Riboflavin transport by rabbit renal basolateral membrane vesicles

The present study examined riboflavin (RF) uptake by isolated rabbit renal basolateral membrane (BLM). RF uptake was linear during the initial 10 seco nds and leveled off thereafter with longer incubation. Studies on RF uptake as a function of incubation medium osmolarity indicated that the BLM RF up take was the results of transport (similar to 45%) into the intravesicular space as well as binding (similar to 55%) to membrane surfaces. The RF bind ing to BLM was Na+-dependent so that replacement of Na+ by other cations el iminated the binding component of RF uptake. The process of BLM RF uptake w as saturable as a function of substrate concentration and was significantly inhibited by cis-addition of its structural analogs, lumiflavin and lumich rome, indicating the involvement of a carrier-mediated process. The BLM RF uptake was affected by changes in extravesicular pH so that, as compared to pH 7.5, RF uptake was lower at pH 6.5 and higher at pH 8.5. The effect of extravesicular pH persisted when the transmembrane H+ gradient was dissipat ed by FCCP, indicating the direct effect of pH on BLM RF uptake. The BLM RF uptake was not affected by alterations of the transmembrane electrical pot ential, induced by either the presence of anions with different membrane pe rmeability (Cl- = NO3- > SO4- > gluconate(-)) or using nigericin (10 mu g/m g protein) with an outwardly or inwardly directed transmembrane K+ gradient . The BLM RF uptake was, however, inhibited by probenecid and p-aminohippur ate, and was enhanced by trans-RF. In summary, these results demonstrate th e existence of a Na+-dependent BLM binding of RF and a membrane-associated carrier system for RF uptake by renal BLM. (C) 1998 Elsevier Science B.V. A ll rights reserved.