638 D. B. COWIE 
enough to compete with the pool amino acids. The kinetics of incorporation of [14C]- 
glutamic acid, shown in Fig. 5 indicate that such is not the case. The exogenous 
amino acid is rapidly incorporated by the cell with significant quantities appearing 
in the TCA-soluble and protein fractions within a few minutes incubation. 
When higher concentrations of exogenous amino acid are available, the quantity 
of amino acids contained in the cold TCA-soluble fraction increases. Table III 
TABLE III 
DISTRIBUTION OF RADIOCARBON AMONG POOL AND PROTEIN 
AMINO ACIDS 
Cells grown from light inoculum to approx. 2.3 mg wet weight 
of cells per ml medium. 

GOnTEniV AEN pmoles [!C \threonine/g dry wt. cells* 



of exogenous se eee 
(4C\threonine TCA Die : Cold ; 
(uwmoles/ml medium) A-soluble fraction TCA-precipitable fraction 
52:8 HAZ 520 
Pe 394 446 
0.7 268 328 
5:0 190 272 
5A 104 149 
1.7 2 ep 
0.8 9 41 

* Calculated on the basis that all the radiocarbon incorporated re- 
mained [!4C|threonine. 
shows the #C content of the cold TCA-soluble fraction for various concentrations 
of exogenous [!4C]|threonine. Other amino acids may be also concentrated in the 
cold TCA-soluble fraction as shown in Table IV. When these accumulations are 
compared with the steady-state amino acid concentrations in the internal pool 
shown in Table V, it is apparent that not only are exogenous amino acids rapidly 
incorporated (Fig. 5) but that accumulation to levels greatly in excess of the internal- 
pool concentrations are possible. 
TABLE IV 
AMINO ACID ACCUMULATION OF THE COLD TCA-SOLUBLE FRACTION 
In each experiment, cells grown exponentially for 150 min in C medium containing 
the labeled amino acid. 



~ eat Cold 
Experiment right eae id ( second # See ; TCA-soluble fraction 
[Camino acta ponoles|ml medium ) (umoles|g dry wt. cells) 
I Arginine Ge 158 
2 Proline a7) 170 
3 Valine Way, 225 

References p. 645 
