Substrate specificity of the intestinal brush-border proline/sodium (IMINO) transporter

L-proline uptake via the intestinal brush-border IMINO carrier was tested for inhibition by 41 compounds which included sugars, N-methylated, alpha-, beta-, gamma- and epsilon- amino and imino acids, and heterocyclic analogs of pyrrolidine, piperidine and pyridine. Based on competitive inhibitor constants (apparent Ki' 's) we find that the IMINO carrier binding site interacts with molecules which possess a well-defined set of structural prerequisites. The ideal inhibitor must 1) be a heterocyclic nitrogen ring, 2) have a hydrophobic region, 3) be the L-stereoisomer of 4) an electronegative carbonyl group which is 5) separated by a one-carbon atom spacer from 6) an electropositive tetrahedral imino nitrogen with two H atoms. Finally, 7) the inhibitor conformation determined by dynamic ring puckering must position all these features within a critical domain. The two best inhibitors are L-pipecolate (apparent Ki' 0.2 mM) and L-proline (apparent Ki' 0.3 mM).