Hopfer U
Horiz Biochem Biophys. 1976;2:106-33.
The molecular basis of intracellular metabolism of nutrients and its control is quite well understood in animal cells. Comparable knowledge about solute entry into cells is still lacking, as, in contrast to metabolism, no chemical reactions seem to be directly associated with the known nutrient transport. Nevertheless, translocations of sugars and amino acids across the plasma membrane are specific and controlled processes, biologically as well as chemically. Recent advances in techniques for isolation of plasma membranes have made it feasible to study transport properties of animal cells without the complications encoutered in viable cells. This approach has been applied to sugar and amino acid transport in plasma membranes of several tissues, and intact transport systems for D-glucose, D-fructose, neutral L-amino acids, and dipeptides have been demonstrated. This demonstration of intact transport systems in an in vitro setting accomplishes the first step in the direction of molecular isolation of transport systems. Furthermore, the information obtained about the transport mechanism catalyzed by some systems has settled controversies on active nutrient transport. For example, electrogenic cotransport of sodium and D-glucose or of sodium and neutral L-amino acids has been shown to form the basis for active, sodium-dependent absorption of these nutrients. A consequence of this type of mechanism is interaction between sugar and amino acid transport via the common charged cosubstrate sodium. Moreover, different types of transport systems for the same substrate have been demonstrated in the luminal and contraluminal regions of the plasma membrane of epithelial cells, which explains unidirectional transepithelial transport. The luminal membrane contains sodium-dependent, active transport systems, and the contraluminal membrane passive, facilitated diffusion systems. In vivo, the lower intracellular sodium potential would result in concentrative nutrient uptake from the lumen, but would not influence exit on the contraluminal side. Variations in the electrical components of the sodium potential, which have not been measured, may explain apparently contradicting results on active sugar and amino acid transport with various tissue preparations.
在动物细胞中，营养物质细胞内代谢的分子基础及其调控已得到相当深入的了解。然而，与代谢不同，由于似乎没有化学反应直接与已知的营养物质运输相关联，关于溶质进入细胞的类似知识仍然匮乏。尽管如此，糖类和氨基酸跨质膜的转运是特定且受调控的过程，无论在生物学还是化学层面皆是如此。质膜分离技术的最新进展使得研究动物细胞的转运特性成为可能，而无需面对活细胞中遇到的复杂情况。这种方法已应用于多种组织质膜中的糖类和氨基酸转运研究，并且已证实存在针对D - 葡萄糖、D - 果糖、中性L - 氨基酸和二肽的完整转运系统。在体外环境中对完整转运系统的这一证实，朝着转运系统的分子分离迈出了第一步。此外，通过某些系统获得的关于转运机制的信息解决了有关营养物质主动转运的争议。例如，钠与D - 葡萄糖或钠与中性L - 氨基酸的电生性协同转运已被证明是这些营养物质主动的、钠依赖性吸收的基础。这种机制的一个结果是糖类和氨基酸转运之间通过共同的带电荷共底物钠发生相互作用。此外，上皮细胞质膜的管腔面和对管腔面区域已证实存在针对同一底物的不同类型转运系统，这解释了单向跨上皮转运现象。管腔膜含有钠依赖性主动转运系统，而对管腔膜含有被动的易化扩散系统。在体内，较低的细胞内钠电位会导致从管腔中集中摄取营养物质，但不会影响在对管腔侧的排出。尚未测量的钠电位电学成分的变化，可能解释了使用各种组织制剂进行的主动糖类和氨基酸转运研究中明显相互矛盾的结果。
