Cerebral glucose-6-phosphatase and the movement of 2-deoxy-D-glucose across cell membranes

Glucose-6-phosphatase (glucose-6-phosphohydrolase and its associated phosphotransferase activities) was determined in brain tissue and in several preparations derived from brain tissue. These included purified capillaries and established cell lines of neuronal or glial origin. Since it has been suggested that glucose-6-phosphatase may be involved in sugar transport, the characteristics of that process were examined in these preparations. The pattern of uptake of 2-deoxy-D-glucose in four cell lines was shown to involve transport of the analog across the cell membrane that was more rapid than the subsequent phosphorylation of the sugar in the intracellular compartment. In the remaining cell lines and in purified capillaries, phosphorylation of 2-deoxy-D-glucose was at least as rapid as uptake. No differences could be found between the cells in these two categories with respect to amount or localization of glucose-6-phosphatase, ability to phosphorylate 3-O-methyl-D-glucose, or ability to phosphorylate extracellular and intracellular 2-deoxy-D-glucose. In the course of these experiments, it was found that there was a rapid efflux of 2-deoxy-D-glucose from cells that had taken up this sugar. The efflux involves a dephosphorylation step catalyzed by intracellular phosphatase that releases free sugar in the cytoplasm. Glucose-6-phosphatase thus probably has no major role in the phosphorylation of glucose in brain cells, but acts in the more conventional sense, i.e. as a phosphohydrolase.