PROTEIN SYNTHESIS AND TURNOVER IN HUMAN CELL CULTURES 703 
each of these external concentrations. Fig. 5 shows the rate of growth, and thus of 
protein synthesis, as a function of these intracellular concentrations. There was a 
critical threshold level of amino acid (0.01-0.04 mM) below which protein synthesis 
proceeded at an insignificant rate. In excess of this critical level, the growth rate 
increased sharply to reach maximal levels at internal pool concentrations only two 
or three times the minimal effective level. Further increase in the intracellular amino 
acid concentration had no effect on the rate of protein synthesis. 
Amino acid pools and virus synthesis 
In the synthesis of poliovirus by HeLa cells, the only metabolites required by the 
cell for maximal production of poliovirus were found to be glucose and glutamine”’. 
Clearly, the cell can provide from its own substance everything else needed for the 



Oo 
O [ == = a ao 
a2 OO} 
wo 
a 
= 
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= 80} 
x< 
a 
= 
2 
O 60;— 
WW 
a 
a 
ul 
ve le 
i 40 
a 
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Se) | | | | Fig. 5. The rate of growth of four human cell lines 
© -0! 02 05 | -@ mM asa function of the intracellular concentration of 
INTRACELLULAR CONCENTRATION valine, lysine and threonine (from EAGLE AND 

OF SPECIFIC AMINO ACID jenuDyA 
Symbol Cell strain Amino acid 
ww, Conjunctiva Valine 
e KB Valine 
O HeLa Valine 
& KB Lysine 
O Intestine Lysine 
x HeLa Threonine 
synthesis of viral protein and nucleic acid. What that early study did not make clear 
was whether the pool amino acids and nucleotides were being used, or whether in- 
stead the cells’ own protein and nucleic acid were being broken down to provide the 
materials for viral synthesis. It was subsequently found that if the cell population 
density were reduced in an amino acid-free medium, and the cellular amino acid 
pool thereby drastically depleted, the cells did not elaborate poliovirus. The com- 
petence of the cell to make virus was totally restored if amino acids were added 
back to the medium?”?. 
References p. 705 
