Riboflavin transport by rabbit renal brush border membrane vesicles

The present study examined riboflavin (RF) uptake by isolated rabbit renal brush border membrane (BBM). RF uptake was linear for up to 30 s and leveled off thereafter reaching an equilibrium with longer incubation. Studies on RF uptake as a function of incubation medium osmolarity indicated that the uptake was the results of transport (61.4%) into the intravesicular space as well as binding (38.6%) to membrane surfaces. The process of RF uptake was saturable as a function of substrate concentration with an apparent Km of 25.7 +/- 7.6 microM and Vmax of 75.6 +/- 14.7 pmol/mg protein/10 s. cis-Addition of unlabeled RF and its structural analogues, lumiflavin and lumichrome, inhibited the uptake of [3H]RF significantly, indicating the involvement of a carrier-mediated process in RF uptake by renal BBM. RF uptake by renal BBM was partly Na+-dependent so that when Na+ was replaced by potassium, choline, lithium or tetramethylammonium, the RF uptake was reduced to ca. 60% of the control. This Na+-dependency was unlikely to be due to Na+-cotransport mechanism because RF uptake occurred without the characteristic 'overshoot' phenomenon as for other Na+-cotransport systems and the elimination of transmembrane Na+-gradient by preloading Na+ to the intravesicular space did not affect RF uptake. In contrast, removal of Na+ eliminated the binding component of RF uptake, suggesting the requirement of Na+ for RF binding to BBM. The RF uptake was not affected when extravesicular pH was varied within the physiological pH range of 6.5 to 8.5. No effect on BBM [3H]RF uptake was found when the transmembrane electrical potential was altered by either the presence of anions with different membrane permeability (Cl- = NO3- > SO4- > gluconate-) or by using nigericin (10 microg/mg protein) with an outwardly or inwardly directed transmembrane K+ gradient. The uptake of RF by BBM vesicles was, however, inhibited by probenecid and organic anion transport inhibitors, 4,4-diiso-thiocyanatostilbene-2,2-disulfonic acid (DIDS, 1 mM) and 4-acetamido-4-isothiocyanatostilbene-2,2-disulfonic acid (SITS, 1 mM). In summary, these results demonstrate the existence of a membrane-associated, and organic anion inhibitor-sensitive, carrier system for RF uptake by renal BBM.