Renal sugar transport in the winter flounder. II. Galactose transport system

The structural specificity of the transport of 0.5 mM D-galactose and 2-deoxy-D-galactose by teased renal tubules of the winter flounder Pseudopleuronectes americanus was investigated. The sugar uptake reflects preponderantly transport at the antiluminal membrane of tubular cells. Both sugars compete for the transport sites, indicating the sharing of a common carrier. 1) The structural requirements for the hexose-carrier interaction were defined by an inhibiton analysis using 12 structurally analogous sugars (5 mM): a (pyranose) ring structure; a free hydroxyl on C1; free hydroxyls (equatorial configuration) on C3 and C4; an oxygen on C6. Close packing in the vicinity of C2 is indicated. 2) Consonant with the requirement of a free C1-OH, beta-methyl-D-galactoside is not transported 3) Increasing concentrations (0.05-0.5 mM) of phlorizin and phloretin inhibit sugar uptake by lowering cellular levels of phosphorylated hexoses, whereas the levels of free sugars are not depressed. The possibility of an interaction of the sugar at C1-0H with a phosphorylated group at the carrier as the first step in the translocation process of both hexoses across the basal membrane is raised.