INTESTINAL SUGAR TRANSPORT - IONIC ACTIVATION AND CHEMICAL SPECIFICITY

The mode of absorption of several monosaccharides was studied in vitro with segments and everted sacs of hamster small intestine. 1. 1. Na+ reduced the apparent Km of transport of the actively transported d-galactose, 3-methyl-d-glucose, α-methyl-d-glucoside, d-xylose and l-glucose. The 'nontransported' d-arabinose, l-arabinose, l-rhamnose, l-mannose and l-fucose showed Michaelis-Menten kinetics, but their Km's were not significantly altered by Na+. The νmax of all the sugars tested was identical and independent of Na+. Inhibition of entry of nontransported sugars by d-galactose and by phlorizin was demonstrated. 2. 2. By the criteria of Na+ dependence, inhibition by phlorizin and ouabain and inhibition by transported sugars, d-mannose and d-fructose were also shown to interact with the joint sugar carrier. The transport of d-mannose appears to be active, but rapid metabolism of the two sugars precluded a quantitative determination of transport parameters. 3. 3. The data suggest a dual specificity of a joint sugar carrier: In the absence of Na+ its affinity for many diverse sugars is low; the presence of Na+ increases the affinity for only some of these sugars. The potential for active transport depends on the extent of this activation by Na+ and varies with different sugars from negligible to very great. 4. 4. With low concentrations of sugar the asymmetry of transport increases towards a limiting value which depends on the ratio Km(Na+ free) Km(Na+). Active transport occurs with sugars where this ratio >3-5, provided their concentration is below a critical value which also parallels the Km ratio. © 1969.