Significant Amounts of Glycogen are Synthesized from 3‐Carbon Compounds in Astroglial Primary Cultures from Mice with Participation of the Mitochondrial Phosphoenolpyruvate Carboxykinase Isoenzyme

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Abstract
The incorporation was studied of the gluconeogenic substrates lactate, alanine, aspartate and glutamate into glycogen of astroglial primary cultures derived from mouse brain. The incorporation was inhibited by 3‐mercaptopicolinate, an inhibitor of one of the characteristic gluconeogenic enzymes, phosphoenolpyruvate carboxykinase. Only the mitochondrial isoenzyme of phosphoenolpyruvate carboxykinase was detectable in the astroglial primary cultures. After the incubation of glucose‐starved cells with medium containing a mixture of [6‐3H]glucose and [U‐14C]glucose, the newly synthesized glycogen showed a 3H/14C ratio which was approximately 15% less than the isotope ratio for the medium. The decrease of the isotope ratio was not significantly inhibited by 3‐mercaptopicolinate, indicating a cycling of approximately 15% of the glucose to the level of the triose phosphates before its incorporation into astroglial glycogen. During the initial phase of glycogen resynthesis, the contribution of the gluconeogenic substrates appeared to be higher. This was in agreement with the accumulation of fructose 2,6‐bisphosphate during refeeding. A participation of gluconeogenic substrates in glycogen metabolism was also detectable when the glycogen content was not changing significantly.