FREE NITROGEN COMPOUNDS IN PLANTS 683 
reagent to study its effect on ion uptake and on respiration. Cells which have pre- 
viously developed a salt-induced respiration by contact with KCl have this com- 
ponent unaffected by the chloramphenicol but concurrently their ability to accumu 
late ions is decreased. Therefore, SUTCLIFFE believes that protein synthesis and turno- 
ver in these cells is directly associated with ion uptake, though not with respiration in 
the general manner which has been postulated (cf. STEWARD AND SUTCLIFFE”). 
WRIGHT AND ANDERSON”? have studied the incorporation of [?°S|methionine into 
the free amino acid pool and into the protein of a slime mold. This is a very illuminat- 
ing study, and the results are very relevant to this discussion. Briefly, these in- 
vestigators find that the [°°S|labeled methionine readily enters into the protein of the 
slime mold, and this protein is regarded as in active turnover, even though the total 
protein in the organism is declining during its development. It is estimated that the 
rate of turnover of the protein in the slime mold is about 7° of the total protein per 
hour, as compared to such other rates of turnover as 5°%, for resting bacterial cultures, 
1~2°% for mammalian cells in culture. These figures may be compared with the rate 
of turnover of tobacco or of corn leaf protein which is estimated at 1/,-1% from 
unpublished data in the laboratory of one of us (R.G.S.B.). 
Furthermore, WRIGHT AND ANDERSON now distinguish between two broad classes 
of protein in the slime mold. These classes are distinguished as the ethanol soluble 
and trichloroacetic acid precipitable protein on the one hand the ethanol-insoluble 
trichloroacetic acid-precipitable protein on the other. *°S-labeled methionine enters 
the protein of both these classes, but the interesting feature is that the ethanol in- 
soluble moiety “turns over” much more rapidly than the ethanol soluble fraction. In 
both of these classes of protein the incorporation of the [%°S|methionine occurs even 
though the total protein is on the decline. This is evidence for turnover. But it is also 
important that the incorporation of the [°°S|methionine into the insoluble fraction 
is very much greater than that in the soluble fraction; and this ratio, which achieves 
a maximum value of 4, is a function of the development of the slime mold. Further- 
more, WRIGHT AND ANDERSON point out that the incorporation of the [?°S|methionine 
into the more actively metabolizing protein moiety, that is the alcohol-insoluble 
fraction, which is a function of its turnover, seems also to be related to such other 
vital activities of the slime mold as oxygen consumption and certain enzymatic 
activities. 
One can, therefore, conclude that a variety of evidence from a variety of plants 
and botanical systems all points in the general direction that cyclical turnover of 
plant protein certainly occurs, although it must be recognized that the absolutely 
final and critical proof may still remain to be produced. This can only emerge from 
the study of the specific activities of amino acids which have been derived from 
single and purified proteins, isolated from the cell, and from the changes which 
these incur during metabolism. Experiments to provide this type of evidence are 
now being planned. 
In animal systems the synthesis of single proteins has been approached by selecting 
those proteins that have either enzymatic or hormonal properties and which can, 
therefore, be readily identified and purified. SCHAPIRA e¢ al.** refer to rat muscle al- 
dolase and can be quoted as follows: “It seems that a direct demonstration of the 
turnover of a well-defined protein has never been given. In our study we tried to make 
such a demonstration from muscle aldolase. The finding of an exponential decrease 
References p. 692/693 
