734 R. B. LOFTFIELD 
specific activity of the free leucine in a rat’s liver remains high and nearly constant 
for one day and is reduced only by 50% after 3 days of continuous infusion of large 
amounts of non-radioactive leucine. Protein breakdown to amino acids and resyn- 
thesis are such active processes in many tissues that experimental determinations 
are necessary to establish the specific activities of amino acid pools. No amount of 
exogenous amino acid can reduce the specific activity of the intracellular amino acid 
to zero. 
A third type of observation which has been interpreted as suggesting that peptides 
or larger protein fragments may participate in protein synthesis is the reversible 
incorporation of certain amino acids such as glutamic acid or alanine into “protein” 
under conditions where net protein synthesis was not taking place?°. As Dr. HALVor- 
SEN pointed out this morning many of these experiments are now invalidated because 
the incorporation of radioactive amino acid was into cell wall material rather than 
protein??. 
A fourth source of support for peptide intermediates comes from teleologic reason- 
ing. RAACKE™ found in developing pea seeds an increase in peptide material followed 
by a drop when protein synthesis became rapid. TuBor AND Huzino?3 have found 
that several peptides are activated by enzymes similar to those that activate amino 
acids. KONINGSBERGER™ has isolated peptide-nucleotide compounds from yeast. 
HANES, HIRD AND ISHERWOOD?? observed that a variety of peptides can be formed 
by the enzymatic reaction of various amino acids with glutathione and FRuToN?6 
has discovered many systems in which transamidation reactions can yield peptides 
of considerable length. It is appropriate to inquire: “What significance do these 
observations have?” But it is a non sequitur to conclude, if you can think of no 
other meaning, that they have a bearing on protein synthesis. 
A final type of argument in favor of peptides as intermediates in protein synthesis 
involves those experiments in which a given labeled amino acid is found to be in- 
corporated into different parts of a protein molecule at different rates. If every 
amino acid residue in a newly formed protein is derived from the same amino acid 
pool and if the time required for the synthesis of a protein molecule is short relative 
to the rate of change of specific activity of the amino acid pool, all residues must 
have the same specific activity. There are several reports to the contrary?7*!. In 
most of these it is possible to suggest sources of artifact. Suffice it to say that in the 
most carefully conducted experiment of this type, ASkonAs ef al.® found, as men- 
tioned above, constant activities of some 30 valine and lysine containing peptides 
isolated from goat milk. There are, of course, circumstances in which non-uniform 
labeling of a protein is to be expected because the duration of the labeling experiment 
was of the same order as the time required for the synthesis of a single molecule. 
Thus in the schematic synthesis shown here (Fig. 1) valine residues close to one end 
of the chain will be more radioactive than residues isolated from the other end until 
the time when there are many more fully labeled protein molecules than partly 
labeled molecules. The kinetics of labeled amino acid incorporation do indeed cor- 
respond to this scheme** and not to a scheme that involves peptide intermediates. 
Recently Dinrzis*8 has shown that, as predicted by this scheme, hemoglobin peptides 
isolated from reticulocytes after brief exposure to labeled leucine are not equally 
labeled but that after longer exposure all peptides become equally radioactive. 
These experiments might be construed to suggest that peptides are intermediates in 
References p. 737 
