Leucine Transport in Membrane Vesicles From Chironomus riparius Larvae Displays a Mélange of Crown-Group Features

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 (Km 33 ± 5 mM and Vmax 22.6 ± 6.8 pmol/ 7s/mg protein) and alkaline (Km 46 ± 5 mM and Vmax 15.5 ± 2.5 pmol/7s/mg protein) pH values. At alkaline pH, external sodium increased the affinity for leucine (Km 17 ± 1 mM) and the maximal uptake rate (Vmax 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 a-methylated amino acids, histidine and lysine, but has a low affinity for phenylalanine, b-amino acids, and N-methylated amino acids. At neutral pH, the transport has a decreased affinity for lysine, glycine, and a-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.