AMINO ACID TRANSPORT IN MICROORGANISMS 591 
activating enzymes function as transport catalysts is the large difference in substrate 
specificity observed for the two processes. 
A number of restrictive findings similar to the results of our nutritional studies 
may be cited. For example, BOYER AND STULBERG® studied the transfer of !80- 
labeled amino acids from growth medium to cell protein in Leuconostoc mesenteroides 
and concluded, on the basis of a large retention of isotope in the protein amino acids, 
that transfer across the cell membrane probably did not require reactions in which 
oxygen loss from the carboxyl group was possible. Since the exchange observed 
could be accounted for by the activation step presumably preceding incorporation 
into protein, one is tempted to conclude that transport involving separate catalysts 
would not lead to formation of bonds which permit exchange in the carboxyl group. 
Alternately, one could suggest that activating enzymes are involved in transport but 
that the activated amino acids proceed directly through the protein-synthetic reactions 
without equilibrating with the free amino acid pool in exponentially dividing cells. 
This concept of transport predicts a by-pass of the pool during protein synthesis 
which in fact has been observed in S. cereviseae™ but not in E. coli!3. In the latter 
case, this conclusion rests on the behavior of very small pools having an uncertain 
relation to the large pools found in Gram-positive organisms. In view of this uncer- 
tainty it would be of interest to determine directly whether accumulation of amino 
acids in non-growing cells leads to exchange. CHRISTENSEN has concluded that in 
Ehrlich ascites carcinoma and in the rat there is no significant loss of carboxyl-18O 
from labeled a-aminoisobutyric acid despite entry, presumed to be active, into many 
tissues of the animal!’, 
BorEzzI AND DE Moss observed that there was not an increase in tryptophane 
activating enzyme, tryptophane synthetase or tryptophane-a-ketoglutarate trans- 
aminase in cells of E. colt which had undergone a large adaptive increase in trypto- 
phane accumulation activity, therefore, apparently excluding these enzymes as com- 
ponents of the accumulation system’. Glutamine has repeatedly been proposed as a 
possible intermediate in glutamate uptake #1 8°. Recently we have isolated a mutant 
strain of L. avabinosus with an absolute growth requirement for glutamine. The 
organism nevertheless accumulates glutamic acid at a normal rate compared with 
its parent. The mandatory intervention of the glutamine synthetase, therefore, 
would seem to be excluded in this system. 
In other transport systems more definitive progress has been achieved in relating 
cellular enzymatic activities and solute accumulation or permeation. Potassium and 
sodium movement in nerve!® and erythrocyte: ** may be associated with the 
operation of a potassium and sodium stimulated ATPase located in the surface 
structures of these cells. Bacterial mutants have been isolated which are defective 
in the ability to accumulate potassium ion®* 10%, SoLomon!®® has now shown 
that the activity of a potassium-dependent ATPase normally present in the mem- 
brane is greatly reduced in this mutant compared to wild-type strains. Such obser- 
vations support the recurrent models suggesting that accumulation systems may 
consist of, or at least include, enzymatic catalysts located in the cell membrane. It 
should be noted that, in contrast, BRITTEN AND McCCLuRE’s analysis of experimental 
findings on amino acid accumulation in F. coli has led to the formulation of a model 
containing a mobile relatively small molecular weight intracellular catalyst??. 
There is extensive evidence showing that net accumulation (although not neces- 
References p. 592/594 
