Leucine Uptake in Brush-border Membrane Vesicles from the Midgut of a Lepidopteran Larva, Philosamia Cynthia

A potassiumor sodium-activated cotransport of leucine occurs in brush-border membrane vesicles prepared from the midgut of larvae of Philosamia cynthia (Drury). The potassium chemical gradient can drive a twofold accumulation of leucine, which is greatly increased under experimental conditions that presumably provide an electrical potential difference (AT/;). Kinetic parameters show that leucine transport is improved by these conditions and by a pH gradient similar to that occurring in vivo. However, these gradients cannot drive an intravesicular accumulation of leucine in the absence of potassium. The potassium-dependence of leucine uptake shows that 20% of the transport is potassium-independent and that Kx and Vmax are 30.3±3.2mmoll~ 1 and 2584±148pmol 7 s ^ m g " 1 protein, respectively. The potassium-independent component of leucine transport is also carrier-mediated and some evidence is reported suggesting that the same carrier can cross the membrane as binary (carrier and leucine) or ternary (carrier, leucine and potassium) complexes, each having a different mobility.