Substrate structure and amino acid/K+ symport in brush-border membrane vesicles from larval Manduca sexta midgut

The effects of amino acid sidechain length, substituent position and c chirality on amino acid/K+ symport have been examined in rapid filtration experiments on brush-border membrane vesicles prepared from larval Manduca sexta midgut. Cis-inhibition and trans-stimulation protocols were used to examine the effects of amino acid analogs on the uptake of alanine, phenylalanine, leucine and lysine, which are cotransported with K+ by a zwitterionic symporter at the high pH characteristic of the midgut in vivo. The symporter was found to translocate both L- and D-stereoisomers of alanine, leucine and lysine, but only the L-form of phenylalanine. Alterations to substrate structure that leave the charge distribution unchanged do not affect symport. Thus, moving the methyl group from C-3 to C-5 in the sequence isoleucine, leucine and norleucine has no effect on their ability to inhibit leucine symport. Increasing sidechain length among alanine homologs has little effect on their ability to inhibit alanine uptake, but increasing the sidechain length of lysine homologs from 1 to 3 methylene groups enhances cis-inhibition and trans-stimulation of lysine symport. The substantial difference in molecular charge distribution among aminobutanoic acid isomers has a large impact on alanine symport with only alpha- (or 2-) aminobutanoic acid functioning as an alanine analog. Only those changes in substrate structure that are coupled to the molecular charge distribution seem to affect symport. The tolerance of the symporter may reflect a balance mandated by the conflicting demands of selectivity and throughput.