LETTERS TO THE EDITORS a-AMINOADIPIC ACID : A PRODUCT OF LYSINE METABOLISM*t sirs: As part of a study of protein and peptide metabolism lysine was synthe-

As part of a study of protein and peptide metabolism lysine was synthesized with Cl4 in the e position and resolved into the L and D isomers. 10 mg. of labeled lysine dihydrochloride (either Lor D-) and 0.66 gm. (wet weight) of guinea pig liver homogenate were added to a reaction mixture containing 1.3 per cent of an amino acid mixture corresponding to the composition of casein except for lysine and 0.01 M cr-ketoglutarate, all in a final volume of 4 ml. of isotonic saline solution.’ The reaction was carried out under oxygen for 6 hours at 38”. After precipitation of the proteins by boiling at pH 5.0, a fraction of the non-protein filtrate was chromatographed on filter paper.2 Ninhydrin gave a lysine spot which was radioactive and another radioactive spot in the glutamic acid region. This indicated the presence of a dicarboxylic a-amino acid different from glutamic acid, since to obtain glutamic acid from the labeled lysine would entail removing the radioactive e-carbon. The chromatographic behavior of this radioactive substance on filter paper and on Lloyd’s reagent suggested, in view of the source of the C14, that the unknown substance might be a-aminoadipic acid. Accordingly the latter was synthesized; it gave the same filter paper chromatogram as the unknown. Radioactive cr-aminoadipic acid (probably mixed with glutamic acid) was isolated from the reaction mixture as a barium salt by the following procedure. After hydrolysis of the non-protein filtrate with hydrochloric acid and chromatography on Lloyd’s reagent,3 whereby the hexone bases were removed, the non-basic amino acid fraction was precipitated with barium from 75 per cent ethanol. The barium was removed from the precipitate, and the dicarboxylic acids again precipitated with barium. 99 per cent of the radioactivity originally present in the fraction was found in