title: Topographical similarities between harmaline inhibition sites on Na+-dependent amino acid transport system ASC in human erythrocytes and Na+-independent system asc in horse erythrocytes.
abstract: Na+-dependent system ASC and Na+-independent system asc are characterized by a common selectivity for neutral amino acids of intermediate size such as L-alanine and by their interactions with dibasic amino acids. For system ASC, the positive charge on the dibasic amino acid side chain is considered to occupy the Na+-binding site on the transporter. We report here the use of harmaline (a Na+-site inhibitor in some systems) as a probe of possible structural homology between these two classes of amino acid transporter. Harmaline was found to inhibit human erythrocyte system ASC noncompetitively with respect to L-alanine concentration, but approximated competitive inhibition with respect to Na+ concentration (apparent Ki = 2.0 and 0.9 mM, respectively). Similarly, harmaline noncompetitively inhibited L-alanine uptake by horse erythrocyte systems asc1 and asc2 (apparent Ki = 2.0 and 1.9 mM, respectively). In contrast, harmaline functioned as a competitive inhibitor of L-lysine uptake by system asc1 (apparent Ki = 2.6 mM). It is concluded that harmaline competes with Na+ for binding to system ASC and that a topographically similar harmaline inhibition site is present on system asc. This site does not however bind Na+, the asc1 transporter exhibiting normal L-alanine and L-lysine influx kinetics in the total absence of extracellular cations.
mesh_terms: D000409:Alanine / Q000097:blood; D000470:Alkaloids / Q000494:pharmacology*; D000596:Amino Acids / Q000097:blood*; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D004912:Erythrocytes / Q000187:drug effects / Q000378:metabolism*; D006246:Harmaline / Q000494:pharmacology*; D006736:Horses; D006801:Humans; D007700:Kinetics; D008239:Lysine / Q000097:blood; D012964:Sodium / Q000494:pharmacology*; D013045:Species Specificity
chemical_list: D000470:Alkaloids; D000596:Amino Acids; D012964:Sodium; D006246:Harmaline; D008239:Lysine; D000409:Alanine