A novel regulatory mechanism for amino acid absorption in lepidopteran larval midgut

Abstract

A number of methyl and ethyl esters of naturally occurring amino acids exert a potent stimulatory effect on the cotransport system responsible for the absorption of most essential amino acids along the midgut of the silkworm Bombyx mori. l-Leucine methyl ester (Leu–OMe), one of the most effective activators, induces a large increase of the initial rate of leucine uptake in midgut brush border membrane vesicles (BBMV) from the anterior–middle (AM) region, and a small effect in BBMV from the posterior (P) region. Nonetheless, the methyl ester causes in both regions a relevant K +-, Δ ψ- and pH-independent increase of the intravesicular accumulation of the amino acid. The activation by Leu–OMe proves that amino acid absorption can be modulated all along the B. mori larval midgut and that the AM region, where the ability to transport and concentrate the substrate is very low, is more susceptible than the P region. Leucine uptake in AM-BMMV can be activated by amino acid methyl esters with definite structural requisites, with the following order of potency: l-leucine> l-phenylglycine> l-methionine> l-phenylalanine> l-norleucine≫ l-isoleucine. The activation is stereospecific and occurs also with some ethyl esters (e.g. leucine and phenylalanine). No activation was observed with esters of amino acids with short hydrophobic or polar side-chains. The activation mechanism here described plays a fundamental role in larval growth since silkworms reared on artificial diets supplemented with leucine or methionine methyl esters reach maximum body weight 12–18 h before control larvae and spin cocoons with a larger shell weight. This novel regulatory mechanism of an amino acid transport protein appears to be widespread among lepidopteran larvae.