536 H. N. CHRISTENSEN et al. 
for one side of the membrane than for the other. JACQUEz!8 has recently reported a 
similar failure of accumulated glycine to accelerate the subsequent uptake of L-tryp- 
tophane, although tryptophane and azaserine were mutually effective in driving ex- 
change. 
A prior, exchange rates could prove to be valueless for measuring affinities. 
HEINZ AND WALSH proposed that exchange occurs through a dissociation—association 

Distribution ratio at 5 min 
iN 
-— 
ho 
i 
Valine O 
PECLOCCie pats aa siol= = eas ae 
GLYCINE AIB METHIONINE CYCLOLEUCINE 
Fig. 7. Effect of valine previously accumulated from a 1o-mM solution on the uptake of several 
amino acids at 1-mM levels by the Ehrlich ascites tumor cell. 
sequence (Fig. 6, upper diagram). But if exchange with another amino acid occurs 
without the preliminary dissociation of the carrier-solute complex (lower diagram), 
factors quite distinct from relative affinities may determine the rate. We know for 
example that certain highly stable chelates will exchange their constituent parts 
more quickly than other less stable ones. 
Nevertheless we have analyzed the exchange relationships between five amino 
ABER AV: 
STIMULATING ACTION OF METHIONINE PREVIOUSLY TAKEN UP ON THE UPTAKE OF AMINO ACIDS 
BY HUMAN ERYTHROCYTES 
To heparinized whole blood was added t-methionine, 20 mM/l. After incubation at 37° for 
40 min, the cells were separated and washed twice with ice-cold plasma and then resuspended 
in 10 parts of plasma containing the indicated “C-labeled amino acid at a 5-mM level. After 
incubation the cells were extracted with trichloroacetic acid and the “C measured by liquid- 
scintillation counting. 
Distribution ratio cells|/plasma 
Amino acid After © min After 5 min 
Control Preloaded Control Preloaded 
Glycine 0.21 0.18 0.18 0.19 
Valine 0.38 0.53 0.81 1.00 
Methionine 0.42 1.07 0.81 1.28 
References p. 538 
