The “dimeriser” hypothesis for sugar permeation through red cell membrane: Reinvestigation of original evidence

The original evidence for the operation of a “dimeriser” site in the human erythrocyte surface, in connection with facilitation of the passage of simple sugars through the cell membrane, has been experimentally re-examined and considerably extended. The course of total sugar uptake was followed photometrically, in mixtures of 21 pairs among nine monosaccharides, as a function of the varying proportions of the two sugars at a constant total level. The observed behavior was found to adhere closely to the quantitative predictions computed by numerical integration of rate equations derived from the simple monovalent carrier model proposed by Widdas, on the basis of previously estimated affinity constants for the various sugars, and a common rate constant for all.

The most crucial of Stein's dissonant conclusions from similar experiments, involving a systematic deviation of the initial cell swelling rates from this pattern, are shown to be accountable by errors inherent in the methods by which these data were analyzed. It is concluded that, although this type of experimental approach does not definitively exclude the possibility that the transport sites may be divalent for the substrate sugars, it provides no basis whatever for advancing this hypothesis.