METABOLIC IMPORTANCE OF AMINO ACID—LIPID COMPLEXES 753 
suggested to minimize the possible occurrence of amino acid derivatives as contami- 
nants. In the latest paper’, WREN has decided that although some amino acid-lipid 
complexes are artefacts, others are undoubtedly real. WREN has defined artefact to 
include those complexes that form when amino acid or amino acid analogs are 
added to a metabolically inactive form of the tissue being worked up. Although it is 
wise to exercise caution in the study of these complexes, I believe that WREN is not 
justified in applying the label of artefact to these complexes. His criterion is the fact 
that the complexes are formed too efficiently, that is by simply extracting the tissue 
in the presence of the amino acid. I can see no reason why this high affinity of the 
non-polar lipid for highly polar amino acids cannot be used in the normal metabolic 
sequences of tissues. If we are considering the possibility of certain stages of metab- 
olism occurring in membranes or non-polar cellular media, then why should we 
decide a priori that complexes which are formed so easily are artefacts as opposed 
to real metabolic intermediates? Other criteria must be employed before making 
such a categorization. 
TRIA, BARNABEI AND FERRARI have described a lipid-soluble “peptidylphospha- 
tide” isolated from rat liver. Upon perfusing the isolated rat liver with heparinized 
rat blood containing [C)jamino acids, it was found that at early time points, the 
specific activity of the peptide isolated from the lipid fraction was considerably 
higher than the specific activity of the average protein!®. 
In recent experiments with the hen oviduct, we have been concerned with further 
criteria for the metabolic importance of these complexes and have studied the 
structural and optical specificity requirements for the amino acid. We have tried to 
dilute a small quantity of radioactive t-valine with unlabeled t-valine, p-valine 
and L-serine respectively. If the formation of the amino acid—lipid complex required 
L-valine then dilution could be accomplished with L-valine, but not with p-valine or 
L-serine. If the specificity was directed only towards the gross aspects of the valine 
TABLE I 
A TEST OF SPECIFICITY OF LIPID—AMINO ACID SITES 


Relative dilution of valine Relative radioactivity 
specific activity by Relate in lipids 
- — --— — internal - - 
rere ss ne aaa 
L-valine L+p-valine t ae valine concentra Lo Total silicic CHCl, peak 
+ serine of valine s 4 from 
acid peaks... . F 
stlicic acid 
Expt. x 
L-(4C]Valine (83 ug) 1.0 1.0 1.0 1.0 [.0 1.0 
+L-[{12C]Valine (6 mg) ime) 10 ime) 8.9 0.93 0.70 
+p-[1?C}Valine (6 mg) 0.97 5.4 5-4 6.6 1.0 1.48 
Serine 
+.-[!2C|Serine (6 mg) 0.75 0.75 13.6 5-5 1.0 0.92 
Expt. 2 
Valine 
L+D-(14C]Valine (4 mg) L.0 1.0 1.0 1.0 1.0 1.0 
+1L-[1#2C]Valine (20 mg) 8.4 6.2 6.2 4.7 0.43 0.57 
+p-[{!#C]Valine (20 mg) 1.0 4.0 4.0 4.7 1.33 1.29 
Serine 
+1-[#2C]Serine (20 mg) 0.84 0.88 5-4 II.4 0.91 0.97 
References p. 758 
