Transmembrane exchange of fluorosugars: characterization of red cell GLUT1 kinetics using ¹⁹F NMR

Abstract

We developed a novel approach for quantifying the equilibrium-exchange kinetics of carrier-mediated transmembrane transport of fluorinated substrates. Our method is based on an adapted kinetic theory describing the concentration dependence of the transmembrane-exchange rates of two simultaneously transported species. Using the new approach, we quantified the kinetics of membrane transport of both anomers of three monofluorinated glucose analogues in human erythrocytes (red blood cells: RBCs) using 19 F nuclear magnetic resonance (NMR) exchange spectroscopy (EXSY).An inosine-based glucose-free medium was shown to promote survival and stable metabolism of RBCs over the duration of the experiments (a few hours). Earlier NMR studies only yielded the apparent rate constants and transmembrane fluxes of the anomeric species, whereas we were able to categorize the two anomers in terms of the catalytic activity (specificity constants) of the glucose transport protein GLUT1 towards them. Differences in the membrane permeability of the three glucose analogues were qualitatively interpreted in terms of local perturbations in the bonding of substrates to amino-acid residues in the active site of GLUT1. The methodology of this work will be applicable to studies of other carrier-mediated membrane transport processes, especially those with competition between simultaneously transported species. The GLUT1-specific results will apply to the design of probes of glucose transport, or inhibitors of glucose metabolism in cells including those exhibiting the Warburg effect.

Structure, function and regulation of mammalian glucose transporters of the SLC2 family

“…Studies on C1-fluoro glucoses (which do not interconvert) suggest that the β anomer binds more strongly than the α isomer [ 14 ] and is transported more rapidly by GLUT1 [ 135 ]. However, additional experiments in which glucose anomer mixtures are studied [ 130 , 189 ] suggest that the α anomer is preferred. These experiments are complicated by rapid interconversion of α and β glucose anomers and by hetero exchanges (αβ and βα, exchanges).…”

“…The latter dominates the transport kinetics and particularly the maximum rate of transport, which is the sum of the maximum exchange rates for α + β, no matter which anomer is being followed [ 170 ]. However, the rate of transport at low substrate concentrations (or the ratio of apparent K m over apparent V max ) is clearly faster for the α anomer [ 189 ]. Furthermore, in the crystal structure of GLUT3, both α and β glucose are present but α glucose is more abundant [ 55 ].…”