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  • Title: The comparative specificity of the inner and outer substrate transfer sites in the choline carrier of human erythrocytes.
    Author: Deves R, Krupka RM.
    Journal: J Membr Biol; 1984; 80(1):71-80. PubMed ID: 6481794.
    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.
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