738 DYNAMIC ASPECTS — AMINO ACID POOL TURNOVER 
DISCUSSION 
Chairman: HARRY EAGLE 
HENDLER: Dr. LOFTFIELD has very clearly presented the generally accepted scheme for incor- 
poration of amino acids into proteins. I think that it would be appropriate at this time to indicate 
several of the points on which I disagree with this scheme and the reasons for disagreement. 
Peptide intermediates were ruled unlikely by showing the inadequacies of experimental schemes 
which purported to show their existence. I agree with his conclusion that these schemes do not 
show that there are peptide intermediates. But I think we can use the very same approach and 
analyze the validity of the experimental evidence which was cited to show that there are no 
peptides. For instance, there is the experiment in which the protein of certain cells is made 
radioactive, the free amino acids are depleted and then cold amino acid is introduced. The pro- 
tein subsequently synthesized is unlabeled. This could say that if peptide intermediates exist, 
they are present only in very low quantities and are turning over rapidly. But it cannot say that 
they do not exist. In addition, if the peptides interchange rapidly with free amino acid while 
the depletion of radioactive free amino acid is being carried out, their activity might also be 
depleted. 
It is not entirely a question of semantics, to say that the amount of peptide would have to be 
rather small and rapidly turning over. The question is, can we rule out the existence of such 
peptides which are difficult to demonstrate? The incomplete peptides growing on the template 
surface can be considered as peptides, but I am really talking about peptides that have not yet 
reached the template, and I think that the possibility of their occurrence deserves further con- 
sideration. 
The second argument is that peptides having a sequence known to exist in a protein are not 
incorporated. Well, even if peptide pools were involved in the formation of that protein, there 
would be an astronomical number of possible peptides which could exist in the pool. You may 
have picked a particular dipeptide; perhaps you should have taken the tripeptide. Another 
problem is permeability barriers. How do you know that the peptide is reaching the site of syn- 
thesis? Another is that even though these peptides may be intermediates before they reach the 
template they may be intermediates in another bound sense, and that you are not able to ap- 
proach this binding by introducing the peptides. 
Another series of experiments mentioned to show that there are no peptide intermediates was 
based on the observation that one could add “specific inhibitors” for a given amino acid and find 
that the incorporation of all the other amino acids into the protein was inhibited. If there were 
a pool of peptide intermediates, the argument is that these other amino acids should at least be 
able to supply the requirements for the protein until the peptides have been depleted. But 
it seems very possible that if you throw in a specific analog blocking the incorporation of a 
given amino acid that all these interrelated actions may also come to a halt. The fact that 
interference with a single amino acid has blocked incorporation of all the others does not rule 
against peptides. 
The argument was also raised that there is enough of a problem for enzymes to determine the 
specificity of a single amino acid, let alone having enzymes handle the recognition of all the pep- 
tides. But peptide intermediates do not have to work without a template. You could still have 
the template, and the job of recognizing the final sequence of the protein could still be handled 
by whatever sequence mechanism handles it for the template. 
The argument also was advanced that since we have a well-established scheme for explaining 
protein synthesis, it is unlikely that peptides are involved because there is no place for them in 
this scheme, as we understand it. It would be superfluous at this point, apparently, to introduce 
a stage with peptides going through the same sequence. I think that if you critically examine the 
currently accepted scheme for protein synthesis as I have done, there is no doubt that all of these 
reactions occur, and the specificity as indicated occurs. But the only “evidence” that I have 
been able to find for the involvement of this scheme in protein synthesis is the query “Why 
else are these things around?” 
I want to make it clear that I am not questioning the very fine experiments showing that 
