RIBOFLAVIN UPTAKE BY RAT-LIVER BASOLATERAL MEMBRANE-VESICLES

The present study examined riboflavin (RF) uptake by purified rat liver basolateral membrane vesicle (BLMV). Uptake of RF was found to be Na+- and pH-independent in nature. Studies on RF uptake by BLMV as a function of incubation medium osmolarity have indicated that the uptake is the result of transport (66.5%) into the intravesicular space as well as binding (33.5%) to membrane surfaces. The process of RF uptake by BLMV was saturable as a function of substrate concentration with an apparent K-m of 3.55 +/- 0.70 mu M and V-max of 39.89 +/- 3.24 pmol/mg protein/5 s, respectively. cis-Addition of unlabeled RF and its structural analogs lumaflavin and lumichrome inhibited the uptake of [H-3]RF while trans-addition of unlabeled RF stimulated the efflux of [H-3]RF from preloaded vesicles. No effect on RF uptake was found by the membrane transport inhibitors probenecid, 4,4-diisothiocyanotostilbene-2,2-disulfonic acid (DIDS) and 4-acetamido-4-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), Induction of a transient positive intravesicular space led to a slight stimulation of RF uptake, while induction of a negative intravesicular space led to a slight inhibition in RF uptake. These results demonstrate the existence of a membrane-associated carrier system for RF uptake by liver BLMV, This system appears to be Na+- and pH-independent and is influenced to a certain degree by changes in transmembrane electrical potential.