EVIDENCE FOR AN INTERMEDIATE STEP IN CARRIER-MEDIATED SUGAR TRANSLOCATION ACROSS BRUSH BORDER MEMBRANE OF HAMSTER SMALL INTESTINE

The following observations have been made about the interactions of 6-deoxy-l-galactose with the Na+-dependent sugar transport system of hamster small intestine. 1. 1. 6-Deoxy-l-galactose does not appreciably enter the intracellular spaces of incubated intestinal segments and phlorizin is devoid of action on this minimal entry. 2. 2. 6-Deoxy-l-galactose is a competitive inhibitor of the Na+-dependent sugar transport system with a Ki of about 20 mM. 3. 3. 6-Deoxy-l-galactose does not elicit counterflow of a known substrate, l-glucose, under conditions where counterflow is elicited by a substrate, 1,5-anhydro-d-glucitol, of approximately equivalent affinity. 4. 4. The i of 6-deoxy-l-galactose increases with the reduction in Na+ concentration as does the Km of 3-O-methylglucose. Consequently, the ratio Km 3-O-methylglucose/Ki 6-deoxy-l-galactose remains constant over a 5-fold range of Na+ concentration; that is, the competitive interaction of 6-deoxy-l-galactose is Na- dependent. 5. 5. 6-Deoxy-l-galactose acts like mannitol in transmural potential studies; that is, it induces a streaming potential only. It does not induce Na+ movement despite the fact that its carrier interaction is Na+-dependent. From these observations the inference is drawn that mobility of the carrier-substrate complex which results in translocation of a substrate involves a non-covalently bonding transformation of the complex; a transformation which cannot take place when 6-deoxy-l-galactose is bound to the carrier, 6-Deoxy-l-galactose forms an abortive complex. © 1969.