Transport of d-fructose and its analogues by Trypanosoma brucei

Abstract

Kinetic parameters for entry of d-fructose into Trypanosoma brucei brucei have been determined. The net uptake of d-fructose was found to be rapid and occurred at a rate which was comparable with that observed for uptake of d-glucose. The K m and max were 3.91 ± 1.58 mM and 69.1 ± 7.2 nmol min −1 (mg protein) −1. d-Fructose was metabolized to pyruvate under aerobic conditions and to pyruvate and glycerol under anaerobic conditions in a manner similar to d-glucose. Comparisons of the kinetic parameters for d-fructose transport and metabolism indicated that uptake was rate limiting. Inhibition constants ( K i) for inhibition of 6-deoxy- d-glucose by d-fructose and d-fructose transport by 6-deoxy- d-glucose were consistent with the K m values for these two substrates. These interactions indicate that d-fructose and 6-deoxy- d-glucose share a single common transporter. 1,5-Anhydro- d-glucitol and 1,5 anhydro- d-mannitol (the fused pyranose ring analogues of d-glucose and d-mannose) have been found to interact well with the transporter, while l-sorbose (a d-fructose analogue with a pyranose ring) had only low affinity. However, 2,5-anhydro- d-mannitol (a fused furanose ring analogue of d-fructose) inhibited both 6-deoxy- d-glucose and d-fructose transport with a K i of approx. 0.8 mM. The high affinity for 2,5-anhydro- d-mannitol (2-deoxy- d-fructofuranose) indicates that d-fructose is transported in the furanose ring form. This was directly demonstrated by measuring inhibition of 6-deoxy- d-glucose transport byβ- d-fructopyranose solutions during the approach to anomeric equilibrium withβ- d-fructofuranose. We therefore propose that the trypanosome transporter accommodates d-glucose as a pyranose ring and d-fructose as a furanose ring by binding to hydrogen bonds directed at the C-3, C-4 and ring oxygen positions and that a bulky group may be tolerated at C-2. The ring oxygen is however not essential, as 2 R,5 R-dihydroxymethyl-3 R4 R-dihydroxypyrollidine (a fructofuranose analogue with a ring nitrogen) was also accepted by the trypanosome transporter.