DISCUSSION 7O7 
‘TI should have mentioned earlier (and actually the observation is consistent with either view 
of protein turnover) that with appropriate isotope labeling experiments you can show that at 
any one moment in growing cells 2—3 per cent of the cell amino acid pool has derived from the 
cell protein, either by a process of degradation and resynthesis, or by a process of amino acid 
exchange. 
Lucy: In this discussion of the control of protein turnover, Dr. HALVorson suggested that in- 
creased protein breakdown may result from repression or induction of a latent degradative system 
that is normally present in the bacterial cell. As an example of the activation of a latent degra- 
dative system, he mentioned the liberation of ribonuclease that occurs on breakdown of the ri- 
bosomes of E. colt. 
I would like to extend this suggestion to the control of protein breakdown and turnover in 
animal cells. The lysosome particles of animal cells contain a number of hydrolytic enzymes, 
including the cathepsin proteases. These particles provide a preexisting system that can, upon 
suitable activation, yield an active proteolytic enzyme. 
At the Strangeways Laboratory in Cambridge, England, we have been studying a system that 
may possibly be important in the control of proteolysis in animal cells. It has been found that 
the addition of vitamin A to a suspension of lysosomes from rat liver results in the release of a 
protease from these particles. As an example of the possible function of vitamin A in controlling 
proteolytic activity, we have considerable evidence which indicates that the loss of polysaccharide 
from embryonic chick cartilage that occurs on treatment with excess vitamin A in tissue culture 
is the result of increased protease activity. The observations indicate that, in this instance, excess 
vitamin A alters the permeability of the lysosomes thereby liberating a proteolytic enzyme that 
attacks the protein—polysaccharide complex of the cartilage matrix. 
ARONOFF: I would like to ask Dr. StEwarD two questions. The idea has occasionally been sug- 
gested here today that parts of proteins may metabolize, and others not. I would like to ask him 
initially, does he have evidence for this for a specific protein? Secondly, as a corollary to this, 
does the glutamine of TMV protein metabolize while the proline does not? 
STEWARD: In answer to the first question, I was rather careful to point out that we could not 
distinguish between these two possibilities; that is, whether the whole protein is turning over 
or whether parts of it were being lopped off leaving an inaccessible part which would not be so 
removed. At first, when we obtained these data, we thought in terms of a turnover of a distinct 
protein, and I still think this may be a possibility. What we are actually doing, however, is to 
try to purify these substances, and by critical electrophoresis on the new acrylamide gels we are 
getting very characteristic patterns of bands. What we now hope to be able to do is to localize 
the labeling in a homogenous component, of which there are very many. Then we may be able 
to distinguish between these two possibilities. However, I think if you will ask Dr. POLLARD this 
question, he may say that he thinks in terms of a central part of a protein which receives the 
proline but, for some reason, is not accessible to turnover; for my part, I just don’t know which 
possibility will on the evidence prove to be correct. 
As for the second question, I was using the turnover of TMV as an example of a protein which 
did not turn over, so far as we know and which had different degrees of accessibility to different 
soluble components in the cell as potential sources of carbon. 
