PROTEIN SYNTHESIS AND TURNOVER IN HUMAN CELL CULTURES 699 
100 Teel T T pes] 
50 A. EXTERNAL CONCN. = |OmM 

nm 
{e) 
TE 
! 
B.EXTERNAL CONCN=0.lmM 
fo) 
Oo — 
T T 
| | 
(e) 
ia) 
= 
| 
INTRACELLULAR CONCENTRATION OF SPECIFIC AMINO ACID,mM 
¢ ( ie) oO ro) 
T a] eae, eal 
= | 



Or | 2 4 8 
TIME IN HOURS 
Fig. 2. The concentrative uptake of phenylalanine (©, @) and threonine (@) by HeLa cells. 
Suspension cultures of HeLa cells were resuspended in a phenylalanine-(or threonine-) free 
medium for th at 37°. The amino acid was then added at a concentration of 0.1 or 10 mM, 
and the intracellular concentration of free amino acid determined on aliquot portions of the 
suspension after varying intervals at 37°. The rapid uptake, reaching essential equilibrium in 
less than 1 h, is shown in the figure. 





(itl eaasAcl T 
1S) ° 
re 20 Pek —| 
re 
a 10 F }\ = 
” (= A 
; == 
5 SP} : ; 
} S: 
z 1 
os a \= My - 
BE (ae a B 
a oe _e—__.. 
a SS 
Se | aa 
FZ 
a ° O 
uJ Oro = 
S93 | 
8 = | 
0.2 - = 
aoa \ 
s Oia ees c Z 
= — SS 
: 
7H 0.05 ae D = 
Se) ne 
< = 
Site igre = oe eee 
Fe 
aa a 1 1 a] een 
ot 1 2 4 8 
TIME IN DAYS 
Fig. 3. The rapid loss of free amino acids from the cell to the medium. HeLa cells were preloaded 
with uniformly labeled [!4C|phenylalanine at 10 or 0.1 mM, in a complete growth medium. 
Aliquot portions were then diluted 1 : 10 (curves A, C), or r : 100 (curves B, D), in a phenyl- 
alanine-deficient medium, and the cell free amino acid content determined at the intervals 
indicated in the figure. In curve E, the cells were resuspended in a phenylalanine-deficient me- 
dium. The rapid loss to the medium, reaching essential equilibrium in considerably less than 1 h, 
is apparent in the curves, 
References p. 705 
