Inhibition by N-acetyl-aspartate of aspartate binding to a proteolipid fraction from rat cerebral cortex

N-acetylaspartate in neuropsychiatric disorders

N-Acetyl aspartate (NAA) is the second most abundant amino acid in the human brain. NAA is synthesized by L-aspartate N-acetyl transferase or by cleavage from N-acetyl aspartyl glutamate by N-acylated alpha-linked L-amino dipeptidase (NAALADase); and it is catabolized to acetate and aspartate by N-acetyl aspartate amino hydrolase (amino acylase II). NAA is localized primarily to neurons, where it is concentrated in the cytosol. Although NAA is devoid of neurophysiological effects, it serves as an acetyl donor, an initiator of protein synthesis or a carbon transfer source across the mitochondrial membrane. The concentration of NAA in human brain increases 3-fold between midgestation and adulthood. In Canavan's Disease, an autosomal recessive disorder due to a null mutation in amino acylase II, NAA levels in brain are markedly increased and disrupt myelination. NAA levels have been found to be reduced in neurodegenerative disorders, including Alzheimer's Disease and Huntington's Disease. Since endogenous NAA can be readily detected in human brain by magnetic resonance spectroscopy, it is increasingly being exploited as a marker for functional and structural integrity of neurons in an expanding number of disorders.

N-acetyl-L-aspartic acid: a literature review of a compound prominent in 1H-NMR spectroscopic studies of brain

N-acetyl aspartic acid (NAA), discovered in 1956 by Tallan, is the major peak seen in water-suppressed NMR proton (hydrogen) spectroscopy. NAA makes up about one thousandth of the wet weight of human brain and appears to be limited solely to neurons. This compound has been shown to be relatively stable for a period of twenty-four hours post-mortem and the concentration of NAA is not changed by insulin-induced hypoglycemia. MAO inhibitors lower its concentration while reserpine and other drugs increase it. NAA has been implicated in many processes of the nervous system: it may be involved in the regulation of neuronal protein synthesis, myelin production, or the metabolism of several neurotransmitters such as aspartate or N-acetyl-aspartyl-glutamate. It is involved in the neurologic disorder Canavan disease and has grown to be a vital component of in vivo 1H-NMR spectroscopic studies.

A re-examination of the interaction of N-acetyl-L-aspartyl-L-glutamate with a subpopulation of rat brain membrane L-[3H]glutamate binding sites

The neuropeptide, N-acetyl-L-aspartyl-L-glutamate (NAAG) has been reported to act at a subpopulation of putative quisqualate receptors on the basis of its competitive inhibition of specific binding of L-[3H]glutamate and on the basis of quisqualate-sensitive binding of [3H]NAAG radiolabeled on the glutamate moiety. Recently, a membrane-bound metallopeptidase, N-acetylated alpha-linked acidic dipeptidase (NAALADase), which cleaves NAAG to N-acetyl-aspartate (NAA) and glutamate, has been characterized and has been shown to exhibit optimal activity under incubation conditions used to measure NAAG binding sites. Accordingly, we have examined whether NAALADase mediated release of glutamate from NAAG might account for the receptor binding results. Insertion of empirically derived kinetic constants for NAALADase hydrolysis of NAAG into a theoretical model for peptide-derived glutamate inhibition of [3H]glutamate binding reveals that NAAG can appear to displace, with high affinity, a subpopulation of [3H]glutamate labeled sites, and yet have little or no intrinsic activity for these sites. Furthermore, empirical data relating time and protein concentration to NAAG displacement of [3H]glutamate binding are more consistent with a proteolytic mechanism rather than an equilibrium interaction of the peptide with membrane receptors. Coupled with recent findings attributing the Cl- -dependent glutamate binding to a sequestration phenomenon, these results demonstrate that the inferred action of NAAG at glutamate synaptic receptors through previous radioligand binding studies is probably incorrect. Furthermore, these studies offer a general caution regarding the conclusions about subpopulations of receptors drawn from receptor binding assays conducted with ligands, which may be structurally modified by enzymatic processes.

13C n.m.r. study of the structure and the metal ion binding sites of neuropeptides composed of L-Asp and L-Glu

13C NMR spectral data are presented for a variety of possible neuropeptides composed of L-Asp, Ac-L-Asp, and L-Glu which contain alpha and beta peptide linkages. The data for the various compounds are compared to the data presented for Ac-Asp-Glu, a known neuropeptide, in order to gain structural information about the related compounds. Indications are that for compounds 1 and 5, the cis peptide bond configuration exists due to the interaction of zwitterionic species. This interaction appears to be eliminated when the beta peptide bonds are formed, as in the case of compounds 3 and 7. Spin-lattice relaxation times were used to confirm the structures. Electron-nuclear relaxation rates are also used to elucidate the metal ion binding sites of these species.