Citric acid

About: Citric acid is a research topic. Over the lifetime, 17745 publications have been published within this topic receiving 277125 citations. The topic is also known as: citrate & H3cit.

The effect of composition on mechanical properties of brushite cements.

Abstract: Due to a fast setting reaction, good biological properties, and easily available starting materials, there has been extensive research within the field of brushite cements as bone replacing material. However, the fast setting of brushite cement gives them intrinsically low mechanical properties due to the poor crystal compaction during setting. To improve this, many additives such as citric acid, pyrophosphates, and glycolic acid have been added to the cement paste to retard the crystal growth. Furthermore, the incorporation of a filler material could improve the mechanical properties when used in the correct amounts. In this study, the effect of the addition of the two retardants, disodium dihydrogen pyrophosphate and citric acid, together with the addition of β-TCP filler particles, on the mechanical properties of a brushite cement was investigated. The results showed that the addition of low amounts of a filler (up to 10%) can have large effects on the mechanical properties. Furthermore, the addition of citric acid to the liquid phase makes it possible to use lower liquid-to-powder ratios (L/P), which strongly affects the strength of the cements. The maximal compressive strength (41.8 MPa) was found for a composition with a molar ratio of 45:55 between monocalcium phosphate monohydrate and beta-tricalcium phosphate, an L/P of 0.25 ml/g and a citric acid concentration of 0.5 M in the liquid phase.

Mechanism of the citrate transporters in carbohydrate and citrate cometabolism in Lactococcus and Leuconostoc species

Abstract: Citrate metabolism in the lactic acid bacterium Leuconostoc mesenteroides generates an electrochemical proton gradient across the membrane by a secondary mechanism (C. Marty-Teysset, C. Posthuma, J. S. Lolkema, P. Schmitt, C. Divies, and W. N. Konings, J. Bacteriol. 178:2178–2185, 1996). Reports on the energetics of citrate metabolism in the related organism Lactococcus lactis are contradictory, and this study was performed to clarify this issue. Cloning of the membrane potential-generating citrate transporter (CitP) of Leuconostoc mesenteroides revealed an amino acid sequence that is almost identical to the known sequence of the CitP of Lactococcus lactis. The cloned gene was expressed in a Lactococcus lactis Cit− strain, and the gene product was functionally characterized in membrane vesicles. Uptake of citrate was counteracted by the membrane potential, and the transporter efficiently catalyzed heterologous citrate-lactate exchange. These properties are essential for generation of a membrane potential under physiological conditions and show that the Leuconostoc CitP retains its properties when it is embedded in the cytoplasmic membrane of Lactococcus lactis. Furthermore, using the same criteria and experimental approach, we demonstrated that the endogenous CitP of Lactococcus lactis has the same properties, showing that the few differences in the amino acid sequences of the CitPs of members of the two genera do not result in different catalytic mechanisms. The results strongly suggest that the energetics of citrate degradation in Lactococcus lactis and Leuconostoc mesenteroides are the same; i.e., citrate metabolism in Lactococcus lactis is a proton motive force-generating process.

Comparative analysis of mitochondrial citrate synthase gene structure, transcript level and enzymatic activity in acidless and acid-containing Citrus varieties

Abstract: Most citrus (Citrus L. spp.) fruits accumulate a considerable amount of citric acid in the vacuoles of the juice sac cells. As part of research aimed to understand the mechanism of acid accumulation, we compared the gene structures and transcript levels of citrate synthase in sour lemon (high acid, C. limon (L.) Burm.), ‘Shamouti’ orange (moderate acid, C. sinensis (L.) Osbeck) and sweet lime (acidless, C. limmetioides Tan.). Southern analyses suggested that a single gene for citrate synthase was present in the genomes of all three Citrus varieties. The gene structures seemed to be very similar, with minor differences in Shamouti orange. Overall, the transcript levels of citrate synthase were similar in sweet lime and sour lemon, and about 2-fold lower in Shamouti orange. The enzymatic activity of citrate synthase was compared between sour lemon and sweet lime. In sour lemon, the specific activity of the enzyme was induced early in fruit development and, in parallel with the increase in acid content, reached a maximal level, and did not diminish significantly towards fruit maturation; the pattern and level of activity detected during sweet lime fruit development were similar. These results suggest that the difference in acid accumulation between acidless and acid-containing fruits may not be attributed to changes in the activity of citrate synthase.