Concerted Hexose Transport Curb by Tunicamycin is Rendered Irreversible by Glucose or Allose in Medium Containing L-glutamine.

The hexose transport in a hamster fibroblast mutant (DS7), unable to use glucose for generation of energy, is nevertheless subject to a marked down-regulation ("curb") after prolonged incubation of monolayer cultures with glucose; fructose is unable to exert any curb. D-Allose, an all-cis hexose, mediates a vigorous curb of the transport system. Moreover, prolonged coincubation of glucose or allose with tunicamycin (TM) brings about an additional effect that is not an inhibition of the transport system, which we shall call the "concerted" transport curb. This type of concerted transport curb requires L-glutamine in the maintenance medium; moreover, addition of cycloheximide prevents the development of this TM effect. Apparently, cellular protein synthesis or protein turnover or both are required for the development of the TM-concerted transport curb. The concerted transport curb can be reversed in sugar-free or in fructose-containing medium, even upon readdition of TM. In contrast, the sole readdition of glucose or D-allose renders the concerted curb irreversible. This raises the question of whether the cells under the condition of the concerted curb somehow have internalized the TM into the membrane.