Separation of two distinct Na+/D-glucose cotransport systems in the human fetal jejunum by means of their differential specificity for 3-O-methylglucose.

Based on kinetic arguments, we have recently proposed the existence of two distinct Na+/D-glucose cotransporters in brush-border membrane vesicles isolated from the human fetal jejunum (Biochim. Biophys. Acta 938 (1988) 181-188). In order to further test this hypothesis, inhibition studies of the zero-trans influx of substrate have been performed under Na(+)-gradient and voltage-clamped conditions. Initial rates of D-glucose uptake were totally abolished by D-glucose, D-galactose, alpha-methylglucose and phlorizin while 3-O-methylglucose and phloretin induced only a 65% inhibition even at the highest concentrations used. The residual activity of D-glucose uptake is thus compatible with substrate flux through a low-affinity transport system which is insensitive to phloretin and does not accept 3-O-methylglucose as substrate. This substrate specificity has been used to separate kinetically the two putative pathways for glucose transport. The data obtained are compatible with the existence of the following two systems: (i) a low-affinity, high-capacity system with a Km of 4.7 mM and a Vmax of 22 nmol/min per mg of protein, and; (ii) a high-affinity, low-capacity system with a Km of 0.57 mM and a Vmax of 10.7 nmol/min per mg of protein. These data thus demonstrate clearly the existence of two distinct Na(+)-dependent D-glucose carriers in the human jejunum during the early gestation period since these systems can be differentiated not only by their kinetic properties but also by their differences in both substrate and inhibitor specificities.