Na+-dependent biotin transport into brush-border membrane vesicles from human kidney cortex

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Abstract
Renal reabsorption of biotin was investigated in human kidney by means of the isolated brush-border membrane vesicle technique. Biotin uptake into the vesicles was sodium-dependent producing a typical overshoot when incubated under sodium-gradient conditions (external concentration greater than internal). This effect was not observed in the presence of gradients of KCl, LiCl or choline-chloride, nor in the absence of any salt. Using the K+/valinomycin voltage-clamp method biotin uptake remained uninfluenced, i.e. was electroneutral, whereas glucose uptake (which is known to be electrogenic in kidney of other species) was greatly increased. When biotin transport was investigated as a function of external sodium concentration a stoichiometic coupling factor of 1 for the Na+-biotin− cotransport was determined. Increasing the biotin concentration in the incubation medium up to 200 μmol/l led to saturation with the kinetic parameters of 31 μmol/l for the apparent Michaelis constant and 82 nmol g protein−1 30 s−1 for the maximal transport rate. Uptake was not saturable in the concentration range of 0.001–1 μmol/l. Inhibition of the biotin uptake (25 μmol/l) was observed in the presence of 250 μmol/l dethiobiotin, bisnorbiotin, thioctic acid, and probenecid, whereas biocytin, propionic acid, lactic acid, succinic acid, citric acid, ascorbic acid, primidone and carbamazepine had no effect. We conclude that renal biotin reabsorption in human kidney is specifically sodium-dependent, saturable and electroneutral. It therefore fulfills the requirements for a secondary active carrier-mediated transport system. The results suggest that biocytin is not an inhibitor of renal biotin reabsorption.

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