Said H M, McCloud E, Yanagawa N
Department of Medicine, University of California School of Medicine, Irvine 92717, USA.
Biochim Biophys Acta. 1995 Jun 14;1236(2):244-8. doi: 10.1016/0005-2736(95)00054-7.
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.
本研究检测了纯化的大鼠肝脏基底外侧膜囊泡（BLMV）对核黄素（RF）的摄取情况。发现RF的摄取本质上不依赖于Na⁺和pH。关于BLMV对RF摄取与孵育培养基渗透压关系的研究表明，摄取是转运至囊泡内空间（66.5%）以及与膜表面结合（33.5%）的结果。BLMV摄取RF的过程随底物浓度呈饱和状态，表观Km分别为3.55±0.70微摩尔，Vmax为39.89±3.24皮摩尔/毫克蛋白/5秒。顺式添加未标记的RF及其结构类似物鲁米诺黄素和鲁米诺铬可抑制[³H]RF的摄取，而反式添加未标记的RF则刺激预加载囊泡中[³H]RF的外流。膜转运抑制剂丙磺舒、4,4 - 二异硫氰酸根合芪 - 2,2 - 二磺酸（DIDS）和4 - 乙酰氨基 - 4 - 异硫氰酸根合芪 - 2,2'-二磺酸（SITS）对RF摄取无影响。诱导短暂的正性囊泡内空间会导致RF摄取略有刺激，而诱导负性囊泡内空间则会导致RF摄取略有抑制。这些结果证明肝脏BLMV存在一种与膜相关的RF摄取载体系统。该系统似乎不依赖于Na⁺和pH，并在一定程度上受跨膜电位变化的影响。
