Leucine transport in brush border membrane vesicles from freshwater insect larvae

Leucine transport across brush border membrane vesicles prepared from four insect species common to European freshwater streams has been characterized. The species studied were: Ephemera danica (Ephemeroptera: Ephemeridae), Isoperla grammatica (Plecoptera: Perlodidae), Hydropsyche pellucidula (Trichoptera: Hydropsychidae), and Hybomitra bimaculata (Diptera: Tabanidae). The transport differed among the studied taxa for several features, including pH and sodium dependence, substrate affinity and specificity, and efficiency. In H. pellucidula and E. danica, leucine uptake was higher at pH 7.4 than at more alkaline or acidic pH values, whereas in I÷ grammatica and H. bimaculata, the uptake was rather constant when pH varied from 5.0 to 7.4, then strongly decreased at pH 8.8. All but E. danica displayed a transient intravescicular leucine accumulation in the presence of sodium, suggesting the existence of a cation-leucine symport mechanism. The sodium dependence ranged according to the following order: H. pellucidula > I. grammatica > H. bimaculata > E. danica. Moreover, in H. pellucidula and I. grammatica, the sodium-dependence was stronger at pH 8.8 than at pH 7.4. In E. danica, leucine uptake was sodium-independent at all pH values. The highest value of Vmax (45.3 pmol·s-1·mg proteins-1) was in E÷ danica, which, however, displayed the lowest affinity (Km 137 mM) when compared to the kinetic parameters of other taxa. The Vmax and Km values were: 40 and 52.5, 32.1 and 12.5, and 4.5 and 230 for H. bimaculata, H. pellucidula, and I. grammatica, respectively. The obtained results are discussed within our current knowledge of amino acid transport systems in insects.