IN VIVO LABELLING OF ACETYL‐ASPARTYL PEPTIDES IN MOUSE BRAIN FROM INTRACRANIALLY AND INTRAPERITONEALLY ADMINISTERED ACETYL‐l‐[U‐14C] ASPARTATE

— Following intracranial and intraperitoneal injection of acetyl‐l‐[U‐14C]aspartate into mice about 5% and 0.7% of the radioactivity, respectively, was recovered from the brain after 30 min. On chromatographic separation of the cationic and anionic compounds on a Dowex 50 column, the former fraction contained about 60% of the radioactivity, predominantly as labelled aspartate and glutamate. The anionic compounds, containing 20% of the labelled compounds, were fractionated in several chromatographic systems and resolved into a great variety of labelled peptidic compounds of which five acetyl‐[U14‐C]aspartyl peptides, containing two to four amino acids, were purified. One of these, acetyl‐aspartyl glutamine, has not previously been found in brain.