R. Parthasarathy et al., SUBSTRATE STRUCTURE AND AMINO-ACID K-BORDER MEMBRANE-VESICLES FROM LARVAL MANDUCA-SEXTA MIDGUT( SYMPORT IN BRUSH), Journal of Experimental Biology, 197, 1994, pp. 237-250
The effects of amino acid sidechain length, substituent position and c
hirality on amino acid/K+ symport have been examined in rapid filtrati
on experiments on brush-border membrane Vesicles prepared from larval
Manduca sexta midgut. Cis-inhibition and trans-stimulation protocols w
ere used to examine the effects of amino acid analogs on the uptake of
alanine, phenylalanine, leucine and lysine, which are cotransported w
ith K+ by a zwitterionic symporter at the high pH characteristic of th
e 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 charg
e distribution unchanged do not affect symport. Thus, moving the methy
l group from C-3 to C-5 in the sequence isoleucine, leucine and norleu
cine has no effect on their ability to inhibit leucine symport. Increa
sing sidechain length among alanine homologs has little effect on thei
r ability to inhibit alanine uptake, but increasing the sidechain leng
th of lysine homologs from 1 to 3 methylene groups enhances cis-inhibi
tion and trans-stimulation of lysine symport. The substantial. differe
nce in molecular charge distribution among aminobutanoic acid isomers
has a large impact on alanine symport with only alpha- (or 2-) aminobu
tanoic acid functioning as an alanine analog. Only those changes in su
bstrate structure that are coupled to the molecular charge distributio
n seem to affect symport. The tolerance of the symporter may reflect a
balance mandated by the conflicting demands of selectivity and throug
hput.