DYNAMIC ASPECTS — AMINO ACID POOL TURNOVER 759 
DISCUSSION 
Chairman: HARRY EAGLE 
RouseEr: I would like to know how Dr. HENDLER and Dr. MircueE ti define a lipid. This is not 
always easy, but one thing that generally is excluded in the definition of a lipid is a complex of 
lipid with some other substance held together by ionic or secondary forces. Such complexes can 
be dissociated without breaking covalent bonds. The data presented thus far indicate to me that 
the combinations of amino acids and lipids under discussion can be referred to most appropriately 
as lipo—amino acid complexes, but not as lipids containing amino acids or abbreviated and called 
lipids. If amino acids are covalently bound into a molecule that contains a long hydrocarbon 
chain (of a long chain fatty acid or fatty aldehyde) or some cyclic hydrocarbon, then the sub- 
stance can be called a lipid (containing amino acids). If the amino acid or peptide is bound 
ionically to a true lipid, the combination should be designated as a lipid—amino acid or lipid— 
peptide complex. 
The significance of this differentiation is important for metabolic considerations. If amino acid 
is bound by covalent bonds into a lipid, the metabolism of lipids and amino acids is inter- 
related. If lipid and amino acids or peptides are merely associated by ionic bonds, then the 
complex may only be related to the method of isolation and not necessarily to metabolic inter- 
relationships. 
Solubility criteria are by no means adequate to establish covalent linkage because of the well- 
known ability of lipids to solubilize water-soluble compounds. Clear solutions of lipids and amino 
acids can be prepared with solvents in which the amino acids are insoluble. Also, some peptides 
may be appreciably soluble in organic solvents and thus appear with lipids. Amino acids present 
in lipid extracts can be eluted from silicic acid columns along with lipids, and thus, silicic acid 
column chromatography alone is not a reliable indicator that amino acids occur in covalent 
linkage in a lipid molecule. Is there any evidence then that the amino acids in the lipo—amino 
acid complexes are linked by covalent bonds to hydrocarbon material? Stated another way, 
are there data to show that a compound has been isolated that maintains a constant composition 
when chromatographed in several different systems, when dialyzed, and when subjected to 
electrophoresis ? 
HENDLER: My interest in these compounds has been in their possible metabolic significance. 
If we are considering that reactions of significance may occur in a non-polar area within the cell, 
then so far as I can see the most important criterion is its solubility, and if this is not satis- 
factory in terms of defining lipids in general, I think that is another question. In my analysis of 
these systems attention has been placed on solubility because we are considering their reacting 
in non-polar areas of the cell. 
As for the relation to Dr. AXELRop’s system, there is no reason why this compound should neces- 
sarily be characteristic of the major compounds I have been studying. Now that I know the 
properties of this material perhaps I will be able to isolate it. However, for any given amino acid, 
I find a number of different complexes. We are studying not one but several substances. 
L. MILLER: Just by way of trying to get some notion of the possible biological implication of 
these lipid—amino acid complexes, to what extent have these materials been isolated from living 
animals? To what extent do they turn over rapidly or not so rapidly? What kind of evidence 
do we have that they have any role in biology as we understand it? 
AXELROD: That is easy for me to answer, because I have made no claim for these things. We 
have not attempted to isolate a known compound from natural material. We have only worked 
with the labeled material which we obtained incidental to the protein in our first attempt to 
incorporate phenylalanine into lipid material, and I have really no idea what the physiological 
importance of this material is. I think I did say that we wished that we could have attributed 
something of great value to it, but as things stand, we will just have to let someone else see if 
they can find a place for it. 
Rouser: Dr. MrrcHELL, would you agree with what Dr. HENDLER said? I want to get back 
to this definition. Are you willing to state that these bound amino acids have lipid residues? 
MitTcHELL: Well, I can say I agree with everybody! First, I should say that I have been slightly 
