Thiamine transport in human placental brush border membrane vesicles.
Thiamine transport in human placental brush border membrane vesicles.
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人胎盘刷状缘膜囊泡中的硫胺素转运。
DOI：
10.1016/s0005-2736(98)00019-4
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发表时间：
1998
期刊：
影响因子：
--
通讯作者：
Grassl,SM
中科院分区：
文献类型：
--
作者：
Grassl,SM
使用通过二价阳离子沉淀分离的刷状缘膜囊泡研究了人胎盘上皮转运硫胺素的途径。通过[3H]-硫胺素示踪剂通量测量来评估介导Na+-硫胺素共转运、质子/硫胺素交换和促进扩散的硫胺素转运机制的存在。囊泡内硫胺素积累的程度不受施加向内定向的钠梯度的影响，这表明不存在介导刷状缘膜Na+-硫胺素共转运的机制。在存在和不存在有利于内部负性钾扩散电位发展的条件下进行测量时，囊内硫胺素的积累是无法区分的。观察到的传导性硫胺素摄取的缺失表明胎盘刷状缘膜中不存在介导硫胺素促进扩散的机制。观察到施加内部酸性 pH 梯度会诱导硫胺素集中积累至超过平衡的水平，表明胎盘刷状缘膜质子/硫胺素交换机制的存在。在没有膜电位的情况下，观察到质子载体诱导的内部酸性 pH 梯度的消散消除了硫胺素的集中积累，这表明质子和硫胺素通过介导的交换机制进行直接化学偶联。与通过有机阳离子交换介导硫胺素转运的机制预期的功能特性一致，与不存在向外导向的硫胺素浓度梯度相比，在存在时测量时，囊泡内[3H]-硫胺素积累的速率和幅度增加。质子/硫胺交换机制的底物特异性研究表明，嘧啶环四位的胺，而不是羟乙基侧链或未修饰的噻唑鎓环，是与转运蛋白底物结合位点相互作用的重要化学决定因素。底物特异性研究进一步表明可能存在三种独立的有机阳离子交换机制，介导硫胺素、胍和 MIA 穿过胎盘刷状缘膜的转运。
Pathways for transport of thiamine by the human placental epithelium were investigated using brush border membrane vesicles isolated by divalent cation precipitation. The presence of thiamine transport mechanisms mediating Na+-thiamine cotransport, proton/thiamine exchange and facilitated diffusion was assessed from [3H ]-thiamine tracer flux measurements. The magnitude of intravesicular thiamine accumulation was unaffected by the imposition of an inwardly directed sodium gradient suggesting an absence of a mechanism mediating brush border membrane Na+-thiamine cotransport. Intravesicular thiamine accumulation was indistinguishable when measured in the presence and absence of conditions favoring the development of an inside-negative, potassium diffusion potential. The observed absence of conductive thiamine uptake suggests the absence of a mechanism mediating facilitated diffusion of thiamine in placental brush border membrane. The imposition of an inside-acid pH gradient was observed to induce concentrative accumulation of thiamine to levels exceeding equilibrium, suggesting the presence of a placental brush border membrane proton/thiamine exchange mechanism. Protonophore- induced dissipation of an imposed inside-acid pH gradient in the absence of membrane potential was observed to abolish concentrative accumulation of thiamine, suggesting a direct chemical coupling of protons and thiamine via a mediated exchange mechanism. Consistent with the functional properties expected for a mechanism mediating thiamine transport by organic cation exchange, the rate and magnitude of intravesicular [3H ]-thiamine accumulation was increased when measured in the presence compared to the absence of an outwardly directed thiamine concentration gradient. Substrate specificity studies of the proton/thiamine exchange mechanism suggest that the amine at position four of the pyrimidine ring, but not the hydroxyethyl side chain or an unmodified thiazolium ring, is an important chemical determinant for interaction with the transporter substrate binding site(s). Substrate specificity studies further suggest the possible presence of three separate organic cation exchange mechanisms mediating transport of thiamine, guanidine and MIA across placental brush border membrane.
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