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Leucine transport in membrane vesicles from Chironomus riparius larvae displays a mélange of crown‐group features
Author(s) -
Parenti Paolo,
Forcella Matilde,
Pugliese Anna,
Giacchini Roberto,
Rossaro Bruno,
Hanozet Giorgio M.
Publication year - 2001
Publication title -
archives of insect biochemistry and physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.576
H-Index - 66
eISSN - 1520-6327
pISSN - 0739-4462
DOI - 10.1002/arch.1057
Subject(s) - leucine , amino acid , biochemistry , chironomus riparius , biology , vesicle , phenylalanine , histidine , glycine , membrane , midge , larva , botany
Leucine uptake into membrane vesicles from larvae of the midge Chironomus riparius was studied. The membrane preparation was highly enriched in typical brush border membrane enzymes and depleted of other membrane contaminants. In the absence of cations, there was a stereospecific uptake of l‐leucine, which exhibited saturation kinetics. Parameters were determined both at neutral ( K m 33 ± 5 μM and V max 22.6 ± 6.8 pmol/7s/mg protein) and alkaline ( K m 46 ± 5 μM and V max 15.5 ± 2.5 pmol/7s/mg protein) pH values. At alkaline pH, external sodium increased the affinity for leucine ( K m 17 ± 1 μM) and the maximal uptake rate ( V max 74.0 ± 12.5 pmol/7s/mg protein). Stimulation of leucine uptake by external alkaline pH agreed with lumen pH measurements in vivo. Competition experiments indicated that at alkaline pH, the transport system readily accepts most L‐amino acids, including branched, unbranched, and α‐methylated amino acids, histidine and lysine, but has a low affinity for phenylalanine, β‐amino acids, and N‐ methylated amino acids. At neutral pH, the transport has a decreased affinity for lysine, glycine, and α‐methylleucine. Taken together, these data are consistent with the presence in midges of two distinct leucine transport systems, which combine characters of the lepidopteran amino acid transport system and of the sodium‐dependent system from lower neopterans. Arch. Insect Biochem. Physiol. 48:51–62, 2001. © 2001 Wiley‐Liss, Inc.