CHARACTERIZATION OF TRYPTOPHAN TRANSPORT IN HUMAN PLACENTAL BRUSH-BORDER MEMBRANE-VESICLES
CHARACTERIZATION OF TRYPTOPHAN TRANSPORT IN HUMAN PLACENTAL BRUSH-BORDER MEMBRANE-VESICLES
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DOI：
10.1042/bj2380201
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发表时间：
1986-08-15
影响因子：
4.1
通讯作者：
GANAPATHY, V
中科院分区：
文献类型：
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作者：
GANAPATHY, ME;LEIBACH, FH;GANAPATHY, V
研究了人胎盘刷状缘膜囊对色氨酸的摄取特性。这些囊泡中色氨酸的摄取主要是Na+非依赖性的。色氨酸的摄取与短期孵育测量发生完全由载体介导的过程，但显着的结合，这种氨基酸的膜囊泡观察到较长的孵育。载体介导的系统服从Michaelis-Menten动力学，表观亲和常数为12.7 ±。1.0最大速度为91 μ M。5 pmol/15 s/mg蛋白质。在存在和不存在Na+梯度的情况下，动力学常数相似。竞争实验表明，除脯氨酸、羟脯氨酸和2-（甲氨基）异丁酸外，许多中性氨基酸都能有效抑制色氨酸的摄取。抑制性氨基酸包括芳香族氨基酸以及其它系统1特异性氨基酸（根据氨基酸转运系统的最新命名法，系统1指经典L系统）。该转运系统对D-异构体的亲和力很低，不受根皮素或葡萄糖的影响，但受对叠氮苯丙氨酸和N-乙基马来酰亚胺的抑制。在4.5-8.0范围内，pH值变化对摄取率的影响极小。色氨酸摄取显着响应反式刺激，和氨基酸能够引起反式刺激包括所有的氨基酸与系统-1-特异性。各种氨基酸抑制色氨酸和亮氨酸摄取的模式非常相似。我们的结论是系统t，这是特定的芳香族氨基酸，是不存在的人胎盘和色氨酸运输在该组织中发生通过系统1，它具有非常广泛的特异性。
The characteristics of tryptophan uptake in isolated human placental brush-border membrane vesicles were investigated. Tryptophan uptake in these vesicles was predominantly Na+-independent. Uptake of tryptophan as measured with short incubations occurred exclusively by a carrier-mediated process, but significant binding of this amino acid to the membrane vesicles was observed with longer incubations. The carrier-mediated system obeyed Michaelis-Menten kinetics, with an apparent affinity constant of 12.7 .+-. 1.0 .mu.M and a maximal velocity of 91 .+-. 5 pmol/15 s per mg of protein. The kinetic constants were similar in the presence and absence of a Na+ gradient. Competition experiments showed that tryptophan uptake was effectively inhibited by many neutral amino acids except proline, hydroxyproline and 2-(methylamino)isobutyric acid. The inhibitory amino acids included aromatic amino acids as well as other system-1-specific amino acids (system 1 refers to the classical L system, according to the most recent nomenclature of amino acid transport systems). The transport system showed very low affinity for D-isomers, was not affected by phloretin or glucose but was inhibited by p-azidophenylalanine and N-ethylmaleimide. The uptake rates were only minimally affected by change in pH over the range 4.5-8.0. Tryptophan uptake markedly responded to trans-stimulation, and the amino acids capable of causing trans-stimulation included all amino acids with system-1-specificity. The patterns of inhibition of uptake of tryptophan and leucine by various amino acids were very similar. We conclude that system t, which is specific for aromatic amino acids, is absent from human placenta and that tryptophan transport in this tissue occurs via system 1, which has very broad specificity.