Cx-aminoadipic Acid as a Constituent of a Corn Protein*

In a study of transaminase systems Braunstein (1) learned that cr-aminoadipic acid takes part in transamination. Both pigeon breast muscle mince and a purified glutamic transaminase extracted from it were active in transferring an amino group from alanine to a-ketoadipic acid. A plant aspartic transaminase was inactive with the same substrate. Experiments by Borsook and Dubnoff (2) on the synthesis of arginine by rat kidney slices showed that, in addition to the activity of glutamic and aspartic acids in transferring amino groups to citrulline, lysine was active. They suggested at that time that lysine was converted to glutamic acid. Since aminoadipic acid was active in transamination, Braunstein suggested (3) that the lysine activity in arginine synthesis was due to its conversion into aminoadipic acid. Dubnoff and Borsook (4) repeated their previous experiments using aminoadipic acid as a substrate and found that it could aminate citrulline sufficiently well to explain the activity of lysine. That one path of lysine degradation in guinea pig liver homogenate was through ac-aminoadipic acid was proved by Borsook et al. (5) using radioactive L-lysine as a substrate and isolating radioactive a-aminoadipic acid from the reaction mixture. The aminoadipic acid was further degraded to a-ketoadipic acid and glutaric acid (6). Geiger and Dunn (7) and Stevens and Ellman (8) independently found that aminoadipic acid was unable to replace lysine as an essential amino acid in rats and in two bacteria used in bioassays for lysine, Leuconostoc mesenteroides and Streptococcus faecalis. The occurrence of free aminoadipic acid in biological material raises the possibility that this amino acid may be a protein constituent. In the research described herein evidence is presented that a-aminoadipic acid is part of a natural protein.