METABOLIC IMPORTANCE OF AMINO ACID—-LIPID COMPLEXES yl: 
of 11 different amino acids tested were capable of forming such complexes and that 
phenylalanine was unusual in that it was able to form certain types of complexes to 
about ten times greater extent than the other amino acids*. That the amino acids 
were not present in the free form was shown by the fact that as an alcoholic solution 
of these complexes was diluted with water, they were more easily extracted with 
hexane ; radioactive amino acid could be added to the biologically labeled complexes 
and then be separated away by simple extraction or chromatography; free amino 
acids were not observed before hydrolysis either directly or as DNP derivatives (but 
could be observed after hydrolysis), these complexes could be chromatographed on 
silicic acid under conditions where free amino acid could not*4. 
Successful fractionation of amino acid—lipid complexes from hen oviduct was 
achieved by graded counter-current distribution and chromatography on columns of 
alumina-silica and silicic acid. It was found that the structural elements of hen 
oviduct contained a greater proportion of the most non-polar of the mixture of 
amino acid—lipid complexes obtained from this tissue (Fig. 1). 
Incubations of the tissue with radioactive amino acid under conditions in which 
protein synthesis was inhibited yielded mixtures of amino acid—lipid complexes with 
an altered distribution of radioactivity. That is, there was a decrease in the propor- 
tion of radioactivity found in the least polar components. One rather interesting 
observation was that after treatment with 2,4-dinitrophenol, it was found that there 
was an increase in the amount of radioactivity found in the lipid fraction. Upon 
fractionation of the tissue and chromatography of the complexes, it was found that 
there was marked decrease in the radioactivity of the least polar complex obtained 
from the particulate fraction and a more than equivalent increase in the radioactivity 
of the more polar complexes of the supernatant ‘fraction? (Fig. 1). 
HAINING, FUKUI AND AXELROD have studied the formation of lipid—amino acid 
complexes in rat-liver microsome and supernatant preparations® *. Although their 
experiments increase our understanding of the type of compound with which we 
are concerned, I must take some time to examine critically the interpretation of these 
studies as regards protein synthesis. It has never been established that the system 
under study is capable of normal protein synthesis. Aside from the fact that the in- 
corporated radioactivity cannot be demonstrated to be present in the proper internal 
position for normal liver proteins, it is known that at least one type of artifactitious 
incorporation, that of disulfide bonding, takes place in cell-free liver preparations 
that does not take place in intact cells®. In their studies on relative uptake of amino 
acid into protein and lipid as well as the experiments on effects of metabolic poisons, 
the specific activity of the whole lipid fraction was determined. 
In the hen oviduct, it has been observed that many different amino acid—lipid 
complexes are formed for a given amino acid and that the specific activities for each 
vary. Furthermore, when the hen oviduct system was poisoned with 2,4-dinitrophenol, 
there was a marked depression in the labeling of a complex associated with the struc- 
tural elements and an increase in the labeling of a different amino acid—lipid com- 
plex obtained from the supernatant fraction. The overall labeling of the lipids was 
enhanced by this poison. This observation illustrates the danger of drawing con- 
clusions from a study of the total lipid fraction. Many of the data presented by 
HAINING, FUKUI AND AXELROD were obtained with phenylalanine. It has been 
observed that phenylalanine is an unusual amino acid in that it can label certain 
References p. 758 
