Cellular metabolism of glucose is required for stimulation of insulin secretion from pancreatic β cells, but the precise metabolic coupling factors involved in this process are not known. In an effort to better understand mechanisms of fuel-mediated insulin secretion, we have adapted ¹³ C NMR and isotopomer methods to measure influx of metabolic fuels into the tricarboxylic acid (TCA) cycle in insulinoma cells. Mitochondrial metabolism of [U- ¹³ C ₃ ]pyruvate, derived from [U- ¹³ C ₆ ]glucose, was compared in four clonal rat insulinoma cell 1-derived cell lines with varying degrees of glucose responsiveness. A ¹³ C isotopomer analysis of glutamate isolated from these cells showed that the fraction of acetyl-CoA derived from [U- ¹³ C ₆ ]glucose was the same in all four cell lines (44 ± 5%, 70 ± 3%, and 84 ± 4% with 3, 6, or 12 mM glucose, respectively). The ¹³ C NMR spectra also demonstrated the existence of two compartmental pools of pyruvate, one that exchanges with TCA cycle intermediates and a second pool derived from [U- ¹³ C ₆ ]glucose that feeds acetyl-CoA into the TCA cycle. The ¹³ C NMR spectra were consistent with a metabolic model where the two pyruvate pools do not randomly mix. Flux between the mitochondrial intermediates and the first pool of pyruvate (pyruvate cycling) varied in proportion to glucose responsiveness in the four cell lines. Furthermore, stimulation of pyruvate cycling with dimethylmalate or its inhibition with phenylacetic acid led to proportional changes in insulin secretion. These findings indicate that exchange of pyruvate with TCA cycle intermediates, rather than oxidation of pyruvate via acetyl-CoA, correlates with glucose-stimulated insulin secretion.

During continuous cultivation of Yarrowia lipolytica N 1, oxygen requirements for growth and citric acid synthesis were found to depend on the iron concentration in the medium. A coupled effect of oxygen and iron concentrations on the functioning of the mitochondrial electron transport chain in Y. lipolytica N 1 was established. Based on the results obtained in continuous culture, conditions for citric acid production in a batch culture of Y. lipolytica N 1 were proposed. At relatively low pO(2) value and a high iron concentration, citric acid accumulation was as high as 120 g l(-1); the specific rate of citric acid synthesis reached 120 mg citric acid (g cells h)(-1). The mass yield coefficient was 0.87 and the energy yield coefficient was 0.31.

Current Review article summarizes the developments in citric acid production technologies in East and West last 100 years. Citric acid is commercially produced by large scale fermentation mostly using selected fungal or yeast strains in aerobe bioreactors and still remains one of the runners in industrial production of biotechnological bulk metabolites obtained by microbial fermentation since about 100 years, reflecting the historical development of modern biotechnology and fermentation process technology in East and West. Citric acid fermentation was first found as a fungal product in cultures of Penicillium glaucum on sugar medium by Wehmer in 1893 [1]. Citric acid is an important multifunctional organic acid with a broad range of versatile uses in household and industrial applications that has been produced industrially since the beginning of 20th century. There is a great worldwide demand for citric acid consumption due to its low toxicity, mainly being used as acidulant in pharmaceutical and food industries. Global citric acid production has reached 1.4 million tones, increasing annually at 3.5-4.0% in demand and consumption. Citric acid production by fungal submerged fermentation is still dominating, however new perspectives like solid-state processes or continuous yeast processes can be attractive for producers to stand in todays strong competition in industry. Further perspectives aiming in the improvement of citric acid production are the improvement of citric acid producing strains by classical and modern mutagenesis and selection as well as downstream processes. Many inexpensive by-products and residues of the agro-industry (e.g. molasses, glycerin etc.) can be economically utilized as substrates in the production of citric acid, especially in solid-state fermentation, enormously reducing production costs and minimizing environmental problems. Alternatively, continuous processes utilizing yeasts which reach 200-250 g/l citric acid can stand in todays strong competition in citric acid industry and replace the traditional discontinuous fungi processes.