Previous studies have revealed two different kinds of regulation of glucose utilization in cell lines derived from the nervous system (Keller et al., 1981). We found glucose metabolism of C-6 glioma cells to be limited and regulated by membrane transport. In contrast, glucose utilization of C-1300 neuroblastoma (N2A) cells was limited by the known regulatory enzymes of the Embden-Meyerhof pathway. Under the given experimental conditions the "membrane-limited" C-6 glioma cells were characterized by periodically changing glucose transport rates and very low intracellular glucose concentrations, which remained constant in spite of widely differing transport rates. These findings suggest the close functional coupling between transport and phosphorylation required for the regulation of glucose transport by cellular metabolic needs.

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Neuroblastoma cells (N1E-115) and glial cells possess aminopeptidases, dipeptidyl carboxypeptidases and dipeptidyl aminopeptidases, which are located in the plasma membranes. Neuronal cells possess angiotensin-converting enzyme (ACE), which splits the Gly-Phe bond and generates Tyr-Gly-Gly from enkephalin. They have no enkephalinase A. In contrast, glial cells possess enkephalinase A but no ACE. Not only the dipeptidyl carboxypeptidases on neuronal and glial cells are different. These cells have aminopeptidases which have to be distinguished as well. This differentiation is made by using bestatin as an aminopeptidase inhibitor and reveals peptidases of high and poor bestatin-sensitivity.

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