AMINO ACID TRANSPORT IN MICROORGANISMS 575 
washes whereas material accumulated from the medium and present in the so-called 
expandable pool is rapidly lost under these conditions. Such pools appear also to 
be distinguishable by their exchange properties; amino acids in the internal pool do 
not exchange with extracellular amino acids, whereas those in the expandable pool 
do. The tenacious retention of large pools during exposure to buffer at 37° in the 
absence of exogenous amino acids is not explicable by the so-called permease concept 
which predicts a sizable leak under these conditions. However, retention of the pool 
on adsorption sites is not a necessary alternate explanation. As will be seen below, 
studies with L. avabinosus suggest that this property is largely determined by the 
rigidity of the cell wall and that the phenomenon can be accommodated by either 
the adsorption site or membrane transport concepts of the pool. 
Amoles GLUTAMATE /!00mg CELLS 


0 30 60 90 
MINUTES 
Fig. 3. Retention of accumulated glutamic acid by L. avabinosus. Cells were allowed to accumulate 
L-[44Cjglutamic acid for 75 min at 37°. In these and the reincubated cells intracellular glutamic 
acid was measured isotopically. After centrifugation the cells were resuspended in water or 0.12 VW/ 
phosphate buffer (pH 6.5) containing the indicated supplements and incubated at 2° or 37° as 
follows: @— — —@®@, water, 2°; O——-— O, water + L-[!*C]glutamic acid (0.003 M), 2°; 
C) @, water, 37°; O @p butters 7 e N A, buffer + tL-[}2C]glutamic acid 
(0.003 VM), 37°; A A, buffer + 1-[??C]glutamic acid (0.003 WM) + glucose (0.028 M), 37°. 
Separate batches of cells were used for experiments at 2° (-———) and 37° ( 
i 





Accumulated amino acids generally can be eluted by exogenous amino acids and 
their analogs. As indicated previously, amino acids which compete with the test 
amino acid when uptake is measured will also elute previously accumulated amino 
acid from the cell’®: 71, >. In E. coli the studies of COHEN AND RICKENBERG do not 
show whether such displacements are dependent on cell metabolism. BRITTEN et al.}? 
using growing cells have considered extensively the relation between the uptake and 
exchange reactions. They find that the rate of proline uptake into the pool is reduced 
by a factor of 270 when the incubation temperature is reduced from 25° to 0°, whereas 
the exchange rate is reduced only to one-fourth of the control value. Proline exchange 
at o° does not follow a simple exponential curve and Britten concludes that there 
are at least two separate components of the pool which exchange at different rates. 
Both exchange rates appear to be independent of extracellular amino acid concen- 
tration and dependent on the intracellular pool size. HEINz* has reported acceleration 
of glycine uptake into Ehrlich ascites carcinoma cells preloaded with this amino acid. 
A comparable finding using microorganisms has not been reported. 
References p. 592/594 
