Leucine methyl ester is a powerful allosteric activator of the neutral amino acid cotransport system in Bombyx mori larval midgut

We have identified three methyl esters that have a potent stimulatory effect on the cotransport system responsible for the absorption of most essential amino acids in the silkworm Bombyx mori. L-Leucine methyl ester, the most powerful activator, determined a large dose-dependent, K+-independent increase of leucine uptake into midgut brush border membrane vesicles. Kinetic experiments revealed non-essential mixed-type activation, with Ka values of 27±2 and 47±8 μM in the presence and in the absence of K+, respectively. The activation increased Km twofold, and Vmax up to 18-fold depending upon the experimental conditions. Leucine uptake mediated by the amino acid uniport appears to be unaffected by the activator.