704 H. EAGLE AND K. A. PIEZ 
This strongly suggested that the cell was making poliovirus from the amino acid 
pool. This was conclusively shown by DARNELL AND LEVINTOW in labeling experi- 
ments with purified virus?!. With “cold” cell protein and a “hot” amino acid pool, 
the specific activity of the virus protein was essentially as high as that of the pool; 
conversely, with highly labeled protein and relatively unlabeled pool, the viral pro- 
tein was unlabeled. The viral nucleic acid was similarly found to derive from the 
nucleotides of the pool??. 
Protein turnover 
In cultured animal cells, as in bacteria and yeast, there is active protein turnover. 
There are, however, important qualitative and quantitative differences in this 
respect between animal cells and microorganisms. The turnover rate is approx. 
TABLE YW; 
THE CONTRIBUTION OF CELL PROTEIN TO THE AMINO ACID POOL 
Cells were pre-labeled for 7 days in suspension culture in a growth medium containing [14C]valine, 
washed and re-suspended for varying periods in unlabeled medium. The specific activities of the 
free valine in the medium and in the cell pool were then determined. In the table, these specific 
activities have been referred to that of the valine residues in the cell protein as 1oo. 





Time for which Specific Activities of 
labeled cells valine residues in % of free valine in % of protein 
Cell Z DAs ; 
sje were grown in -_— —- - cell pool deriving valine present 
Strain 2 : = = : ; * 
cold medium Cell Cell Median from protein in pool 
(h) protein pool i 
KB 5 100 4.0 22 1.86 0.070 
HeLa 24 100 14.2 10.4 4.2 0.115 
* Protein-derived free valine in pool, referred to total valine in protein. 
1%/h, of the same order of magnitude as that observed in yeast, and less than the 
5%/h rate in bacteria. This is, however, an extraordinarily high turnover rate 
considered in relation to the growth rate. Animal cells grow at a rate of approx. 
3%/h. A 1%, turnover rate per h implies that for every four molecules being syn- 
thesized by the cell, one is being broken down. Teleologically, protein turnover 
could be considered a mechanism for the replacement of inactive enzymes. On this 
basis one may ask why the mammalian cell needs this mechanism to so much larger 
an extent quantitatively than do either bacteria or yeast. An interesting qualitative 
difference between animal cells and microorganisms is the further fact that in the 
former, protein turnover proceeds at essentially the same rate in growing as in resting 
cells. One may estimate from the data of Table V that in cells growing on the me- 
dium of Table I, approx. 2-4°% of the free amino acid of the pool derives directly 
from the cell protein by some turnover mechanism, rather than from the amino 
acids of the medium. The amount of that protein-derived amino acid corresponds 
to approx. 0.1%, of the cell protein. 
Dr. HALvorson has pointed out the possible mechanisms which may be operative 
in this turnover process. The current orthodoxy is that protein turnover represents 
References p. 705 
