Said H M, Redha R, Nylander W
Vanderbilt University School of Medicine, Department of Pediatric Gastroenterology, Nashville, Tennessee 37232.
Am J Physiol. 1987 Nov;253(5 Pt 1):G631-6. doi: 10.1152/ajpgi.1987.253.5.G631.
Transport of biotin across human intestinal brush-border membrane (BBM) was examined using brush-border membrane vesicle (BBMV) technique. Uptake of biotin by BBMV is mostly the result of transport of the substrate into an active intravesicular space with little binding to membrane surfaces. The transport of biotin was carrier mediated and was 1) Na+ (but not K+) gradient dependent with a distinct "over-shoot" phenomenon, 2) saturable as a function of concentration in the presence of a Na+ (but not a K+) gradient with an apparent Km and Vmax for the Na+ gradient-dependent system of 5.26 microM and 13.47 pmol.mg protein-1.20 s-1, respectively, and 3) inhibited by structural analogues and related compounds. Unlike the electrogenic transport of D-glucose, transport of the anionic biotin in the presence of a Na+ gradient (out greater than in) was not affected by imposing a relatively positive intravesicular electrical potential, suggesting that biotin transport is most likely an electroneutral process. These results demonstrate the existence of a carrier-mediated system for biotin transport in human BBM and show that the transport process is Na+ gradient dependent and electrically silent. It is suggested that biotin transport across the BBM is driven by a Na+ gradient most probably through a biotin-Na+ cotransport system.
利用刷状缘膜囊泡（BBMV）技术研究了生物素跨人小肠刷状缘膜（BBM）的转运。BBMV对生物素的摄取主要是底物转运到活跃的囊泡内空间的结果，与膜表面的结合很少。生物素的转运由载体介导，且具有以下特点：1）依赖Na⁺（而非K⁺）梯度，存在明显的“过冲”现象；2）在存在Na⁺（而非K⁺）梯度的情况下，随浓度呈饱和状态，Na⁺梯度依赖系统的表观Km和Vmax分别为5.26 μM和13.47 pmol·mg蛋白⁻¹·20 s⁻¹；3）受结构类似物和相关化合物的抑制。与D-葡萄糖的电生性转运不同，在Na⁺梯度（胞外大于胞内）存在的情况下，阴离子生物素的转运不受施加相对正的囊泡内电位的影响，这表明生物素转运很可能是一个电中性过程。这些结果证明了人BBM中存在生物素转运的载体介导系统，并表明转运过程依赖Na⁺梯度且无电活动。提示生物素跨BBM的转运很可能通过生物素-Na⁺共转运系统由Na⁺梯度驱动。
