REGULATION OF FRUCTOSE-1,6-BISPHOSPHATASE ACTIVITY IN PRIMARY CULTURED ASTROCYTES

In the gluconeogenic pathway, fructose-1,6-bisphosphatase (EC 3. 1. 3. 11) is the last key-enzyme before the synthesis of glucose-6-phosphat e. The extreme diversity of cells present in the whole brain does not facilitate in vivo study of this enzyme and makes it difficult to unde rstand the regulatory mechanisms of the related carbohydrate metabolis m. It is for instance difficult to grasp the actual effect of ions lik e potassium, magnesium and manganese on the metabolic process just as it is difficult to grasp the effect of different pH values and the inf luence of glycogenic compounds such as methionine sulfoximine. The pre sent investigation attempts to study the expression and regulation of fructose-1,6-bisphosphatase in cultured astrocytes. Cerebral cortex of new-born rats was dissociated into single cells that were then plated . The cultured cells were flat and roughly polygonal and were positive ly immunostained by anti-glial fibrillary acidic protein antibodies. C ultured astrocytes are able to display the activity of fructose-1,6-bi sphosphatase. This activity was much higher than that in brain tissue in vivo. Fructose-1,6-bisphosphatase in cultured astrocytes did not re quire magnesium ions for its activity. The initial velocity observed w hen the activity was measured in standard conditions was largely incre ased when the enzyme was incubated with Mn2+. This increase was howeve r followed by a decrease in absorbance resulting in the induction, by the manganese ions, of a singular kinetics in the enzyme activity. Pot assium ions also stimulated fructose-1,6-bisphosphatase activity. When the enzyme was exposed to different pH values ranging from 6 to 9 uni ts, the highest activity was observed at pH 6. When the cultured astro cytes were incubated with methionine sulfoximine, the fructose-1,6-bis phosphatase activity increased. This increase was quick and depended o n the dose of methionine sulfoximine. These results show that cultured astrocytes are able to maintain fructose-1,6-bisphosphatase activity. With the exception of the higher level activity associated acidic pH ranges, the properties of the enzyme resemble those of the in vive enz yme. Methionine sulfoximine has a direct effect on astrocytes in its a ctivation of fructose-1,6-bisphosphatase. It is concluded that the exp ression and the regulation of fructose-1,6-bisphosphatase activity in cultured astrocytes look like those in the brain. Astrocytes are proba bly the principal cells that express this activity in the brain in viv o.