Metabolism of 3-13C-Malate in Primary Cultures of Mouse Astrocytes

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Abstract
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.