Rate and affinity in human red blood cell sugar transport

Abstract

After addition of any of six penetrant aldoses to suspensions of washed human erythrocytes at body temperature, serial samples were taken for analysis of sugar distribution between cells and medium. The progress of equilibration was analyzed in terms of a simple carrier model previously proposed for monosaccharide transport in these cells, by which the sugar movements are characterizable by two constants, one of which defines the affinity of the sugar for the transport system's reactive site, the other expressing the rate of diffusion of the sugar-carrier complex through the cell membrane. The experiments show the latter constant to be nearly invariant among the several sugars, although the affinity constants and the net transfer times cover a wide range. This finding implies that the identity of the passenger sugar molecule does not seriously influence the diffusivity of the sugar-carrier complex. Certain contrary results recently published are questioned on the basis of an oversimplification in the formal analysis by which the data were interpreted.