K(+)-neutral amino acid symport of Bombyx mori larval midgut: a system operative in extreme conditions.
The K(+)-dependent symporter for leucine and other neutral amino acids expressed along the midgut of the silkworm Bombyx mori operates with best efficiency in the presence of a steep pH gradient across the brush-border membrane, with external alkaline pH values up to 11, and an electrical potential difference (delta psi) of approximately 200 mV. Careful determinations of leucine kinetics as a function of external amino acid concentrations between 50 and 1,000 microM, performed with brush-border membrane vesicles (BBMV) obtained from the middle and posterior midgut regions, revealed that the kinetic parameter affected by the presence of a delta pH was the maximal rate of transport. The addition of delta psi caused a further marked increase of the translocation rate. At nonsaturating leucine concentrations in the solution bathing the external side of the brush-border membrane, leucine accumulation within BBMV and midgut cells was not only driven by the gradient of the driver cation K+ and delta psi but occurred also in the absence of K+. The ability of the symporter to translocate the substrate in its binary form allows the intracellular accumulation of leucine in the absence of K+, provided that a pH gradient, with alkaline outside, is present. The mechanisms involved in this accumulation are discussed.
Giordana
B
B
Department of Biology, University of Milan, Italy.
Leonardi
M G
MG
Casartelli
M
M
Consonni
P
P
Parenti
P
P
D000596:Amino Acids / Q000502:physiology*; D000818:Animals; D012831:Bombyx; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport / Q000502:physiology; D007700:Kinetics; D008564:Membrane Potentials; D011188:Potassium / Q000502:physiology*; D017494:Proton Pumps
D000596:Amino Acids; D017494:Proton Pumps; D011188:Potassium