Jessen H, Sheikh M I
Institute of Medical Biochemistry, University of Aarhus, Denmark.
Biochim Biophys Acta. 1991 May 7;1064(2):189-98. doi: 10.1016/0005-2736(91)90301-n.
The uptake of taurine by luminal membrane vesicles from pars convoluta and pars recta of rabbit proximal tubule was examined. In pars convoluta, the transport of taurine was characterized by two Na(+)-dependent (Km1 = 0.086 mM, Km2 = 5.41 mM) systems, and one Na(+)-independent (Km = 2.87 mM) system, which in the presence of an inwardly directed H(+)-gradient was able to drive the transport of taurine into these vesicles. By contrast, in luminal membrane vesicles from pars recta, the transport of taurine occurred via a dual transport system (Km1 = 0.012 mM, Km2 = 5.62 mM), which was strictly dependent on Na+. At acidic pH with or without a H(+)-gradient, the Na(+)-dependent flux of taurine was drastically reduced. In both kind of vesicles, competition experiments only showed inhibition of the Na(+)-dependent high-affinity taurine transporter in the presence of beta-alanine, whereas there was no significant inhibition with alpha-amino acids, indicating a beta-amino acid specific transport system. Addition of beta-alanine, L-alanine, L-proline and glycine, but not L-serine reduced the H(+)-dependent uptake of taurine to approx. 50%. Moreover, only the Na(+)-dependent high-affinity transport systems in both segments specifically required Cl-. Investigation of the stoichiometry indicated 1.8 Na+: 1 Cl-: 1 taurine (high affinity), 1 Na+: 1 taurine (low affinity) and 1 H+: 1 taurine in pars convoluta. In pars recta, the data showed 1.8 Na+: 1 Cl-: 1 taurine (high affinity) and 1 Na+: 1 taurine (low affinity).
对兔近端小管曲部和直部管腔膜囊泡摄取牛磺酸的情况进行了研究。在曲部，牛磺酸的转运由两个依赖钠离子的系统（Km1 = 0.086 mM，Km2 = 5.41 mM）和一个不依赖钠离子的系统（Km = 2.87 mM）所表征，在存在内向氢离子梯度的情况下，该不依赖钠离子的系统能够驱动牛磺酸转运进入这些囊泡。相比之下，在直部管腔膜囊泡中，牛磺酸的转运通过一个双重转运系统（Km1 = 0.012 mM，Km2 = 5.62 mM）进行，该系统严格依赖钠离子。在酸性pH条件下，无论有无氢离子梯度，牛磺酸的钠离子依赖性通量都会大幅降低。在这两种囊泡中，竞争实验仅表明在存在β-丙氨酸时，钠离子依赖性高亲和力牛磺酸转运体受到抑制，而α-氨基酸则无明显抑制作用，这表明存在一个β-氨基酸特异性转运系统。添加β-丙氨酸、L-丙氨酸、L-脯氨酸和甘氨酸，但不包括L-丝氨酸，可使牛磺酸的氢离子依赖性摄取降低约50%。此外，仅两个节段中的钠离子依赖性高亲和力转运系统特别需要氯离子。化学计量学研究表明，在曲部为1.8个钠离子:1个氯离子:1个牛磺酸（高亲和力）、1个钠离子:1个牛磺酸（低亲和力）和1个氢离子:1个牛磺酸。在直部，数据显示为1.8个钠离子:1个氯离子:1个牛磺酸（高亲和力）和1个钠离子:1个牛磺酸（低亲和力）。
