-fold over the dark phase of the cycle. Glycolate dehydrogenase activity assayed by dye reduction increased over the light and remained constant during the dark phase, but when determined by the phenylhydrazone method, an assay dependent upon the presence of a natural hydrogen acceptor, activity decreased in the dark phase. The acceptor decayed in the dark in all cell-free extracts and 3-(3,4-dichlorophenyl)-1, 1-dimethylurea inhibited light regeneration.Enzyme activity regulated flow of carbon via the glycolate pathway; with cells at end of light phase and early dark phase, high levels of glycolate dehydrogenase activity were accompanied by an early and rapid labeling of glycerate. Degradation of early samples gave uniformly labeled glycolate and glycerate while phosphoglycerate was predominantly carboxyl labeled. At these stages in the division cycle, glycerate was formed from glycolate via the glycolate pathway, all the enzymes of the pathway being recorded in Euglena. In contrast to cells from end of dark phase and early light phase, times of maximum phosphoglycerate phosphatase activity, glycerate was predominantly carboxyl labeled as was phosphoglycerate, but glycolate was still uniformly labeled.Glycolate excretion varied over the cycle, being maximal at end of dark phase, decreasing throughout the light phase until not detectable in early dark phase. When alpha-hydroxy-2-pyridinemethanesulfonate was used as a measure of glycolate biosynthesis, sufficient glycolate dehydrogenase was present to oxidize the glycolate produced at all stages over the cycle.

Journal Article↗
PubMed1970

The pathway of glycollate utilization in Chlorella pyrenoidosa.

1. Exogenous glycollate was rapidly metabolized in both the light and the dark by photoautotrophically grown Chlorella pyrenoidosa. 2. The incorporation of (14)C from [1-(14)C]glycollate by these cells was inhibited by the tricarboxylic acid-cycle inhibitors monofluoroacetate, diethylmalonate and arsenite, and also by alpha-hydroxypyrid-2-ylmethanesulphonate and isonicotinylhydrazine. 3. Short-term kinetic experiments showed over 80% of the total (14)C present in the soluble fraction from the cells to be in glycine and serine after 10s. This percentage decreased with time whereas the percentage radioactivity in glycerate increased for up to 30s then remained steady. The percentage of the total radioactivity present in citrate increased over the experimental period. Malate was the only other tricarboxylic acid-cycle intermediate to become labelled. 4. The kinetic and inhibitor experiments supported the following pathway of glycollate incorporation: glycollate --> glyoxylate --> glycine --> serine --> hydroxypyruvate --> glycerate --> 3-phosphoglycerate --> 2-phosphoglycerate --> phosphoenolpyruvate --> pyruvate --> acetyl-CoA. 5. The specific activities of the enzymes catalysing this metabolic sequence in cell-free extracts were great enough to account for the observed rate of glycollate metabolism of 0.25mumol/h per mg dry wt. of cells in the light.

Carbon Isotopes↗