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Evidence for an intermediate step in carrier-mediated sugar translocation across the brush border membrane of hamster small intestine

1969; Elsevier BV; Volume: 193; Issue: 1 Linguagem: Inglês

10.1016/0005-2736(69)90070-4

1879-2642

Wolfgang F. Caspary, Nancy R. Stevenson, Robert K. Crane,

Amino Acid Enzymes and Metabolism

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. 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. 6-Deoxy-l-galactose is a competitive inhibitor of the Na+-dependent sugar transport system with a Ki of about 20 mM. 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. 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 Km3-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. 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.