Transport of .alpha.- and .beta.-D-glucose by the intact human red cell

Abstract

The kinetics of .alpha.- and .beta.-D-glucose mutarotation and transport of these anomers by intact human red cells were determined at 0.6 and 36.6.degree. C. The mutarotation coefficients for .alpha.- .beta.-D-glucose in cell-free tris(hydroxymethyl)aminomethane medium (pH 7.4) at 0.6.degree. C are (2.25 .+-. 0.2) and (1.73 .+-. 0.42) .RTM. 10-3 min-1, respectively), and at 36.3.degree. C are (69 .+-. 12) and (75 .+-. 5) .RTM. 10-3 min-1, respectively. These values are in good agreement with previous estimates. AT 0.6.degree. C, the red cell contains no detectable mutarotase activity. Initial rates of sugar uptake were measured by using radiolabeled D-glucose and time courses of uptake by turbidimetry. The time courses of .alpha.- and .beta.-D-glucose and an equilibrium mixture of .alpha.- and .beta.-D-glucose inifite-cis entry are identical at 0.66.degree. C (n = 41) where negligible mutarotation is observed. The apparent Ki values for inhibition of radiolabeled D-glucose initial uptake by unlabeled .alpha.- or .beta.-D-glucose at 0.6.degree. C are identical (1.6 mM). The calculated Vmax paramteres for uptake of the radiolabeled anomers at this temperatures are also distinguishable. The time courses of inifinite-cis .alpha.- and .beta.-D-glucose uptake at 36.66.degree. C are identical(n = 40). While D-glucose mutarotation is more rapid at this temperature, the anomers of D-glucose are not transported differently by the red cell hexose transfer system. These findings confirm the suitability of the use of the integrated rate analysis for infinite-cis entry kinetics and support rejection of the symmetric and asymmetric carrier models for red cell sugar transport.