682 F. C. STEWARD AND R. G. S. BIDWELL 
All these facts show that the rate of synthesis of non-radioactive glutamine from 
protein was even greater in the cells which were more rapidly growing. Thus, protein 
breakdown was also greater when synthesis was also greater. In other words, the 
pace of the protein turnover cycle had been increased in the more rapidly growing cells. 
It was in fact shown that the chief effect of the coconut milk, which induced 
growth in the otherwise resting cells, was to cause first a net synthesis of protein, 
partly at the expense of the stored reserves in the cell and partly by the use of exo- 
genous nitrogen, and also to accentuate the pace of protein turnover. The coconut 
milk thus acts to stimulate vital activity inasmuch as it accelerates the operation of 
the protein cycle of synthesis and breakdown, which in turn furnishes carbon sub- 
strates for respiration and causes a demand for new carbon for the re-formation of the 
protein so broken down. In this way the pace of the “protein cycle” draws carbon 
from sugar into the metabolism of nitrogen compounds. 
The study showed that the amino acids which were present in the protein hydro- 
lyzate fell into two groups. In one group, of which proline and hydroxyproline were 
the prominent examples, the increment of bound amino acids caused by the coconut 
milk was simply in proportion to the increase in the bulk of total protein. In the 
other group of amino acids, of which glutamic and aspartic acid, and threonine were 
prominent examples, the increase as indicated above far exceeded the increase of 
total. These data, briefly summarized in Table V, suggested that there were two 
distinct ways in which amino acids were entering protein in these cells. 
In the case of proline, it seemed that the free proline which existed in the soluble 
phases of the cel! could be directly incorporated into the synthesized protein; whereas 
other amino acids, which existed in bulk in the soluble constituents of the cells, did 
not appear to be the immediate precursors of the corresponding amino acid in the 
synthesized protein, for, in these cases, the carbon of these amino acids in the protein 
seemed to come more directly from sugar. 
Thus the investigation of carrot explants led to the idea that there were two 
functional types of protein in the cell. Some protein, which incorporated certain 
amino acids directly, seemed to be structural and non-metabolized; whereas other 
protein seemed to be in a sufficiently active state that it was metabolized by break- 
down and re-synthesized at rates which were linked to respiration. It should be re- 
cognized, and indeed it was implicit in the original thought, that one cannot at this 
point distinguish between metabolically active protein as an entire molecule, com- 
pletely broken down or completely re-synthesized, and an alternative view that 
there is a metabolically active part of the protein which is merely a fraction of a 
larger molecule, so that the amino acids which are not susceptible to re-metabolism 
(e.g. proline) would be in the metabolically inaccessible part. Until work is done 
with isolated and completely purified proteins, these possibilities cannot be dis- 
tinguished. However, the present working hypothesis is that there is some structural 
moiety of the protein of growing carrot cells which does not actively participate in 
metabolism and which incorporates proline directly. 
Some indirect evidence which relates to protein turnover also accrues from the 
following. SuTCLIFFE® has returned to the idea (cf. summary in STEWARD AND 
SUTCLIFFE*S; also see p. 424) that the intake and accumulation of ions by discs of 
tissue (carrot and beet) is linked to protein synthesis and turnover. It has been shown 
that chloramphenicol inhibits protein synthesis in bacteria, and SUTCLIFFE utilized this 
References p. 692/693 
