Anand R J, Kanwar U, Sanyal S N
Department of Zoology, Panjab University, Chandigarh, India.
Res Exp Med (Berl). 1996;196(1):29-43.
Brush border (microvillous) plasma membranes (BBM) and the basal surfaces (BCM) from the syncytiotrophoblast of human term placenta were prepared by a method of sonication, dialysis and differential centrifugation in specific buffer systems. Such plasma membranes formed closed, osmotically active, right-side-out vesicles in which amino acid transport could be studied unidirectionally by a carefully designed membrane filtration assay under reduced pressure. In such vesicles, L-leucine (1 mM) was found to be transported in a time-dependent manner, peak accumulation being attained at 45 s in both BBM and BCM. The accumulation of L-leucine in the vesicles was dependent on an inward NaC1 gradient, as replacing the Na+ with K+, Li+ and choline, or replacing the C1- with S0(2-4) failed to influence the amino acid movement. Leucine transport in the vesicles also appeared to be dependent on the substrate concentration, indicating saturation at a higher concentration. The transport process showed a k(t) (affinity constant) of 3.85 and 6.67 mM, while the values recorded for the J max (maximum apparent initial velocity) were 270.27 and 384.62 nmol/mg protein-1 per min in the BBM and BCM respectively. Leucine transport was inhibited by a number of amino acids, among which amino isobutyric acid (AIB) produced the maximum inhibition. The k(i) inhibition constant) for different amino acids has also been listed. Lysine showed the least k(i) value, thus showing it to be the most inhibitory compound. These findings are discussed in relation to the mechanism and regulation of transplacental amino acid transfer.
通过在特定缓冲系统中进行超声处理、透析和差速离心的方法，制备了来自足月人胎盘合体滋养层的刷状缘（微绒毛）质膜（BBM）和基底表面（BCM）。这种质膜形成了封闭的、具有渗透活性的、外翻小泡，在其中可以通过精心设计的减压膜过滤试验单向研究氨基酸转运。在这种小泡中，发现L-亮氨酸（1 mM）以时间依赖性方式转运，BBM和BCM在45 s时均达到峰值积累。小泡中L-亮氨酸的积累依赖于内向的NaCl梯度，因为用K⁺、Li⁺和胆碱替代Na⁺，或用SO₄²⁻替代Cl⁻均未能影响氨基酸的转运。小泡中的亮氨酸转运似乎也依赖于底物浓度，表明在较高浓度下会饱和。转运过程的k(t)（亲和常数）为3.85和6.67 mM，而BBM和BCM中J max（最大表观初始速度）的记录值分别为270.27和384.62 nmol/mg蛋白每分钟。亮氨酸转运受到多种氨基酸的抑制，其中氨基异丁酸（AIB）产生的抑制作用最大。还列出了不同氨基酸的k(i)（抑制常数）。赖氨酸的k(i)值最小，因此表明它是最具抑制作用的化合物。结合胎盘氨基酸转运的机制和调节对这些发现进行了讨论。
