Carbon dioxide fixation in trypanosomatids.

Fixation of carbon dioxide has been demonstrated for extracts from Crithidia fasciculata, Trypanosoma mega and Trypanosoma brucei brucei bloodstream and culture forms. The enzymes involved in this fixation were found to be ADP-stimulated phosphoenolpyruvate carboxykinase (E.C. 4.1.1.32), 'malic' enzyme (E.C. 1.1.138-40) and pyruvate carboxylase (E.C. 6.4.1.1). The subcellular localization of these enzymes has been investigated in all three organisms. Products of short and long term fixation experiments were separated and identified. The importance of carboxylation reactions is discussed in relation to the maintenance of oxidized and reduced coenzyme levels.     

Carbon dioxide fixation in isolated kalanchoe chloroplasts

Chloroplasts isolated from Kalanchoe diagremontiana leaves were capable of photosynthesizing at a rate of 5.4 μmoles of CO₂ per milligram of chlorophyll per hour. The dark rate of fixation was about 1% of the light rate. A high photosynthetic rate was associated with low starch content of the leaves. Ribose 5-phosphate, fructose 1,6-diphosphate, and dithiothreitol stimulated fixation, whereas phosphoenolpyruvate and azide were inhibitors. The products of CO₂ fixation were primarily those of the photosynthetic carbon reduction cycle.     

Carbon dioxide fixation in green sulphur bacteria

1. About one-third of the CO(2) fixed during photosynthesis by washed suspensions of Chlorobium thiosulfatophilum strain 8346 gave rise to alpha-oxoglutarate and branched-chain oxo acids, mainly beta-methyl-alpha-oxovalerate. Another one-third to one-half gave rise to a polyglucose. 2. The fixation of CO(2) was inhibited by fluoroacetate, increasing concentrations up to 1mm stimulating the accumulation of alpha-oxoglutarate and causing a decrease in the formation of the branched-chain oxo acids and polyglucose. 3. Acetate was converted into the same products as was CO(2). 4. Fluoroacetate (1mm) had a negligible effect on the formation of polyglucose from acetate and caused a slight inhibition of the formation of the branched-chain oxo acids and increased accumulation of alpha-oxoglutarate. 5. Iodoacetate (1mm) strongly inhibited polyglucose formation from acetate and caused accumulation of pyruvate. The formation of the branched-chain oxo acids from acetate was only slightly affected by this inhibitor. 6. Pyruvate can be metabolized by this organism in the presence of a suitable electron donor whether CO(2) is present or not. In the absence of CO(2) pyruvate is converted into polyglucose. 7. The accumulation of oxo acids during CO(2) fixation is completely inhibited by NH(4) (+) ions. The formation of the branched-chain oxo acids is considerably decreased by the presence of isoleucine, leucine or valine, or a mixture of these. 8. CO(2) fixation in two other strains of Chlorobium appears to exhibit a similar pattern to that in C. thiosulfatophilum strain 8346. 9. It is concluded that in washed suspensions, CO(2) is fixed mainly by a mechanism involving the reductive carboxylic acid cycle. Acetate, the product of the cycle, is converted into polyglucose via pyruvate synthase and a reversal of glycolysis or into branched-chain oxo acids by an unknown mechanism.     

Carbon dioxide fixation in orchid aerial roots

Acidity fluctuation, CO₂ gas exchange, δ¹³C value, PEP carboxylase and RuBP carboxylase activities in aerial roots of selected thick-leaved orchid hybrids ( Arachnis and Aranthera ) were studied. Both aerial roots and leaves showed acidity fluctuation over a 24 h period. Dark acidification in aerial roots was enhanced at low temperature (15°C). Aerial roots had δ¹³C values close to those of leaves which have been previously demonstrated to possess crassulacean acid metabolism. Variation in δ¹³C values along the length of the roots was observed; the root tip having a less negative δ¹³C value (—13.34%‰) than the older portions of the roots (—14.55%‰). There was no net CO₂ fixation by aerial root, although ¹⁴³²CO₂ fixation was observed in light and in darkness. The pattern of fluctuation in activities of PEP carboxylase and RuBP carboxylase in aerial roots was similar to that obtained for the leaves. In both aerial roots and leaves, PEP carboxylase activity was several times higher than that of RuBP carboxylase.     

Carbon dioxide fixation by immobilized chloroplasts

Chloroplasts of chinese mustard (Brassica campestris L.) were immobilized in polyacrylamide gel. A 8% polymer concentration was suitable for the immobilization. The activity of the carbon dioxide fixation of immobilized chloroplasts was 65% of that of free chloroplasts. The optimum conditions for the carbon dioxide fixation of immobilized chloroplasts were similar to that of native chloroplasts. However, immobilized chloroplasts were more stable under alkaline conditions and high temperatures than native chloroplasts. Light penetration of the gel was not a limiting parameter of the carbon dioxide fixation. The lifetime of immobilized chloroplasts was three times longer than that of free chloroplasts. 3-Phosphoglyceraldehyde and other compounds were produced continuously by immobilized chloroplasts.     

Carbon dioxide fixation by chloroplasts isolated in glycinebetaine

Spinach chloroplasts capable of high rates of CO₂ fixation have been isolated in glycinebetaine as an alternative osmoticum to sorbitol and found to be very stable. Proline was a less satisfactory alternatine. The possible significance of the use of glycinebetaine is discussed as this solute may be the physiological cytoplasmic osmoticum in members of the Chenopodiaceae.     

Carbon dioxide fixation by detached cereal caryopses

Immature detached cereal caryopses from barley (Hordeum vulgare L. var distichum cv Midas) and wheat (Triticum aestivum L. cv Sicco) were shown to be capable of fixing externally supplied ¹⁴CO₂ in the light or dark. Green cross cells and the testa contained the majority of the ¹⁴C-labeled material. Some ¹⁴C-labeled material was also found in the outer, or transparent, layer and in the endosperm/embryo fraction. More ¹⁴C was recovered from caryopses when they were incubated in ¹⁴CO₂ without the transparent layer, thus suggesting that this layer is a barrier to the uptake of CO₂. In all cases, significant amounts of ¹⁴C-labeled material were found in caryopses after dark incubation with ¹⁴CO₂. Interestingly, CO₂ fixation in the chlorophyll-less mutant Albino lemma was significantly greater in the light than in the dark. The results indicate that intact caryopses have the ability to translocate ¹⁴C-labeled assimilate derived from external CO₂ to the endosperm/embryo. Carboxylating activity in the transparent layer appears to be confined to phosphoenolpyruvate carboxylase activity but that in the chloroplast-containing cross-cells may be accounted for by both ribulose-1,5-bisphosphate carboxylase-oxygenase and phosphoenolpyruvate carboxylase activity. Depending on a number of assumptions, the amount of CO₂ fixed is sufficient to account for about 2% of the weight of starch found in the mature caryopsis.     

Carbon dioxide fixation in aquatic animals and its metabolic significance]

In experiments on the carp Cyprinus carpio and freshwater lamellibranch mollusc Anodonta cygnea, tissue and organ peculiarities of carboxylic reactions have been revealed together with their relationship to the temperatute and ionic composition of the incubation medium. It was shown that in fish the highest intensity of fixation of CO2 is exhibited by glandular organs with biosynthetic profile of the metabolism (liver), whereas in molluscs it is exhibited in the mantle which plays the key role in the formation of the shell, containing carbonate compounds of calcium.