BIOSYNTHESIS OF PROTEIN 637 
the total C in the cells was found in the TCA-soluble fraction. To one culture was 
added a mixture of {!C]amino acids at concentrations as shown in Table I. Samples 
were removed from both cultures at intervals following this transfer. 
Table I shows the loss of #C from the TCA-soluble fraction for both the control 
culture and the culture containing the exogenous [!#C]amino acids. No difference 
could be detected in the loss rates in the two cultures. Both cultures continued to 
grow exponentially at the same rate as observed prior to and during the labeling 
period. 
Measurements were made of the radioactivity contained in individual amino 
TABLE I 
EFFECT OF EXOGENOUS [!2C]AMINO ACIDS ON THE APPEARANCE 
OF RADIOCARBON IN PROTEIN AMINO ACIDS 
Radioactivity in protein amino acids 
Prove niamninolaHid (radioactivity in counts|sec) 


[*C]Amino acids present* No amino acids added 
Glutamic acid Fi 7.0 
Threonine 0.5 6.8 
Aspartic acid 13.0 II.5 
Arginine 4.5 Ar 
Alanine 7.0 6.5 
Glycine 8.2 8.3 

* (12C)Amino acids present at concentrations as shown in Table I. 
acids of the proteins obtained from cells at the end of the experiment. Table II 
shows that no significant differences exist in the distribution of #C in these amino 
acids of the two cultures. 
This experiment demonstrates that the exogenous [!?C]amino acids have no 
effect upon the transfer of pool radiocarbon to the protein amino acids. The com- 
petitive effects of exogenous amino acids observed when both [“C}fructose and [12C]- 
amino acids are present during the labeling period therefore occur either before, or 
at the time the amino acids enter the internal pool. Those amino acids already 
associated in the pool are not affected by the presence of the exogenous competitors. 
More direct evidence confirming this conclusion may be obtained by following 
the fate of the carbon of a single pool amino acid. When exponentially growing cells 
briefly immersed in medium containing carrier-free high-specific radioactive glutamic 
acid were washed and transferred to fresh C medium containing no radioactive 
supplement, there was a rapid loss of the labeled glutamic acid of the pool as shown 
in Fig. 4. This radiocarbon eventually appeared as protein [!4C|glutamic acid, {!4C]- 
arginine and [!4C|proline. 
The same rate of loss of pool {!4C}glutamic acid occurs whether or not [!#C arginine 
is present. Similar evidence that the exogenous competitor does not influence the flow 
of the pool material was obtained using cells labeled with carrier-free [!4C|aspartic 
and exogenous [!2C|threonine, also shown in Fig. 4. 
It may be argued that the exogenous amino acids do not enter the cell rapidly 
References p. 645 
