074 F. C. STEWARD AND R. G. S. BIDWELL 
SECTION III. SOLUBLE POOLS IN RELATION TO PROTEIN SYNTHESIS AND BREAKDOWN 
The classical view is that all carbon for protein enters via the soluble amino acids 
which occur free in the cell. Much evidence indicates that this is probably true for 
animals; but in plants, which possess a much greater reservoir of soluble nitrogen 
compounds, this is by no means as obvious, for the constituents of this pool may 
need to be extensively re-worked prior to protein synthesis. Also, the early products 
of photosynthesis seem to be especially accessible to protein formation, in fact much 
more so than the stored amino acids. 14CO, is stored during photosynthesis to a very 
surprising degree in the protein, or at least in the alcohol-insoluble fraction and, 
therefore, presumably in protein; and, concurrently, very few soluble amino acids 
are formed in the light in such a way that they receive the label. A suggestive quota- 
tion is the following taken from BENSON AND CALVIN (cf. ref. 2, p. 30): “The major 
portion of the insoluble products formed in the first few minutes by algae ... was 
protein. ... Protein obtained from longer experiments (5-10 min) contained more 
activity than that found in several amino acids present in the cell extracts. Glutamic 
acid, by far the largest amino acid reservoir, was not converted into protein in amounts 
commensurate with its concentration”. By contrast, however, the conditions of dark 
fixation produce a great variety of amino acids in such a way that they become 
labeled, and it was this contrast that led CALVIN et al. for a while to assume, erro- 
neously, that respiration in the light virtually ceased’. This indicates at the outset, 
therefore, that the carbon which enters protein in the light must do so by a more 
direct route than via the soluble amino acids which are stored in bulk in the cell. 
Recent experiments by this group*® have led to the conclusion that protein amino 
acids are synthesized rather directly from the intermediates of the carbon reduction 
cycle. It has been shown that protein synthesis occurs, not at the expense of the 
total pool of free amino acids, but from small “active” pools which are separated in 
compartments from the larger “inactive” pools. 
An earlier Russian experiment is interesting: ANDREEVA! added 1C-labeled 
sugars, !©N-labeled ammonium sulphate and non-isotopic carbon dioxide to tobacco 
leaves in light. He found that whereas N rapidly entered proteins, #C did so much 
more slowly. This indicated that the carbon precursors of protein synthesis are derived 
more or less directly from photosynthetic carbon, and are not readily mixed with 
those derived from soluble compounds in the cell. 
Similar conclusions emerged from experiments on carrot tissue cultures in which 
the availability of the carbon in various exogenously applied and labeled substrates 
for protein synthesis was investigated. The compounds used consisted of labeled 
sugar and two labeled nitrogen compounds, namely glutamine and y-AB. Prior to 
these experiments one might have anticipated that the labeled sugar on absorption 
would have intervened directly in the oxidative reactions of respiration and have 
produced carbon dioxide which would tend to approach the specific activity of the 
source. Similarly, the nitrogen compounds might have been held to enter into the 
nitrogen metabolism in such a way that their carbon would have entered promptly 
into the protein which was being synthesized in the growing cells. In point of fact, 
the exact opposite occurred, for it was shown that the carbon of the labeled sugar 
entered the protein much more readily than that of the labeled amino acids. Further- 
more, the specific activity of certain of the protein amino acids (e.g. glutamic acid) 
Refevences p. 692/693 
