PROTEIN SYNTHESIS AND TURNOVER IN HUMAN CELL CULTURES 695 
by these two methods, and they are not distinguished in the following presentation. In 
contrast to the results reported yesterday by Dr. RoBerts, who found significant and 
important differences in the amino acid pools of normal and tumor tissues, we have 
found no demonstrable differences between cultured cells deriving from such tissues!, 
Again in contrast to some of the systems which have here been described, the 
amino acid pool of these cells is not an intrinsic part of the cell, the composition of 
which is largely independent of the environment, but is instead in immediate dynamic 
equilibrium with the medium. The absolute amounts of amino acids within the cells 
must therefore be considered in relation to the composition of the medium at the 
time the cells were harvested. 
Amino acid metabolism 
Table I summarizes the growth medium used in the present experiments, and 
describes the minimum requirements for the serial propagation of mammalian cells 
in culture. Every factor there indicated is essential, and if any one is removed the cells 
die. Although eight amino acids suffice for nitrogen balance in man, these cultured 
ANB I sl 
BIOSYNTHESIS OF AMINO ACIDS IN MAMMALIAN CELL CULTURES 
% of 
0 
% of carbon skeleton deriving from uniformly C-labeled a-Amino N 
Amino acid - io 2 ; . = ; ; . ; R oe =5 + Guta. Gluta-— Argi- from (15)- 
synthesized Glucose Serine Glycine Ribose Pyruvate nee EERE Winee glutamic 
acid 
Alanine 7I-10Il 0.4—1.4 0.2 0.8—4 85 i) 7 = 80 
Serine jot 22-102 1O 63-60 E77 2 (OVO) ID 
Glycine 69-106 25-99 50 58-60 1.9 7.0 — 76 
Asparagine 4 
Aspartic acid 8-23 0.1I-1.3 <0. | ©.48—12 4.7 71-87 100 — 78 
CEES II 0.02—0.2 <JO1 0.31—0.6 I IOI 8 83 
: : = -UL—-U.2 . . Soto’ . a Cc 
Glutamic acid + 3 3s” 4 s 
Proline 3-18 0.09—-0.44 O15  ©.2—0:3 Ber 59 27 26 58 
* In basal growth medium, containing glucose. 
** In medium lacking glucose. 
*** Tn medium lacking glutamine. 
cells require a minimum of 13: the eight “essential” amino acids plus arginine; 
cystine, glutamine, histidine and tyrosine. The remaining amino acids are synthesized 
from glucose and glutamine. 
Glutamine occupies a central role in mammalian cell metabolism, and is the im- 
mediate precursor for the synthesis of glutamic acid, proline, aspartic acid and as- 
paragine? 3. To a minor extent, the proline derives also from arginine, by way of 
ornithine’ ®. Glucose provides the carbon skeleton for serine, glycine and alanine. 
If ribose and pyruvate are used in lieu of glucose, the alanine derives from the pyru- 
vate, and to only a minor extent from ribose; conversely, serine and glycine derive 
from ribose, but not from pyruvate’. In these cell cultures there is no significant 
References p. 705 
