title: Riboflavin transport by rabbit renal brush border membrane vesicles.
abstract: 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.
authors.affiliation: Division of Nephrology, Sepulveda Veterans Administration Medical Center, CA 91343, USA.
authors.forename: N
authors.initials: N
authors.lastname: Yanagawa
authors.forename: O D
authors.initials: OD
authors.lastname: Jo
authors.forename: H M
authors.initials: HM
authors.lastname: Said
mesh_terms: D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid / Q000494:pharmacology; D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002352:Carrier Proteins / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007668:Kidney / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D008297:Male; D008564:Membrane Potentials; D008871:Microvilli / Q000187:drug effects / Q000378:metabolism; D011817:Rabbits; D012256:Riboflavin / Q000493:pharmacokinetics*; D012964:Sodium / Q000378:metabolism
chemical_list: D002352:Carrier Proteins; D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid; D012964:Sodium; D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid; D012256:Riboflavin