SIGNIFICANT AMOUNTS OF GLYCOGEN ARE SYNTHESIZED FROM 3-CARBON COMPOUNDS IN ASTROGLIAL PRIMARY CULTURES FROM MICE WITH PARTICIPATION OF THE MITOCHONDRIAL PHOSPHOENOLPYRUVATE CARBOXYKINASE ISOENZYME

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 glucon eogenic enzymes, phosphoenolpyruvate carboxykinase. Only the mitochond rial isoenzyme of phosphoenolpyruvate carboxykinase was detectable in the astroglial primary cultures. After the incubation of glucose-starv ed cells with medium containing a mixture of [6-H-3]glucose and [U-C-1 4]glucose, the newly synthesized glycogen showed a H-3/C-14 ratio whic h was approximately 15% less than the isotope ratio for the medium. Th e decrease of the isotope ratio was not significantly inhibited by 3-m ercaptopicolinate, indicating a cycling of approximately 15% of the gl ucose to the level of the triose phosphates before its incorporation i nto astroglial glycogen. During the initial phase of glycogen resynthe sis, the contribution of the gluconeogenic substrates appeared to be h igher. This was in agreement with the accumulation of fructose 2,6-bis phosphate during refeeding. A participation of gluconeogenic substrate s in glycogen metabolism was also detectable when the glycogen content was not changing significantly.