732 DYNAMIC ASPECTS — AMINO ACID POOL TURNOVER 
FREE AMINO ACIDS AS OBLIGATORY INTERMEDIATES 
IN-PRO@TEIN SYNTHESIS* 
ROBERT B. LOFTFIELD 
John Collins Warren Laboratories Huntington Memorial Hospital of Harvard University, 
Massachusetts Geneval Hospital, Boston, Mass. (U.S.A .) 
After two days of discussing amino acids, it is appropriate to consider that function 
of amino acids which makes them uniquely important in metabolism. The particular 
aspect of protein synthesis I would like to comment on is the question of whether all 
protein synthesis proceeds from free amino acids or whether there are mechanisms 
that involve intermediates no smaller than peptides. 
During the last decade there had been a series of experiments on a number of 
biological systems which persuasively indicated that no substantial fraction of 
protein synthesis involves the incorporation into protein of fractions larger than 
monomeric amino acids. Monop!: 2? and SpIEGELMAN?: * had demonstrated that no 
part of newly synthesized (-galactosidase is derived from the pre-existing protein 
of Escherichia colt. ASKoNas et al.° isolated some 30 peptides from partial hydrolysates 
of casein and f-lactoglobulin after 7m vivo labeling with [14C]valine and [14C]lysine 
in a goat. The two amino acids possessed constant specific radioactivity in every 
peptide; an impossibly unlikely coincidence unless all residues were derived from a 
single pool, namely, the free amino acid pool. Entirely different experimental designs 
by SIMPSON AND VELICK® and HEIMBERG AND VELICK’? showed that the incorporation 
of several amino acids, im vivo, into purified rabbit-muscle proteins must proceed 
through a single pool rather than the multiple pools which must be postulated for 
peptide intermediates. In my laboratory we’ were able to show that the valine and 
leucine residues of pure newly synthesized rat-liver ferritin possessed exactly the 
same specific activity as the free valine and leucine inside the liver cells. Thus, when 
I accepted the invitation to speak here I expected to be able to discuss critically 
some of the consequences of these conclusions. As I reacquainted myself with the 
recent literature I realized that many workers du not consider the problem to be 
settled. In fact, since there have been so many suggestions of “proof” that peptides 
participate in protein synthesis I cannot even discuss the individual experiments. 
Nevertheless I feel it may be worthwhile to discuss the various lines of experimen- 
tation and argument and to consider whether these are unambiguous. 
One argument for peptide intermediates involves the many cases in which it has 
been shown that the amino acid residues of a labeled protein or peptide appear to 
be incorporated into new protein more rapidly than the free amino acid. MEDVEDEV 
AND CHIANG-HSIA® 10, BABSON AND WINNICK" and others have described such ex- 
periments. In general, a comparison is made between incorporation of a labeled amino 
acid and a labeled peptide into the proteins of an intact animal or an isolated tissue. 
* This is publication No. 1066 of the Cancer Commission of Harvard University. 
References p. 737 
