744 B. AXELROD, J. L. HAINING, T. FUKUI 
and lipid fractions from microsomes is shown in Table II. Phenylalanine and trypto- 
phane surpassed all of the other radioactive amino acids in their capacity to label the 
lipid as compared to the protein. A similar pattern of incorporation was noted in the 
lipid obtained from the particle-free supernatant. Leucine was relatively the most 
effective in labeling the protein. 
Finding activity in a lipid fraction however, does not on the face of it prove that a 
radioactive amino acid, as such, has actually become joined into a lipo—amino acid 
compound. The existence of fat-soluble substances containing amino acids derived 
by extraction from biological systems has often been the subject of controversy 
TABLE Ti 
RELATIVE INCORPORATION OF SEVERAL AMINO ACIDS INTO MICROSOMAL PROTEIN 
AND LIPID 
Incubation conditions are similar to those described in Fig. 1. Each vessel contained 1.15 “moles 
of the amino acid specified. The measured specific activities in the protein and lipid fractions 
were adjusted for the differences in the specific activities of the various amino acids 
(shown in the table). 




Asset Radioactive specific Specific 
Amino acid Fees ; ROR ULS pases 
(uC|umole) Lipid Protein (Lipid| protein) 
DL-{3-14C] Phenylalanine 4.34 100 100 0.76 
DL-[3-4C]Tryptophane 1.86 105 94 0.84 
DL-{ 1-!4C] Leucine 5.33 19 140 0.13 
DL-[ 1-!4C]Lysine 3.19 9.5 67 0.10 
[2-14C]Glycine 4.87 9 22 ONsZ 
DL-[ 1-14C] Valine 2.92 9 96 0.07 
DL-[1-!@C] Alanine 4.05 BE2 2 0.07 


* Adjusted specific activity (counts/min/mg) in lipid and in protein separately set equal to 
too for phenylalanine incorporation in order to facilitate comparison of the incorporation of 
each amino acid into each fraction. 
** Ratio of adjusted counts/min/mg of lipid to that of protein to illustrate the absence of any 
simple relationship of amino acid incorporation into the two fractions. 
because of the ease with which natural mixtures of lipids may become contaminated 
with amino acids. The presence of phospholipids in solutions of lipids often permits 
the solubilization of polar substances in apparent violation of the rules which are 
based on studies made with pure solvents. Even if covalent combinations are not 
formed, separation of amino acids from crude natural mixtures of lipids can be 
difficult. WREN AND MITCHELL? as well as GABY AND SILBERMAN® are among those 
who have used paper electrophoresis to establish freedom from contaminating 
amino acids in lipo—amino compounds. The possibility has been shown that cyclized 
amino acids, which have lost much of their polar nature can act as particularly 
tenacious contaminants. WREN’ has considered at some length the formation of 
lipo—amino artifacts by the chemical combination of auto-oxidized lipids with 
amino acids. Such combinations could well be favored with the highly active fatty 
oxidation products such as free radical compounds and carbonyl compounds. Finally, 
References p. 749 
