Na(+)-dependent biotin transport into brush-border membrane vesicles from rat kidney.

The mechanisms of biotin reabsorption in rat kidney cortex were investigated using isolated brush-border membrane vesicles. An inwardly directed Na+ gradient specifically stimulated a transient biotin overshoot. Biotin transport was not affected by a valinomycin-induced K(+)-diffusion potential, and biotin(-)-Na+ stoichiometry was found to be 1:1. As a function of concentration, the uptake showed saturation in the presence of a Na+ gradient with an apparent Michaelis constant (Km) of 55 microM and Vmax of 217 pmol.mg protein-1.25 s-1. Desthiobiotin, 250 microM, norbiotin, bisnorbiotin, thioctic acid, valeric acid, probenecid, and nonanoic acid inhibited the transport of 30 microM biotin, whereas other biotin derivatives, as well as biocytin and organic acids found in the urine of biotinidase-deficient patients, did not. Preloading of the vesicles with biotin, desthiobiotin, norbiotin, and thioctic acid in the presence of Na+ increased initial uptake of biotin from the incubation medium (trans-stimulation). Our results indicate that biotin absorption in rat kidney fulfills the criteria for a specific carrier-mediated and electroneutral Na(+)-biotin- cotransport in a 1:1 ratio. The results are discussed in context with congenital biotinidase deficiency in humans.