Malate, specifically labeled with carbon 13 on C3, was synthesized by chemical means and used to study malate metabolism by primary cultures of mouse cortical astrocytes. 3-13C-Malate in combination with glucose as well as 3-13C-malate alone were used as substrates; the effect of 3-nitropropionic acid, an inhibitor of succinate dehydrogenase and fumarase was also examined. The consumption of malate was only 0.26 μmol/mg of protein, approx. 25-fold lower than the consumption of glucose. Besides lactate, glutamine and fumarate were the two major metabolites released to the medium. Very low and similar levels of isotopic enrichment were detected on C2 and C3 of lactate; glutamine was labeled on C2 and C3 to a similar extent as well and labeling on C4 was only detected when glucose was not added. These labeling studies suggest that cytosolic malic enzyme is not active in primary astrocytes and support the occurrence of pyruvate recycling in astrocytes.
Triazines and phenylureas, mainly used in agricultural applications for the selective control of germinating grasses and broad-leaved weeds, are often found in contaminated groundwater, surface water and the effluents of wastewater treatment plants. The toxicity of these herbicides in eukaryotic cells is poorly understood. Saccharomyces cerevisiae is a promising unicellular organism for the toxicological evaluation of xenobiotics because its metabolism is similar to that of higher-level organisms. Consequently, the aim of this study was to compare the effects of three herbicides on yeast-cell growth and the glutathione cycle. Saccharomyces cerevisiae grown in the presence of 5 or 50 μM atrazine, diuron or isoproturon were compared with control cells grown in a rich medium. The results show that the glutathione-dependent buffer capacity decreased significantly in S. cerevisiae grown in the presence of both levels of any of the three herbicides, except in cells exposed to 50 μM isoproturon. In addition, chlorine herbicides inhibited cell growth, probably due to a decrease in antioxidant power and glutathione reductase activity. Isoproturon at 50 μM induced yeast-cell growth, increasing the buffer capacity mediated by glutathione as well as glutathione reductase and glutathione peroxidase activities of UE-ME3 strain. This strain may be useful in studies of isoproturon degradation.
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