A relationship of n-acetylaspartate biosynthesis to neuronal protein synthesis / D. D. Clarke, S. Greenfield, E. Dicker, L.J. Tirri, and E.J. Ronan Chemistry Department, Fordham University, Bronx, NY 10458, U.S.A

A detailed time study of the incorporation of label from sodium-[lC]acetate, [1C]ethanol, and [2C]glucose into the aspartyl moiety of N -acetylaspartic acid (NAA) was conducted. As expected the specific activity of aspartate increased rapidly with time and peaked within 15 20 min after which it fell sharply: but significant!). that of the aspartyl moiety of NAA rose very slowly even after the specific activity of aspartate had fallen to less than I per cent of the peak values. A rat brain microsomal free supernatant preparation was shown enzymatically to incorporate label from sodium-[lC]acetate into the t-RNA fraction from which was isolated N-[lC]acetylaspartic acid. From these observations we were inclined to speculate that NAA-t-RNA may serve as an initiator of neuronal protein synthesis. ALTHOUGH N-acetylaspartate (NAA) has been found to occur in large quantities in brain, 5-7 Jln1o1/g (TALLAN et al., 1956; TALLAN, 1957; GEBHARD & VLLDSTRA, 1964)~ its functional and metabolic importance is as yet unclarified. Many investigators reported that within short periods of time after the administration of labelled precursors. very little radioactivity is incorporated into the aspartyl moiety of NAA, as compared to that in other amino acids, suggesting that endogenous NAA might be metabolically inert (JACOBSON, l958; MARGOLIS et al., 1960; BERL et al., 1961; O'NEAL & KOEPPE, 1966; REICHELT & KVAMME, 1967). However, the slow labelling of NAA indicated to us that (1) NAA is not metabolically inert and (2) its metabolism 1 A preliminary report was presented at the 1972 Meeting of The American Society for Neurochemistry, Seattle, Washington. 2 This investigation was supported by Research Grant NS-07890, ~ational Institutes of Health, Public Health Service. Dr. Ronan is recipient of NASA Traineeship NSG-T121. 3 Present address: Institute of Medical Biochemistry, University of Aarhus, DK-8000C, Aarhus, Denmark. 4 Present address: The Saul R. Korey Department of Neurology, Albert Einstein College of Medicine of Yeshiva University, 1300 Morris Park Avenue, Bronx, NY 10461, U.S.A. 5 Present address: Department of Pharmacology, Mt. Sinai School of Medicine, New York, NY l 0029. U.S.A. Ahhreviations used: NAA, N-acetylaspartic acid; TCA, trichloroacetic acid; HOAc, acetic acid. ~C. 24 3 I 479 is not directly related to that of free aspartic acid, but rather to that of protein bound aspartate. In accordance with this hypothesis, we investigated the incorporation of label from sodium-[ l14C]acetate, [1-C]ethanol, and [2-C]glucose into the aspartyl moiety ofNAA for time periods up to 2 h. We also sought evidence for the existence of an enzyme system capable of synthesizing NAA-t-RNA which would relate the biosynthesis ofNAA to that of protein bound aspartate. MATERIALS AND METHODS