Movement of biotin across the rat intestinal basolateral membrane. Studies with membrane vesicles.

The characteristics of the transport process of the water-soluble vitamin biotin across the basolateral membrane (BLM) of rat intestine, i.e. the exit of biotin from rat enterocyte, were examined using established BLM vesicles (BLMV) techniques. Results of osmolarity and temperature studies have indicated that the majority of biotin taken up by these vesicles is the result of transport of the vitamin into the intravesicular spaces, with little membrane binding. Transport of biotin in these vesicles was found to be: (1) higher in BLMV of the jejunum than those isolated simultaneously from the ileum, (2) Na(+)-independent in nature, (3) saturable as a function of concentration, with an apparent Km of 3.51 microM and a Vmax. of 16.49 pmol/10 s per mg of protein, (4) inhibited by high concentrations of unlabelled biotin and its related compounds, but not by the unrelated taurocholate, (5) sensitive to the anion transport inhibitors 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) and 4-acetamido-4-isothiocyanostilbene-2,2'-disulphonic acid (SITS), and (6) not affected by imposing a positive or a negative intravesicular space with the use of a valinomycin-induced K+ diffusion potential. Based on these findings, it is concluded that biotin transport across the rat intestinal BLM is mediated via a specialized carrier system which is Na(+)-independent, transports the vitamin by a process which is sensitive to the effects of anion transport inhibitors, and is electroneutral in nature.