PROTEIN SYNTHESIS AND TURNOVER IN HUMAN CELL CULTURES 697 
synthesized by the cell from glutamine and glucose, are concentratively retained 
by the cell to a generally higher degree; and a wide variety of compounds deriving 
in part from amino acids are also present in the pool, as indicated in the bottom 
section of Table III. 
The absolute amounts of the intracellular amino acids there shown depend on 
the composition of the medium at the time that the cells were harvested. The im- 
portant aspect of those data is the fact that there is a concentrative retention of 
newly synthesized amino acids, and a concentrative transport of amino acids from 

= — ——— = _ + 
A= THREON-INTEST. | 

40XF- x= THREON-HELA 
x G=LYSINE -INTEST. 
35X F m=LYSINE - KB 
@- VALINE - KB 
30x L O= VALINE -HELA 
a. @= VALINE -CONJ. 
aa Se er el 
aoe 
SO oe ee 
| 
= ; O | 
as sige Ses S) | Fig. 1. The concentration of thre- 
onine, lysine, and valine by cul- 
(OO OOZ ZOOSREOl 202) 05" el a2 .o | tured human cells (from EAGLE 
EXTERNAL LEVEL OF SPECIFIC AMINO ACID, mM AND PIEz?*). 
DEGREE OF CONCENTRATION BY CELL 
(RATIO OF AA LEVELS IN CELL AND MEDIUM) 
oS 
~< 




the medium. As shown in Fig. 1, the degree to which the amino acids are concen- 
trated by these cells is in some cases (e.g. valine) essentially independent of the ab- 
solute concentration of that amino acid in the medium. With other amino acids, 
as with lysine and threonine, the degree of concentration is concentration-dependent, 
as if there was a transport mechanism which was being saturated at the higher 
levels of amino acid. 
In bacteria, the addition of large concentrations of one amino acid may have an 
inhibitory effect on the concentrative uptake of a second structurally or metabolically 
related amino acid. In these animal cells, although a given amino acid may similarly 
inhibit the transport of another amino acid, there is often no clear biochemical 
relationship between the compounds involved (Table IV). 
As shown in Figs. 2 and 3, the rate of equilibration between the amino acid pool 
and the external environment is extremely rapid. Whether a high concentration of 
amino acid is added to the medium, or whether the external concentration is suddenly 
reduced, essential equilibration is attained within 15—30 minutes. 
Amino acid depletion 
1. When a cell is depleted of any one amino acid, there are profound structural 
changes, visible under the light microscope within 12-24 h. Those structural changes 
are evident in thin-section electron microscopy. Fig. 4(a) shows a normal cell, with a 
highly developed endoplasmic reticulum. Within 24-48 h after the cell is placed in 
References p. 705 
