AMINO ACID TRANSPORT INTO CELLS 535 
proves again for this system a model for methionine or valine, with a high rate of 
transport and a corresponding high inhibitory action. Fig. 5 illustrates that this in- 
teresting amino acid is concentrated approx. 2.7 times into the everted intestinal sac. 
A distinct uphill transport of AIB can also be obtained (Table III) if one uses the 
strategem of JERVIS AND SMYTH!*® to minimize water migration. Ileal gut sacs are 
used, and phosphate buffer is substituted for the bicarbonate system in the medium. 
TABER TL 
CONCENTRATION OF G-AMINOISOBUTYRATE BY THE RAT ILEUM 
Initial level in mucosal phase, 1 mW; in serosal phase, 1 mV in the first 
two experiments, 2 mV in the third. Incubation periods: 60, 110 and 100 
min, respectively. 
Final AIB levels 
Experiment Mucosal phase 

In tissue Serosal phase 
(mM |1) (mM |I cell water) (mM |1) 
I 0.84 3.08 1.39 
2 0.81 3.05 1.42 
3 0.88 2.08 

By this technique JERVIS AND SMYTH were able to show distinct concentration of 
b-methionine by the rat intestine. 
We are currently exploring another line of evidence which at first also suggested 
to us a separate exit process. HEINZ AND WALSH!’ noted a paradoxic failure of accu- 
mulated L-methionine to drive glycine influx, in the way that accumulations of 
glycine, a-aminoisobutyrate and other amino acids do. This failure was attributed 
Oo i 
ACTH. Saeritaen ING 
Exchange - diffusion 
t| | | (Heinz and Walsh) 
AMG CEA 
A_+AC ——— AC | A ‘ 2 
Exchange - diffusion 
by a 
displacement reaction 
Aaa AG ——Ye, AC + 4 

Fig. 6. Schemes for exchange diffusion. Above: as visualized by HEINz AND WALSH!’. Below: 
assuming a displacement reaction without the intermediate release of the transport site or carrier. 
to a particularly strong inhibitory action of escaped methionine on glycine uptake 
at the external membrane surface. But why should the mass-action of methionine 
from outside predominate over its mass-action from inside, when for glycine the 
opposite relationship is found? A suggestion arises here of a different order of affinities 
References p. 538 
