Biotin transport in human liver basolateral membrane vesicles: a carrier-mediated, Na+ gradient-dependent process.

The characteristics of biotin transport into human liver were examined using purified liver basolateral membrane vesicle (BLMV) preparations. Biotin uptake by BLMVs was mostly due to transport of the vitamin into the inside of vesicles. In the presence of an Na+ gradient (out greater than in), biotin transport with time was significantly higher than that in the presence of a K+ gradient and showed transient accumulation (overshoot). High concentrations of unlabeled biotin and related compounds caused significant cis inhibition in biotin transport in the presence of an Na+ (but not a K+) gradient. Transport of biotin as a function of concentration in the presence of an Na+ gradient included a saturable component, while it was lower and linear in the presence of a K+ gradient. Apparent Km and Vmax of the saturable Na+ gradient-dependent component were 1.22 mumol/L and 4.76 pmol.mg protein-1 x 10 s-1, respectively. Induction of a negative or positive intravascular potential using valinomycin-K diffusion methodology did not affect biotin transport into BLMVs. Also, neither the anion-exchange inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid nor 4-acetamido-4-isothiocyanostilbene-2,2'-disulfonic acid caused significant inhibition in biotin transport. These results indicate that biotin transport into human liver occurs via a specialized, carrier-mediated transport system. This system is Na(+)-gradient dependent and transports the vitamin via an electroneutral process.