FREE NITROGEN COMPOUNDS IN PLANTS 679 
cells from CO, in the light does so from a small but metabolically active pool, and 
in so doing it bypasses the larger and more sluggishly metabolized pool of the con- 
ventional soluble compounds which there exist in bulk. 
SECTION IV. PROTEIN TURNOVER AND THE SOLUBLE POOLS 
The classical picture of nitrogen metabolism in plants, dating back to the work of 
PFEFFER, SCHULZE AND PRIANISHNIKOV, involved the idea that protein synthesized 
in one organ may be broken down, translocated and re-synthesized in another. 
Although this is protein “turnover” within the organism, this is a quite different 
phenomenon from what is commonly meant by protein turnover within a cell. Even 
the breakdown of one protein and the re-synthesis of another protein within a cell, 
although this may commonly occur during development, is still not what is commonly 
implied by cyclical turnover. It is true that the loss of one protein and the gain of 
another has been prominently invoked to explain events that occur during develop- 
ment. However, what is here in question is the cyclical breakdown and re-synthesis 
of metabolically reactive protein, this being regarded as a normal concomitant of 
metabolism. 
This idea is by no means new, because it was first given prominence for plants by 
GREGORY AND SEN? as early as 1937 to explain certain data which arose from the 
study of respiration of barley plants as this was affected by nutrition. Briefly, their 
idea was as follows. 
A certain amount of the protein in the organism was regarded as cyclically broken 
down to give amino acids as the immediate products of protein breakdown, the 
nitrogen was re-cycled and synthesized back into protein while the carbon skeletons 
were respired away. The idea of GREGORY AND SEN was that the pace of the protein 
cycle determined in part the pace of the respiration, although the net amount of 
protein present might remain steady. 
With the subsequent availability of ®N, two now classical papers by VICKERY et al.®° 
on the one hand and by CHIBNALL AND WILTSHIRE®® on the other furnished the 
evidence upon which the ideas of cyclical protein breakdown and re-synthesis in 
higher plants became familiar. These papers drew attention to the fact that 1°N, 
supplied originally as ammonia, entered the bulk protein even though the total 
amount of protein present either failed to increase or even decreased. It came, there- 
fore, as a surprise that enough re-synthesis of protein occurred to incorporate so much 
of the 1°N, even in isolated leaves of tobacco and of beans where there was little need 
for bulk synthesis of protein and even where actual breakdown should occur. 
Almost concurrently, work with slices of potato tuber (STEWARD et al.°4, p. 418) 
led to the view that a part of their respiration, when in dilute salt solutions, pro- 
ceeded over pathways in which the respiration was closely linked to protein synthesis 
and the metabolism of soluble nitrogen compounds, whereas another part of this 
respiration seemed to be independent of the nitrogenous metabolism. In fact, there 
was a linear regression of CO, produced on protein synthesized, and this relation 
held despite a wide range of salt and oxygen conditions to which the tissue was 
subjected. 
The work on potato discs required a similar interpretation to that of GREGORY 
AND SEN, namely that the very actively metabolizing cells synthesize protein from 
References p. 692/693 
