Evidence from studies of temperature-dependent changes of d-glucose, d-mannose and l-sorbose permeability that different states of activation of the human erythrocyte hexose transporter exist for good and bad substrates

Abstract

(1) The inhibition constant of l-sorbose flux from fresh human erythrocytes by d-glucose, K i(sorbose) increases on cooling from 50°C to 30°C from 5.15±0.89 mM to 12.24±1.9 mM; the K i(sorbose) of d-mannose increases similarly, indicating that the process is endothermic. (2) The activation energy E a(sorbose) of net l-sorbose exit is 62.9±3.1 kJ/mol; in the co-presence of 5 mM d-glucose E a(sorbose) is reduced to 41.7±1.6 kJ/mol ( P<0.005). (3) Cooling from 35°C to 21°C decreases the K i(inf. cis) of auto-inhibition of d-glucose net exit from 5.2±0.3 mM to 1.36±0.06 mM; the K i(inf. cis) of d-mannose falls from 10.9±1.65 mM to 5.7±0.3 mM. (4) The activation energy of d-glucose zero-trans net exit is 34.7±2.1 kJ/mol and that of d-mannose exit is 69.4±3.7 kJ/mol ( P<0.0025). (5) The exothermic and exergonic processes of auto-inhibition of d-glucose net exit are larger than those for d-mannose ( P<0.03). These data are consistent with d-glucose binding promoting an activated transporter state which following dissociation transiently remains; if an l-sorbose molecule binds within the relaxation time after d-glucose dissociation, it will have a higher mobility than otherwise. Cooling slows the relaxation time of the activated state hence raises the probability that l-sorbose will bind to the glucose-activated transporter. d-Glucose donates twice as much energy to the transporter as d-mannose, consequently produces more facilitation of flux. This view is inconsistent with the alternating carrier model of sugar transport in which net flux is considered to be rate-limited by return of the empty carrier, but is consistent with fixed two-site models.