L- and D-alanine transport in brush border membrane vesicles from lepidopteran midgut: evidence for two transport systems

In brush border membrane vesicles from the midgut of Philosamia cynthia larvae (Lepidoptera) the L- and D-alanine uptake is dependent on a potassium gradient and on transmembrane electrical potential difference. Each isomer inhibits the uptake of the other form: inhibition of L-alanine uptake by D-alanine is competitive, whereas inhibition of D-alanine uptake by L-alanine is noncompetitive. Transstimulation experiments as well as the different pattern of specificity to cations suggest the existence of two transport systems. Kinetic parameters for the two transporters have been calculated both when Kout greater than Kin and Kout = Kin. D-alanine is actively transported also by the whole midgut, but it is not metabolized by the intestinal tissue.