Transport of 3-O-methyl d-glucose and β-methyl d-glucoside by rabbit ileum (original) (raw)

1976, Biochimica Et Biophysica Acta - Biomembranes

The intestinal transport of three actively transported sugars has been studied in order to determine mechanistic features that, (a) can be attributed to stereospecific affinity and (b) are common. The apparent affinity constants at the brush-border indicate that sugars are selected in the order, fl-methyl glucose > D-galactose > 3-O-meth.yl glucose, (the K m values are 1.23, 5.0 and 18.1 raM, respectively.) At low substrate concentrations the K t values for Na + activation of sugar entry across the brush-border are: 27, 25, and 140 mequiv, for fi-methyl glucose, galactose and 3-O-methyl glucose, respectively. These kinetic parameters suggest that Na +, water, sugar and membrane-binding groups are all factors which determine selective affinity. In spite of these differences in operational affinity, all three sugars show a reciprocal change in brush-border entry and exit permeability as Ringer [Na] or [sugar] is increased. Estimates of the changes in convective velocity and in the diffusive velocity when the sugar concentration in the Ringer is raised reveal that with all three sugars, the fractional reduction in convective velocity is approximately equal to the (reduction of diffusive velocity) 2. This is consistent with the view that the sugars move via pores in the brush-border by convective diffusion. Theophylline reduces the serosal border permeability to/#methyl glucose and to 3-O-methyl glucose relatively by the same extent and consequently, increases the intracellular accumulation of these sugars. The permeability of the serosal border to fl-methyl glucose entry is lower than permeability of the serosal border to fi-methyl glucose exit, which suggests that fl-methyl glucose may be convected out of the cell across the lateral serosal borde,'.