FREE AMINO ACIDS IN PROTEIN SYNTHESIS WBS 
It is found that the two substrates are metabolized differently and that added cold 
amino acid has little effect on the incorporation of labeled amino acid from polypep- 
tides. From this it has been easy to conclude that amino acids from one peptide can 
be incorporated into another protein without passing through the free amino acid 
pool. Frequently there has been a failure to prove that, in fact, a new protein has 
been formed; the “isolated” protein may include the intact unchanged starting 
material. Where this is not a proper criticism, there has been no demonstration that 
both the peptide and the amino acid have both reached the intracellular sites of 
synthesis. Thus it is to be noted that peptides may be taken into a cell many times 
faster than amino acids™. Finally, before one can claim that the new protein was 
derived from peptides and not amino acids, it would be necessary to demonstrate 
that the peptide is not hydrolyzed. In every case where hydrolysis has been looked 
for it has been found. BURNETT AND Haurowttz!?*: 4 found that intracellular valyl- 
leucine was g0% hydrolyzed in reticulocytes and LEACH AND SNELL” found that 
alanylglycine was completely hydrolyzed in Lactobacillus casev. 
A second line of argument involves the apparent exchange of one amino acid residue 
for another in experiments using doubly labeled proteins. In a sense this can be 
regarded as the use of a very large intact peptide and only one or two new amino 
acids. 
Thus WicGANs et al.!° have reported that when rat albumin containing both 
labeled valine and labeled leucine is injected into a recipient rat, the circulating 
albumin loses the two amino acids at different rates. Similar observations have been 
made by others (FRIEDBERG AND WALTER!®, FRANCIS AND WINNICK!’, among 
others). These workers interpreted their data as proving that individual amino acids 
could exchange (perhaps on the biosynthetic template) with free amino acids and 
that the rates of such exchange varied with the nature of the amino acid and its posi- 
tion in the protein molecule. The flaw common to all these arguments is that the 
administered doubly tagged protein may be hydrolyzed to its constituent amino 
acids and that these labeled amino acids will be diluted to a varying extent by endo- 
genous free amino acids. Newly formed protein would then contain labeled amino 
acids possessing the relative specific activities of the pool amino acids and hence 
there would appear to be a change in the relative abundance of the two. There is 
certainly an extensive breakdown and resynthesis of protein through the free amino 
acid intermediate. Thus BUCHER AND FRANTZz}§ found that massive amounts of non- 
radioactive alanine or glycine decreased the apparent half-life of glycine or alanine 
labeled serum protein in a rat and, very much to the point, that glycine and alanine 
were not at all equally effective in interfering with the reincorporation of the respec- 
tive labeled residues. PENN, MANDELES AND ANKER!® elegantly showed that the 
half-life of labeled serum protein depends on whether the other proteins of the animal 
are also labeled. Thus the radioactivity was lost much more slowly in the donor 
rabbit than in the recipient of a plasma transfusion. The only interpretation is that 
a rapid breakdown of many labeled tissue proteins results in a relatively highly 
radioactive amino acid pool in the donor and hence the continued synthesis of 
Jabeled serum proteins, whereas in the recipient, amino acids derived from the 
breakdown of labeled plasma proteins are thoroughly diluted by the vast continuing 
supply of non-radioactive amino acids released from the non-labeled body protein. 
We ourselves’ have established that, following a single injection of {Cjleucine, the 
References p. 737 
