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Regulatory and molecular aspects of mammalian amino acid transport.

The transport of amino acids across the plasma membranes of mammalian cells is a process of fundamental physiological importance and has been the object of extensive study. The characteristics, kinetics and regulation of various transport systems have been understood in general terms for many years, but until recently little was known about the structure and molecular properties of amino acid transport proteins. The great advances made in the cloning ofgenes for some of these transport proteins since 1991 have provided the impetus for the present review. The review will concentrate on the recent advances made in this area and also on a major outstanding problem in mammalian amino acid transport, i.e. the regulation of the major inducible amino acid transport system, known as System A. These topics to a large extent reflect the interests of the authors. Various aspects of amino acid transport are comprehensively covered in recent reviews [1,2] and in articles in a recently published book [3]. Initial work on the systematic characterization of amino acid transport systems was performed on Ehrlich ascites cells by Christensen and collaborators in the 1950s and 1960s (e.g. [4,5]) and similar investigations were subsequently performed by these and by other investigators to characterize amino acid transport in many other cell types, notably erythrocytes, hepatocytes and fibroblasts. The following general properties were established: first, amino acid transport systems in general are highly stereospecific, transporting L-amino acids much faster than D-amino acids. Secondly, these transport systems have a rather low substrate specificity when compared with most enzymes; thus a number of different amino acids may be transported on one transport system and in general a number of different transport systems with overlapping specificities may occur in the same cell membrane. Thirdly, transport systems can be divided into two categories, i.e. those that catalyse amino acid uniport and those that catalyse electrogenic Na+-amino acid symport. In the second case the electrochemical potential gradient of Na+ drives amino acid accumulation across the plasma membrane. Kinetic and specificity studies on amino acid transport systems have been widely performed on intact cells and on membrane vesicles. Methods used in these studies have been reviewed elsewhere, as have the criteria for discriminating between different transport systems in the same membrane [6-8]. If two amino acids are transported on a common carrier, and this is the only major route of transport, then the transport of each amino acid should be competitively inhibited by the other and the Ki for inhibition of transport of one amino acid by another should equal the K, value for the transport of the inhibitory amino acid [9]. The importance of establishing these criteria has not always been fully recognized, and in some early reports multiple transport systems were postulated where it is now apparent that fewer systems of broader specificity exist.