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Am J Physiol Regul Integr Comp Physiol 274: R1372-R1375, 1998;
0363-6119/98 $5.00
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Vol. 274, Issue 5, R1372-R1375, May 1998

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

M. G. Leonardi1, M. Casartelli1, P. Parenti2, and B. Giordana1

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


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