title: Riboflavin uptake by rat liver basolateral membrane vesicles.
abstract: 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 Km of 3.55 +/- 0.70 microM and Vmax 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 [3H]RF while trans-addition of unlabeled RF stimulated the efflux of [3H]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.
authors.affiliation: Department of Medicine, University of California School of Medicine, Irvine 92717, USA.
authors.forename: H M
authors.initials: HM
authors.lastname: Said
authors.forename: E
authors.initials: E
authors.lastname: McCloud
authors.forename: N
authors.initials: N
authors.lastname: Yanagawa
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002462:Cell Membrane / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008099:Liver / Q000378:metabolism*; D008564:Membrane Potentials; D009713:Nucleotidyltransferases / Q000378:metabolism; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D012256:Riboflavin / Q000378:metabolism*; D013997:Time Factors
chemical_list: D017853:Phosphotransferases (Alcohol Group Acceptor); C022532:riboflavin kinase; D009713:Nucleotidyltransferases; C020143:FMN adenylyltransferase; D012256:Riboflavin