Contribution of glucose transport to the control of the glycolytic flux inTrypanosoma brucei

The rate of glucose transport across the plasma membrane of the bloodstream form of Trypanosoma brucei was modulated by titration of the hexose transp orter with the inhibitor phloretin, and the effect on the glycolytic flux w as measured. A rapid glucose uptake assay was developed to measure the tran sport activity independently of the glycolytic flux. Phloretin proved a com petitive inhibitor. When the effect of the intracellular glucose concentrat ion on the inhibition was taken into account, the flux control coefficient of the glucose transporter was between 0.3 and 0.5 at 5 mM glucose. Because the flux control coefficients of all steps in a metabolic pathway sum to 1 , this result proves that glucose transport is not the rate-limiting step o f trypanosome glycolysis. Under physiological conditions, transport shares the control with other steps. At glucose concentrations much lower than phy siological, the glucose carrier assumed all control, in close agreement wit h model predictions.