Said H M, Arianas P
Department of Medicine, University of California School of Medicine, Irvine.
Gastroenterology. 1991 Jan;100(1):82-8. doi: 10.1016/0016-5085(91)90586-a.
The transport of riboflavin across the brush border membrane of human intestine was examined using the established brush border membrane vesicle technique. Both osmolarity and temperature studies have concluded that the uptake of riboflavin by these vesicles is mostly the result of transport of riboflavin into an active intravesicular space with less binding to membrane surfaces. When an inwardly directed Na+ gradient was imposed, transport of riboflavin was linear with time for approximately 20 seconds of incubation and was significantly higher than in the presence of an identical K+ gradient. Initial rate of transport of riboflavin as a function of concentration was found to include a saturable component in the presence of an inwardly directed Na+ gradient but was linear in the presence of an identical K+ gradient. The apparent Km and Vmax of the Na+ stimulated transport process were found to be 7.26 mumols/L and 0.97 pmol/mg protein per 10 seconds, respectively. The addition of high concentrations of unlabeled riboflavin and its structural analogue lumiflavin to the incubation medium caused significant inhibition in the transport of 3H-riboflavin in the brush border membrane vesicle incubated in the presence of an inwardly directed Na+ gradient but not in vesicles incubated in the presence of an identical K+ gradient. Inducing a relatively positive intravesicular space with the use of valinomycin and an inwardly directed K+ gradient caused significant inhibition in the transport of riboflavin. On the other hand, inducing a relatively negative intravesicular space with the use of anions of different lipid permeabilities caused significant stimulation in the transport of riboflavin. These results demonstrate that riboflavin transport in human intestinal brush border membrane vesicle is through a carrier-mediated system. This system functions in the presence of a Na(+)-gradient and seems to transport the substrate by an electrogenic process.
采用已确立的刷状缘膜囊泡技术，对核黄素在人小肠刷状缘膜上的转运进行了研究。渗透压和温度研究均得出结论，这些囊泡对核黄素的摄取主要是核黄素转运至活跃的囊泡内空间的结果，与膜表面的结合较少。当施加内向性的Na⁺梯度时，核黄素的转运在孵育约20秒内与时间呈线性关系，且显著高于存在相同K⁺梯度时的情况。发现核黄素转运的初始速率作为浓度的函数，在存在内向性Na⁺梯度时包含一个可饱和成分，而在存在相同K⁺梯度时呈线性关系。Na⁺刺激转运过程的表观Km和Vmax分别为7.26 μmol/L和每10秒0.97 pmol/mg蛋白质。向孵育培养基中添加高浓度的未标记核黄素及其结构类似物黄素单核苷酸，在存在内向性Na⁺梯度的情况下孵育的刷状缘膜囊泡中，对³H - 核黄素的转运产生了显著抑制，但在存在相同K⁺梯度的囊泡中未出现这种情况。使用缬氨霉素诱导相对正的囊泡内空间和内向性K⁺梯度，对核黄素的转运产生了显著抑制。另一方面，使用具有不同脂质通透性的阴离子诱导相对负的囊泡内空间，对核黄素的转运产生了显著刺激。这些结果表明，核黄素在人小肠刷状缘膜囊泡中的转运是通过载体介导的系统进行的。该系统在存在Na⁺梯度的情况下发挥作用，似乎通过电生成过程转运底物。
