AMINO ACID TRANSPORT INTO CELLS 5Bu 
but proline and alanine failed to inhibit the uptake of this amino acid. The prelimi- 
nary indications are that the uptake of a-aminoisobutyrate is probably not as readily 
inhibited as it should be if we are dealing with a simple, single-affinity sequence, 
but certainly no striking inconsistencies are yet evident for the red blood cell. 
We turn then to the Ehrlich ascites cell, for which the uptakes of several amino 
acids have been rather extensively proved concentrative, and which accumulates a 
great variety of amino acids. 
TABLE Il 
INITIAL UPTAKE RATES AND DISTRIBUTION RATIOS AT 30 MIN 
FOR A NUMBER OF AMINO ACIDS IN THE EHRLICH CELL 
The steady-state external levels were in the range 0.5—0.9 mV. 
Uptake rate rst min Distribution ratio 
Amino acid (umoles|/ml cell water|min) at 30 min 
L-Methionine 3.52 1229 
Cycloleucine 2.90 18.7 
pL-Norleucine 2.87 10.2 
L-Leucine 2eB2 3.3 
L-Histidine 2.08 W2e7 
L-Valine 1.55 5.0 
a-Aminoisobutyrate 127 26.3 
L-Proline 1.20 11.8 
p-Alanine 1.0 4.60 
L-Isovaline 0.85 10.1 
p-Isovaline 0.45 8.2 
Glycine 0.67 10.5 

Table II compares the uptake behavior of a number of amino acids in molar 
terms. Here again we see the same sequence of increasing rates: glycine, alanine, 
valine and the leucines, using the 1-min values. The positions taken by L-isovaline 
and a-aminoisobutyrate are slightly different from those of the erythrocyte. But many 
of the very rapidly accumulated amino acids soon reach their steady-state levels, 
while the uptake of others continues much longer, until the inverted order: leucine, 
valine, alanine, glycine is assumed, the more slowly accumulated members of this 
aliphatic series being concentrated the most. This simple relationship does not 
apply outside the simple r-aliphatic series. For example methionine has a rapid 
uptake but also yields a steep gradient. Note that the initial rate for L-isovaline is 
nearly twice that for the p-isomer, whereas the distribution ratios at 30 min are 
quite similar. This is the result predicted above from comparing our results with 
those of PAINE AND HEIN2Z?. 
Fig. 2illustrates the progressive uptake of three contrasting amino acids types: 
leucine, which is taken up quickly but briefly; glycine, which is taken up more 
slowly but continuing until a much higher distribution ratio is reached; and cyclo- 
leucine and methionine which are taken up quickly but to high gradients. 
The aliphatic group in the L-position appears to assist in the initial combination 
with the transport site, but may be unfavorable to the loss of affinity on the hypo- 
thetical site modification. Either a polar atom in this aliphatic group, as in methionine, 
or the shift (as in p-alanine and p-isovaline) or extension of the aliphatic group 
References p. 538 
