AMINO ACID TRANSPORT IN MICROORGANISMS 57a 
an “expandable” osmotically sensitive pool from which exchange is possible and also 
in an “internal” pool which is not readily removed from the cells by osmotic shock, 
does not exchange with external amino acids and is composed of endogenously syn- 
thesized amino acids when growth occurs in the absence of exogenous amino acids. 
HALVORSON AND COHEN® also have encountered evidence for metabolically dis- 
tinguishable amino acid pools in S. cerevisease. ZALOKAR"$ has shown with Neurospora 
that exogenous amino acids can by-pass a portion of the pool during incorporation 
into protein. BRITTEN ef al.!*» 3 using FE. coli have found that amino acids enter a 
very specific pool when extracellular levels are low, whereas at high concentrations a 
much larger and relatively non-specific pool is formed whose size, as indicated above, 
can be increased to very high levels by increasing the tonicity of the extracellular 
medium. The possible heterogeneity of pools in Gram-positive bacteria appears not 
to have been examined critically although HANcock’s findings*® suggest that proline 
pools having different exchange properties exist in S. aureus. 
The relationship between the rate and final capacity of accumulation varies 
greatly. Among Gram-positive bacteria the initial rate at 37° can be maintained for 
thirty to sixty minutes before a limiting capacity is reached for most amino acids. 
Gram-negative bacteria however incubated at 37° achieve a saturating capacity 
usually within one minute. The basis of this difference has not been established but 
may be related to differences in cell wall rigidity. 
Substrate specificity 
Competitive effects by structurally related amino acids and their analogs provide 
additional evidence that amino acids interact with cell components during accumula- 
tion. In GALE’s studies this aspect of the process was not extensively examined, 
although it was shown that asparate reduced glutamate accumulation in S. aureus 
and that it was accumulated under these conditions. A truly competitive interaction 
was not demonstrated. Glutamate accumulation also was inhibited by cysteine, 
alanine and glycine, but the accompanying appearance of extracellular peptide 
suggests that this was not a competitive interaction in the uptake system. COHEN 
AND RICKENBERG”! showed with E. coli that isoleucine, leucine and valine interact in 
a common accumulation process which is relatively unreactive with other amino 
acids. Phenylalanine and methionine accumulation also were reduced only by 
structurally related substances. In all cases the process was specific for L-amino acids. 
A study of valine analogs showed that the amino and carboxyl groups must be 
unsubstituted and that modification of the side chain (substitution of dibutyl for 
dimethyl residues) resulted in an inactive molecule. An additional test of specificity 
was the ability of an amino acid or analogue to displace previously accumulated 
amino acids from the cell. Very good agreement was observed in the ability of a sub- 
stance to reduce accumulation and to displace accumulated amino acid from the pool. 
Glutamate accumulation in L. avabinosus likewise shows a high order of structural 
specificity®2» *4. L-aspartic acid and glutamine are very effective competitors, the 
latter exceeding L-{!2C)glutamic acid in reducing the accumulation rate of L-{1C|- 
glutamic acid. However, p-glutamic acid, asparagine, a-ketoglutaric acid and y- 
aminobutyric acid are all essentially inactive in reducing the rate and the amount of 
glutamate accumulated. 
References p. 592/594 
