Substrate specificity of the brush border K+-leucine symport of Bombyx mori larval midgut

L-leucine uptake into membrane vesicles from Bombyx mori larval midgut was tested for inhibition by 55 compounds, which included sugars, N-methylated, alpha-, beta-, gamma-, delta-, epsilon-amino acids, primary amines, cc-ami no alcohols, monocarboxylic organic acids and alpha-ketoacids, Based on cis -inhibition experiments performed at the high pH (10.8) characteristic of t he midgut luminal content in vivo, we find that the carrier binding site in teracts with molecules which possess a well-defined set of structural featu res. Amino acids are preferentially accepted as anions and the ideal inhibi tor must have an hydrophobic region and a polar head constituted by a chira l carbon atom bearing two hydrophilic groups, a deprotonated amino-group an d a dissociated carboxylic group. Binding is reduced if one of the two hydr ophilic groups is removed. Lowering the pH to less alkaline value (8.8) onl y affects the affinity of delta- and epsilon-amino acids, which are exclude d from binding because of their positively charged side-chain. Modification s of the potassium electrochemical gradient increased the affinity constant values of the molecules, but have little effect on the rank of specificity . Physiological implications of the data reported are discussed. (C) 2000 E lsevier Science Ltd. All rights reserved.