684 F. C. STEWARD AND R. G. S. BIDWELL 
of the radioactivity of rat muscle aldolase, labeled by injection of |!#C|glycine, provides 
the direct demonstration of an intra-cellular dynamic state of this protein.” 
The evidence seems to be as follows. [14C|glycine was allowed to be incorporated 
into muscle, and the content of !4C in the isolated aldolase was determined. There- 
after the animal was maintained in fresh weight and protein balance so that, if the 
protein was stable, its #C content should have remained steady. Either secretion of 
the enzyme into the plasma or the death and renewal of some cells would give an 
appearance otherwise consistent with turnover. However, the authors demonstrate 
that neither of these factors significantly affected their results, which nevertheless 
showed an exponential decrease with time of the {!4C|glycine content of the isolated 
aldolase. This decrease is explained by the breakdown of the first formed /C con- 
taining protein and its re-synthesis from unlabeled precursors. 
SECTION V. PROTEIN SYNTHESIS AND TURNOVER IN RELATION TO DEVELOPMENT 
In much current thinking on development it is implicit that stages of differentiation 
in the organism may be marked by the ascendancy of given protein moieties which 
are characteristic of different stages of development. This may occur by breakdown 
of one kind of protein formed at one stage of development followed by synthesis of 
another type of protein at a later stage of development (though how these modifica- 
tions may be accomplished by changes at the presumed RNA template of synthesis 
is another matter). 
Using roots of higher plants, BRowN et al.6.7 have traced the changing patterns 
of protein and of enzyme content in cells with their ontogenetic development as 
this can be seen along the axis of roots. There is clearly a stage of growth, predomi- 
nantly by cell multiplication, in which the self-duplicating structures of the cell are 
reproduced, and one would expect the protein synthesis at this stage to reflect the 
formation of all the essential constituents of the organelles. Later, however, growth 
may be largely by cell enlargement. In the coleoptile of grasses and in the cells of roots 
it is now accepted that a measure of protein synthesis occurs throughout both these 
broad phases of the growth and development of the cell. However, Brown has 
placed the data of protein or enzyme content upon a cellular basis, as distinct from 
a fresh weight basis and has noted that the content of protein in the cell in- 
creases up to a maximum and it then falls. It is interesting also that Brown finds 
the point of maximum respiratory activity per cell coincides with the point of maxi- 
mum protein content’. Moreover, BRowN sees the possibility that, as cells differen- 
tiate along the axis of a root, a different complement of enzymatic proteins comes 
into play. 
Populations of bacterial cells furnish statistically convenient systems on which to 
seek evidence of synthesis and turnover. Although it had become rather widely 
accepted that protein turnover could occur in the more quiescent or resting cells, it 
has seemed that there was little or no conclusive eyidence from cells in the logarithmic 
phase of growth. 
Such conclusions had been reached for cultured mammal cells by KING ef al.?° 
using exogenous |C|leucine, who believed that there was positively no turnover in 
the actively growing cells. However, JORDAN AND SCHMIDT!® have now repeated 
References p. 692/693 
