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PubMed1983

The repressible metabolism of sorbitol (D-glucitol) by intact cells of the oral plaque-forming bacterium Streptococcus mutans.

Sorbitol metabolism of Streptococcus mutans was studied. Cocci adapted to growth in sorbitol, glucose or both were challenged to grow on and to ferment those carbohydrates in pH-controlled defined media with intact cells capable of metabolic inductions and regulations. Glucose degradation when in high concentration did not depend upon induction of glucose-specific phosphoenolpyruvate-dependent phosphotransferase activity, as it did at low glucose concentrations. Sorbitol utilization was signalled by the induction of sorbitol-specific phosphoenolpyruvate-dependent phosphotransferase and sorbitol-6-phosphate dehydrogenase activities which persisted throughout the growth cycle. However, when even low levels of glucose were present, sorbitol transport and catabolic activities were rapidly repressed and they were not de-repressed until essentially all glucose had been utilized. Metabolism of sorbitol thus relies on the sorbitol phosphotransferase/sorbitol-6-phosphate dehydrogenase pathway whose activity is sensitively repressed in the presence of glucose.

Enzyme Repression↗
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

Effect of haemolysis on the hexose monophosphate pathway in normal and in glucose-6-phosphate dehydrogenase-deficient erythrocytes.

The hexose monophosphate pathway of human glucose-6-phosphate dehydrogenase (EC 1.1.1.49) - deficient erythrocytes is under a severe and unexplained restraint (Gaetani, G.D., Parker, J.C. and Kirkman, H.N. (1974) Proc. Natl. Acad. Sci. U.S. 71, 3584-3587). In this study the hexose monophosphate pathway activity and the NADPH level of normal and glucose-6-phosphate dehydrogenase-deficient erythrocytes were measured soon after haemolysis. The results indicate a prompt increase in 14CO2 evolution and a rise in MADPH levels. Since, in this study, the concentration of the haemolysate is comparable to that of intact erythrocytes, the relief of the restraint on glucose-6-phosphate dehydrogenase through dilution-dependent dissociation from inactivator or inhibitor is excluded. The possibility that the intracellular restraint may result from compartmentalization of glucose-6-phosphate dehydrogenase and substrates or from properties of the intact membrane of the erythrocytes is suggested.

Carbon Dioxide↗