Evidence Against Necessary Phosphorylation During Hexose Transport in Aspergillus nidulans

The transport of 2-deoxy-d-glucose, a nonmetabolizable glucose analogue, into Aspergillus nidulans against a concentration gradient does not appear to require phosphorylation, despite the high levels of sugar phosphates accumulated rapidly within the cell. Two other deoxy analogues of d-glucose, 6-deoxy-d-glucose and 1,5-anhydro-d-glucitol (1-deoxy-d-glucose), although they lack the C-6 and the C-1 hydroxyl groups, respectively, and thus cannot be phosphorylated in those positions, still competitively inhibit the entry of 2-deoxy-d-glucose. Moreover, 6-deoxy-d-glucose can be concentrated against a gradient within the cell without the accumulation of 6-deoxy-d-glucose-phosphate. d-Galactose shows an intracellular ratio of free to phosphorylated sugar similar to that found for 2-deoxy-d-glucose in cells that have galactokinase, but no sugar phosphates are found in a galactokinaseless mutant strain. These data suggest that intracellular kinases are responsible for the sugar phosphate pool; and indeed, a kinase capable of phosphorylating 2-deoxy-d-glucose has been demonstrated. Finally, experiments on the kinetics of labeling of intracellular free sugar and sugar phosphate pools with (14)C-2-deoxy-d-glucose show that radioactivity appears first in the free sugar pool and after a delay enters the sugar phosphate pool.