Evidence for a low-affinity, high-capacity uniport for amino acids in Bombyx mori larval midgut

• Published in: American journal of physiology. Regulatory, integrative and comparative physiology integrative and comparative physiology, 1998-05, Vol.274 (5), p.1372-R1375
• Main Authors: Leonardi, M. G, Casartelli, M, Parenti, P, Giordana, B
• Format: Article
• Language: English
• ISSN: 0363-6119; 1522-1490
• DOI: 10.1152/ajpregu.1998.274.5.r1372

Departments of 1  Biology and 2  General Physiology and Biochemistry, University of Milan, 20133 Milan, Italy We investigated the kinetics of leucine influx as a funtion of external substrate concentration between 0.03 and 16 mM in brush-border membrane vesicles (BBMV) prepared from the middle region of Bombyx mori larval midgut. A detailed kinetic analysis of leucine uptake led to the identification, in parallel with the K + -dependent symporter for neutral amino acids, of a K + -independent, low-affinity, high-capacity system. The parameter values of the Michaelis constant (7.12 mM) and maximal rate of transport (4.48 nmol · 7 s 1 · mg protein 1 ) were not influenced by an external alkaline pH nor by a transmembrane electrical potential difference. The uniporter is poorly specific, as it displayed the following rank of preference: Leu, His, Val, Ile, Phe, Ser > Lys, Arg, Gln > Pro, 2-amino-2-norbornane-carboxylic acid, Ala, Gly. The kinetic analysis performed in BBMV prepared from the posterior midgut portion indicates that the low-affinity, high-capacity uniporter is present along the entire length of the silkworm larval midgut with similar expression and functional properties. neutral amino acid uniporter; specificity; regional distribution