title: The comparative specificity of the inner and outer substrate transfer sites in the choline carrier of human erythrocytes.
abstract: The substrate specificities on the inner and outer surfaces of the cell membrane have been compared by determining the relative affinities, inside and outside, of a series of choline analogs. The results of two different methods were in agreement: (1) the carrier distribution was determined in the presence of a saturating concentration of an equilibrated analog, using N-ethylmaleimide as a probe for the inward-facing carrier; (2) the degree of competition was measured between an equilibrated analog and choline in the external solution. The carrier sites are found to have markedly different specificities: the outer site is more closely complementary to the structure of choline than is the inner, and even a slight enlargement of either the trimethylammonium or hydroxyethyl group gives rise to preferential binding inside. It is also found that a nonpolar binding region, which is adjacent to the outer site, is absent from the inner site. As the transport mechanism involves the exposure of only one site at a time, first on one surface and then the other, it follows that an extensive reorganization of the structure of the substrate site may occur during the carrier-reorientation step, or alternatively that two distinct sites may be present, only one of which is exposed at a time.
mesh_terms: D000588:Amines / Q000097:blood; D001665:Binding Sites; D001667:Binding, Competitive; D001692:Biological Transport; D002463:Cell Membrane Permeability; D002794:Choline / Q000097:blood*; D004910:Erythrocyte Membrane / Q000187:drug effects / Q000378:metabolism*; D005033:Ethylmaleimide / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D008433:Mathematics; D008954:Models, Biological
chemical_list: D000588:Amines; D002794:Choline; D005033:Ethylmaleimide