Transport of glycyl-l-proline in intestinal brush-border membrane vesicles of the suckling rat: characteristics and maturation

Transport of the dipeptide glycine-l-proline (Gly-l-Pro) in the developing intestine of suckling rats and its subsequent maturation in adult rats was examined using the brush-border membrane vesicles (BBMV) technique. Uptake of Gly-l-Pro by BBMV was mainly the result of transport into the intravesicular space with little binding to membrane surfaces. Transport of Gly-l-Pro in BBMV of suckling rats was: (1) Na²⁺ independent; (2) pH dependent with maximum uptake at an incubation buffer pH of 5.0; (3) saturable as a function of concentration (apparent K_m = 21.5 ± 7.9 mM, V_max = 8.6 ± 1.5 nmol/mg protein per 10 s): (4) inhibited by other di- and tripeptides; and (5) stimulated and inhibited by inducing a negative and positive intravesicular membrane electrical potential, respectively. Similarly, transport of Gly-l-Pro in intestinal BBMV of adult rats was saturable as a function of concentration (apparent K_m = 17.4 ± 8.6 mM, V_max = 9.1 ± 2.1 nmol/mg protein per 10 s) and was stimulated and inhibited by inducing a relatively negative and positive intravesicular membrane potential, respectively. No difference in the transport kinetic parameters of Gly-l-Pro was observed in suckling and adult rats, indicating a similar activity (and/or number) and affinity of the transport carrier in the two age groups. These results demonstrate that the transport of Gly-l-Pro is by a carrier-mediated process which is fully developed at the suckling period. Furthermore, the process is H²⁺-dependent but not Na²⁺-dependent, electrogenic and most probably occurs by a Gly-l-Pro/H⁺ cotransport mechanism.