Sugar and amino acid transport in animal cells.

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.