530 H. N. CHRISTENSEN et al. 
forward affinity sequence. Although the apparatus for uphill transport, functional 
at the reticulocyte stage, is probably still complete in the mature cell, the energy 
delivery may very well have become inadequate. In this event very little of the 
hypothetical site modification will occur, and transport in both directions may be 
mediated symmetrically by a single site. Some of you might tend to consider the up- 
take of amino acids by such cells as irrelevant to our problem since “pooling”, if we 
understand the sense in which the term is now often used‘ hardly seems to occur. 
Table I shows a comparison of the entry of a number of amino acids at 0.8—1.2-mM 
levels into human erythrocytes, as observed by “@C-labeling. You will note in this 
table the sequence: glycine, alanine, valine, leucine, in which the rate of uptake 
increases as the length of the hydrocarbon chain increases. Either the N-methyl or 
the a-methyl group decreases the rate, as does the a-trifluoromethyl group. The 
latter derivative, an anionic amino acid, nevertheless enters the erythrocyte faster 
than it does other cells studied, probably because of the facilities for anion transport 
possessed by this cell. A transport advantage of the L- over the D-configuration is 
evident in two cases here, for alanine and isovaline. 
The increase of rate with length of hydrocarbon chain might be interpreted in 
either of several ways. One could suppose that the increasing lipophilic nature 
might permit passage across a lipid barrier, perhaps in the form of a complex in 
which the amino and carboxyl groups are masked. This hypothesis fails to account 
for the advantage of having the longer side-chain in the position characteristic of 
the L-series. Furthermore one doubts that a liquid phase of randomly orientated 
lipid is encountered. More likely London forces, the so-called hydrophobic bond, 
between the hydrocarbon side-chain and a hydrocarbon structure in the membrane 
helps to orientate the amino acid molecule with reference to the two primary bonding 
points. One might propose that the side-chain forms a distinct and essential third 
point of attachment as suggested by PAINE AND HEINz?. Against this idea stands 
the wide variety of side-chains that serve, as shown more extensively for the Ehrlich 
ascites tumor cell, and also the ability of a hydrogen atom to serve in the case of 
glycine. 
Cycloleucine is the name used here for convenience for I-aminocyclopentane-I- 
carboxylic acid*. The rapid uptake of this amino acid by all tissues studied refutes 
any idea that an a-hydrogen is necessary for typical amino acid transport. Its trans- 
port into red blood cells is inhibited by leucine, whereas it in turn inhibits the trans- 
port of a-aminoisobutyrate, as one would expect from the order of their rates of 
uptake. 
Limited further exploration of the inhibitory effects in erythrocytes suggests that 
the other amino acids, except for trifluoromethylalanine, also belong in the same 
transport family. For example the uptake of valine can be inhibited by valine itself, 
or by alanine in a 20 : I excess. The uptake of proline is inhibited by leucine, nor- 
leucine and valine, as would be expected. Leucine and valine, as well as cycloleucine, 
inhibit the uptake of a-aminoisobutyrate (a-methylalanine), as one would expect, 
* This amino acid has recently been shown to be toxic when fed to rats, and to have anti- 
tumor activity®. It does not specifically antagonize the utilization of any of 17 amino acids 
tested in several bacterial species’. Conceivably its transport behavior underlies its toxic action. 
We are investigating its metabolic fate. The labeled amino acid can be obtained from the Cali - 
fornia Corporation for Biochemical Research. 
References p. 538 
