LIPO-AMINO ACID COMPOUNDS 747 
acid. The phenylalanine isolated on hydrolysis was tested with L-amino acid oxidase 
and proved to be the L-isomer, showing that the incorporation reaction was specific 
for the natural form. 
When the lipo—amino acid was reduced with lithium aluminum hydride before the 
hydrolysis at 110°, no radioactive phenylalanine appeared on paper chromatography, 
but a new radioactive component which coincided in position with a-amino-/- 
phenylpropanol was present. It thus appears that phenylalanine is bound through 
its amino group, with its carboxyl group free. This mode of combination was further 
confirmed by the fact that the phenylalanine—lipid compound was hydrolyzed almost 
completely by the prolonged action of carboxypeptidase. It is therefore probable that 
the compound formed from phenylalanine and monoolein is N-oleoylphenylalanine. 
The nature of the lipo—amino acid was further established by chromatography of 
synthetic N-palmitoyl-p1-phenylalanine with the product derived enzymatically 
from phenylalanine and palmitic acid. 
When the isolated lipo-amino acid was incubated with the soluble enzyme, it 
was almost completely hydrolyzed to a radioactive substance which was no longer 
ether-soluble. 
SIGNIFICANCE OF LIPO—AMINO ACID COMPLEXES IN NATURE 
Among the roles suggested for lipo—amino acids (or lipo—amino acid complexes) is 
that of a precursor to proteins. 
However attractive it might be to ascribe a topically important role to the com- 
pounds on which we have lavished so much effort we do not feel that our own studies 
provide evidence for such a view, insofar as the limited system which we have studied 
is concerned. We have been inclined to interpret our kinetic evidence in support of 
the contrary view, but as long as the possibility of heterogenous pools of lipo—amino 
acid complexes exists, we must admit that an absolutely unequivocal interpretation 
is not possible. Our own studies do not provide evidence for this view. This is not to 
deny to possibility that amino acids present in combination with the lipids may be 
released and subsequently transferred via the commonly accepted pathway involving 
nucleotide activation. Nor do these studies have anything to say about the lipid 
portion of the microsome in protein synthesis. 
What is the significance of the acylamino acids? We are not prepared to say. We 
can only say (with respect to the system we have studied) that they are formed by an 
enzymatic system and that they probably exist in the cell. Their formation does not 
require a source of triphosphonucleoside, and if they are in the pathway of protein 
synthesis their formation does not occur subsequent to the well-known AT P-amino 
acid activation. Since the amino acid activation is known to occur directly with free 
amino acids in relatively purified systems, it is difficult to see, how, in our system, 
the acylamino acid can be obligatorily involved. However, we have not worked 
with intact cells and cannot on the ground of this work provide a direct evaluation 
of the claims based on work with more complete systems. 
The best reasons for implicating lipo—amino acids (or complexes) are based on 
work with more elaborate systems such as protoplasts, minced tissue or entire Droso- 
phila. It is emphasized that in all of these cases elaborate proof has been provided 
that the lipo—amino acid complex is not an artifact. Larvae of Drosophila have been 
shown in MiITcHELL’s laboratory! to contain lipo—amino acid complexes, probably 
References p. 749 
