676 F. C. STEWARD AND R. G. S. BIDWELL 
acids in the cell were regarded as having originated largely from the protein break- 
down in phases in the cell which are concerned largely with catabolism. The nitrogen 
from protein breakdown had to be donated in acceptable form to the site of protein- 
re-synthesis, whereas the carbon skeletons of these protein breakdown products 
were respired away as carbon dioxide. Therefore, the point was made that any 
amino acids that occurred as intermediates of protein synthesis at the site of protein 
synthesis were not free to mingle with the large pool of soluble nitrogen compounds 
in the cells. In this sense the free amino acids which exist in quantity in plant cells 
are not to be regarded as the immediate precursors of protein, for the carbon of the 
protein can be derived more readily from the sugar entering the cell than from the 
free amino acids. 
The main idea that emerged, therefore, from these experiments on carrot tissue 
explants is as follows. Since the same substances may exist in different regions of the 
cell as free compounds, the availability of their carbon for incorporation into protein 
will depend greatly on the location in the cell and upon the ease of access to the site 
of synthesis. 
However, even in the carrot tissue, evidence was obtained that some amino acids, 
e.g. proline, were directly incorporated into a protein fraction, albeit into a protein 
fraction which was not subsequently metabolized*: #. This occurred so readily, 
however, that when /C-proline was externally applied it did not accumulate ex- 
tensively in the free soluble fraction, nor did it enter that fraction even when the 
tissue was allowed to metabolize for a further 72h in the absence of exogenous 
labeled proline?®. Thus, in the growing carrot stimulated by coconut milk, the soluble 
pool of amino acids is reduced to a low level, and there is obvious evidence of ready 
withdrawal of the metabolites from the soluble pool into the metabolic system. 
The situation, however, in cells of the Jerusalem artichoke tuber is now known to 
be somewhat different. This was first shown by the analysis of the soluble nitrogen as 
it exists in artichoke tissue explants which were prompted to grow in the presence 
of coconut milk and/or naphthaleneacetic acid. In this work, carried out recently by 
Dr. Roprnson and Dr. DURANTON with one of us (F.C.S.), it is evident that the cul- 
tured artichoke cells retain in the soluble pool a much higher concentration of a 
variety of amino acids and of proline in particular. This strong retention is so appa 
rent that the tissue utilizes exogenous supplies of amino acid to form protein even 
much more readily than it does the endogenous supplies, a result which contrasts 
sharply with that obtained with carrot. 
Other areas of metabolism not specifically involving amino acids and protein 
synthesis nevertheless lead to somewhat similar considerations; namely that there 
are different compartments, or pools, in the cells in which the fate of a given meta- 
bolite may be quite different. VirrorIo, KRoTKOV AND REED® supplied C-labeled 
glucose to tobacco leaves. The specific activity of the sucrose which was formed in 
the leaves and was isolated from them proved to be actually higher than the specific 
activity of the free glucose or fructose of the leaves. This obviously means that there 
was a pool or compartment which contained non-radioactive glucose and which was 
not available to take part in sucrose synthesis. Much more recent work of VICKERY 
AND ZELITCH® leads to somewhat similar ideas. These investigators supplied tobacco 
leaves with labeled pyruvic acid, and they investigated the specific activity of the 
citric acid in the leaves. They were led to conclude that there must be two pools of 
References p. 692/693 
