title: Reaction of internal forms of the choline carrier of erythrocytes with N-ethylmaleimide: evidence for a carrier conformational change on complex formation.
abstract: The choline carrier of human erythrocyte membranes exists in distinguishable outward-facing and inward-facing conformations, and previous studies demonstrated that only the latter reacts with N-ethylmaleimide, producing an irreversible inhibition of transport. We now report experiments to determine the individual reaction rates for the two inward-facing forms: the free carrier and the complex. The pseudo-first-order rate constant for the complex with a substrate analog, di-n-butylaminoethanol, is found to be nearly double that for the free carrier, showing that the carrier conformation is altered following addition of a ligand (with 1 mM N-ethylmaleimide at pH 6.8, 37 degree C, the constants are 0.57 +/- 0.05 min-1 and 0.33 +/- 0.02 min-1, respectively). Hence three different conformational states have been distinguished by experiment: (1) the inward-facing free carrier; (2) the inward-facing complex; and (3) the outward-facing carrier.
mesh_terms: D002794:Choline / Q000097:blood*; D004910:Erythrocyte Membrane / Q000187:drug effects / Q000378:metabolism*; D004912:Erythrocytes / Q000378:metabolism*; D005033:Ethylmaleimide / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D008433:Mathematics; D008954:Models, Biological
chemical_list: D002794:Choline; D005033:Ethylmaleimide