Biotin transport in rat intestinal brush-border membrane vesicles.

Transport of biotin across rat intestinal brush-border membrane was examined using the brush-border membrane vesicle (BBMV) technique. Uptake of biotin by BBMV is the result of transport of the substrate into the intravesicular space with negligible binding to membrane surfaces. In the presence of a Na+ gradient (out greater than in), transport of biotin was higher with a transient 'overshoot' phenomenon. In comparison, transport of biotin in the presence of a choline gradient (out greater than in) was lower with no 'overshoot' phenomenon. In both jejunal and ileal BBMV, the transport of biotin as a function of concentration was saturable in the presence of a Na+ gradient (out greater than in) but was linear in the presence of a choline gradient (out greater than in). Vmax of the Na+-dependent transport system was 0.88 and 0.37 pmol/mg protein per s and apparent Kt was 7.57 and 7.85 microM in jejunal and ileal BBMV, respectively. Structural analogues inhibited the transport process of biotin. Unlike the electrogenic transport of D-glucose, the transport of the anionic biotin was not affected by imposing a relatively positive intravesicular potential with the use of valinomycin and an inwardly-directed K+ gradient, suggesting that biotin transport is most probably an electroneutral process. This suggestion was further supported by studies on biotin transport in the presence of anions of different lipid permeability. The results of this study demonstrate that biotin transport across rat intestinal brush-border membrane is by a carrier-mediated, Na+-dependent and electroneutral process. Furthermore, transport of biotin is higher in the jejunum than the ileum.