Lipid-bound amino acids in broad-bean leaves

Abstract

The lipid fraction of broad-bean leaves contained a number of amino acids. These amino acids could not be removed from the lipid fraction of methods demonstrated to remove adequately admixed free valine, and they remained within the lipid fraction after denaturation of proteolipid. Experiments measuring the binding of valine to the lipid fraction during extraction showed that only a small part of the lipid-bound valine was an artifact derived from the free amino acid. The lipid-bound amino acids were concentrated by methods based on adsorption and molecular-sieve chromatography, and by differential hydrolysis. A fraction was obtained which indicated that most of the lipid-bound amino acids were in a relatively-polar alkali-stable complex of large molecular size. Partial acid hydrolysis of this complex released free amino acids and peptides. The amino acid composition of the complex was compared with that of a sample of proteolipid prepared from broad-bean leaves. The complex was much richer than the proteolipid in phenylalanine, leucine (and/or isoleucine), alanine, and proline. When L-[Cl4]valine was applied to broad-bean leaves in the transpiration stream, a substantial incorporation into leaf protein resulted. Only a very small part of the applied valine was bound to the lipid fraction, and the specific activity of the lipid-bound valine was low. These results are discussed in relation to current theories of the involvement of lipids in protein biosynthesis and in transport.