636 D. B. COWIE 
kinetics are characteristic of a more complex process in which fool glutamic acid is 
converted to pool arginine before protein incorporation occurs. In Candida utilis 
arginine carbon is supplied from glutamic acid?!. Similar pool amino acid inter- 
conversions have been observed among other “parental” amino acids and their 
family “members” and this pool has been designated as the “internal pool” of amino 
acids? °. 
The appearance of !@C in the individual amino acids of the proteins was also 
shown to be directly correlated with the loss of radiocarbon from the corresponding 
amino acids of the internal pool’. The rate of transfer of each pool amino acid to 
protein is therefore dependent upon the protein requirements. 
Isotopic competition studies 
a. Competition between exogenous amino acids and fructose carbon. 
Exogenous amino acids reduce the flow of fructose carbon into protein (ref. I, 
chapt. 13). Thus when a non-radioactive amino acid is supplied to medium con- 
taining {!C)fructose, a reduction occurs in the radiocarbon incorporation in the 
same amino acid in the proteins. When a family “head” amino acid is used to supple- 
ment the radioactive medium, that amino acid and all of its family “members” 
TABLE I 
EFFECT OF EXOGENOUS [!2C]AMINO ACIDS ON THE LOSS OF RADIOCARBON FROM 
THE TCA-SOLUBLE FRACTION IN EXPONENTIALLY GROWING Candida utilis 


Total radioactivity in TCA-soluble fraction 
(radioactivity in counts|sec) 





Time (min) Mg wet wt./ml medium : ps A = 
[°C]Amino acids present* No amino acids present 
5 0.59 20.8 20.8 
17 0.07 22 12.4 
35 0.82 8.2 8.3 
62 1.00 4.7 4.9 
92 1.35 2.9 3.0 

* (l2C\)Amino acids present as competitors (mg/ml medium): aspartic acid 0.75; glutamic 
acid 0.75; alanine 0.50; valine 0.25; glycine 0.25; lysine 0.25; serine 0.25; arginine 0.19; leucine 
0.19; proline 0.12; threonine 0.12; isoleucine 0.12; methionine 0.10; tyrosine 0.10. 
will contain reduced quantities of fructose carbon. The preformed amino acid thus 
effectively competes with the fructose carbon for protein formation. 
b. Competition between exogenous amino acids and internal pool material. 
Since the flow of carbon from fructose has been demonstrated to pass through the 
amino acid internal pool the effects of exogenous amino acids upon pool formation 
and utilization have been investigated. 
An exponentially growing culture of Candida utilis immersed for 10 min in me- 
dium containing [!4C}fructose, was washed, divided into two equal fractions, and 
quickly transferred to fresh C medium containing [!C]fructose. More than 70% of 
References p. 645 
