Borowitz S M, Ghishan F K
Department of Pediatric Gastroenterology, University of Virginia Medical Center, Charlottesville.
Gastroenterology. 1989 Jan;96(1):4-10. doi: 10.1016/0016-5085(89)90757-9.
Phosphate transport was studied in isolated brush-border membrane vesicles prepared from human jejunum. Transport consisted of two components: passive diffusion across the brush border, and sodium-dependent, carrier-mediated uptake. Both carrier-mediated transport and passive diffusion of phosphate were significantly greater at pH 6.1 than at pH 7.4. Sodium-dependent uptake was saturable at both pH values; the apparent Km was not affected by pH, whereas Vmax was significantly greater at pH 6.1 (179.3 vs. 118.4 pM/mg per 15 s). Phosphate uptake was half-maximally stimulated by a sodium concentration of 34 mM at pH 7.4, and a sodium concentration of 76 mM at pH 6.1. At both pH values, Hill coefficients were greater than 1 and were unaffected by pH. Moreover, sodium-dependent phosphate uptake was electroneutral at pH 7.4 and electrogenic at pH 6.1. These data suggest that at acid and neutral pH, both monovalent and divalent phosphate are transported across the human jejunal brush border membrane. The transport of each phosphate molecule is associated with the cotransport of two sodium ions.
对从人空肠制备的分离刷状缘膜囊泡中的磷酸盐转运进行了研究。转运由两个部分组成：跨刷状缘的被动扩散，以及钠依赖性载体介导的摄取。在pH 6.1时，载体介导的转运和磷酸盐的被动扩散均显著高于pH 7.4时。在两个pH值下，钠依赖性摄取均具有饱和性；表观Km不受pH影响，而Vmax在pH 6.1时显著更高（每15秒179.3对118.4 pM/mg）。在pH 7.4时，34 mM的钠浓度使磷酸盐摄取达到最大刺激的一半，在pH 6.1时，76 mM的钠浓度使磷酸盐摄取达到最大刺激的一半。在两个pH值下，希尔系数均大于1且不受pH影响。此外，钠依赖性磷酸盐摄取在pH 7.4时是电中性的，在pH 6.1时是生电性的。这些数据表明，在酸性和中性pH值下，单价和二价磷酸盐均通过人空肠刷状缘膜转运。每个磷酸盐分子的转运与两个钠离子的协同转运相关。
