698 H. EAGLE AND K. A. PIEZ 
TABLE IV 
THE EFFECT OF “PRE-LOADING” CELLS WITH AMINO ACIDS 
ON THE TRANSPORT OF [C]VALINE OR [#4C] THREONINE 
Suspension cultures were depleted of valine (or threonine) by resus- 
pension for 1 h at 37° in a medium from which that amino acid had 
been deleted. A single amino acid was then added in large excess 
(10 mW). After 1 h labeled valine or threonine was added for 5 min 
at 37°, and the culture rapidly chilled to o°. The cells were collected by 
centrifugation in the cold, and the radioactivity of the cell pool 
determined. 
Relative amount of labeled amino acid 
in cell pools after 5 min 







Cell 
a eee! Conjunctiva Intestine 
Label 
Valine (0.1 mM ) Threonine (0.1 mM) 
— 100 100 
Arginine 100 98 
Histidine 360 50 
Lysine 73 LEZ 
Tryptophane 19 90 
Phenylalanine 18 125 
Methionine 54 36 
Leucine 2 62 
Isoleucine 55 84 
Tyrosine 33 76 
Cells exposed to label at 0°: 
a) with no pre-loading —— 26 
b) pre-treated with 10 mJ 
phenylalanine 14 27 
a medium lacking a single essential amino acid, one may observe the profound 
structural alterations illustrated in Fig. 4(b). The endoplasmic reticulum largely dis- 
appears; the cell becomes vacuolated, and huge amounts of electron-dense material 
may accumulate. At this point, if one adds back the missing amino acid, there is a 
rapid recovery process. The normal appearance of the cell may be re-established 
within 24 h, and the cell resumes growth at a normal rate’. 
A surprising aspect of these single amino acid deficiencies is the fact that although 
the essential amino acids are not metabolized by the cell to an important degree, 
and are used primarily for protein synthesis, the microscopic appearance of these 
amino acid-deficient cells varies according to the specific amino acid which is 
deleted, 1, 
2. We have now encountered a number of situations in which cells have a rigorous 
requirement for a metabolite which they can synthesize in amounts which should 
suffice for sustained growth, but in the absence of which the cells nevertheless die. 
In all cases so far examined, this paradoxical requirement is population-dependent, 
disappearing at a sufficiently high population density. 
The first example encountered involved the growth of small numbers of cells. In 
the case of bacteria, small inocula often have specific nutritional requirements over 
and above those necessary for the growth of mass populations, Similarly, although 
References p. 705 
