Proline transport into brush border membrane vesicles from the midgut of Manduca sexta larvae

The unidirectional transport of proline by midgut epithelium cells of the tobacco hornworm, Manduca sexta, was investigated in brush border membrane vesicles. Both K.(+)-stimulated and K(+)-insensitive transport pathways were identified. Analyses of K(+)-dependent proline transport revealed 1:1 ratio of K+ to proline, a Km of 13 mM for K+ and a decrease in both Km (from 18 mM to 3 mM) and Vmax (from 37 nmol/mg protein/min to 10 nmol/mg protein/min) for proline in the presence of a K+ gradient. The profiles of cis-inhibition by other amino acids demonstrated that proline is transported into midgut cells by a transport system that is shared by other neutral amino acids.