High‐affinity phlorizin binding to brush border membranes from small intestine: Identity with (a part of) the glucose transport system, dependence on the Na+‐gradient, partial purification

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In the presence of an NaSCN gradient phlorizin binds with a high affinity (Kd similar or equal to 4.7 micron) to vesicles derived from brush border membranes of intestinal cells of rabbits. The value for Kd corresponds closely to that of Ki determined from phlorizin inhibition of sugar transport. The apparent affinity for phlorizin is decreased if NaCl is substituted for NaSCN and decreased substantially if the gradient of NaSCN is allowed to dissipate prior to the phlorizin binding. The number of high affinity binding sites is about 11 pmol/mg protein. Additional binding to low affinity sites can amount to as much as 600 pmol/mg protein after prolonged exposure to phlorizin (5 min.). The high affinity sites are related to glucose transport based on the similarity of the Kd and Ki values under a variety of conditions and on the inhibition of the binding by D-glucose but not by D-fructose. The transport system and the high affinity phlorizin binding sites can be enriched by a factor of 2-3 by treatment of vesicles with papain, which does not affect the transport system, but considerably hydrolyzes nonrelevant protein.