AMINO ACID TRANSPORT INTO CELLS 537 
acids: valine, methionine, cycloleucine, AIB and glycine, in the manner illustrated 
in Fig. 7. Here valine preaccumulated by the Ehrlich cells from a 10-mM solution, 
is seen to be highly effective during 5 min in stimulating the exchange-uptake of 
methionine, cycloleucine and valine, but without significant influence on the uptake 
of glycine and AIB. In a similar way we find that the presence of 10 mM valine in 
the outside environment stimulates the exchange-loss of cycloleucine, but not of 
glycine or AIB. The relationships are the same from the two sides, indicating that 
exchange-diffusion probably operates on the same site or carrier at both faces of 
the membrane. In this way one of the four possible points for the introduction of 
GLYCINE AIB CYCLOLEUCINE VALINE METHIONINE 







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Fig. 8. Comparison of the ability of five previously accumulated amino acids to stimulate the 
uptake of each other. A single lot of Ehrlich cells was divided into six parts. Each of five of these 
portions was incubated 20 min in a 1o-m/ level in Krebs—Ringer—bicarbonate medium of the 
amino acid listed at the top. The cells were then washed once with ice-cold buffered medium, 
and then suspended for 1 min at 37° in fresh medium containing 1-m/V/ levels of the amino acids 
shown at the bottom of the figure, in labeled form. The amount of radioactivity taken up is com- 
pared here with the amount taken up by the control cells previously incubated in the amino acid 
free medium, setting the latter at 100%. For example the first five bars show that previously 
accumulated glycine failed to stimulate the uptake of any of the five amino acids. 
energy, as outlined by PATLAK!?® in his generalized model, namely to modify the 
solute—carrier combination, is made less probable. 
Fig. 8 illustrates the approximate exchange relationships found among these five 
amino acids, allowing I min at 37°. The three amino acids, valine, cycloleucine and 
methionine, exert very potent drives on each other, whereas they have no notice- 
able tendency to drive counterflux of glycine or AIB. When we extended the period 
of observation, AIB was observed to drive the counterflow of glycine, in agreement 
with HEINZ AND WALSH!’. Table IV shows that the same general situation applies 
for erythrocytes. Preaccumulated methionine is able to cause these cells to con- 
centrate C-methionine or valine, but not glycine. 
This result again places glycine and AIB in a separate class. This separation no 
doubt arises from the marked difference in affinity between the two sets. An amino 
acid with a high affinity responds by counterflow to another also with high affinity; 
they tend to change places with high efficiency. In contrast the relatively slowly 
References p. 538 
