D-Aspartic acid

The binding of D-[3H]aspartate to sections of rat brain was examined in an autoradiographic assay. Binding was entirely dependent on the presence of sodium ions, but not chloride ions, and was optimal at 2 degrees C. D-Aspartate bound rapidly, reached equilibrium within 20 min and remained stable for 45 min. The rate of dissociation was relatively rapid with a t1/2 of 56 s, but was not as fast as anticipated, perhaps because of some sequestration of ligand. Binding had a Kd of 6.8 +/- 1.2 microM and a Bmax of 49.4 +/- 8.6 pmol/mg protein. The high Bmax value may further indicate some sequestration of D-aspartate. L-Glutamate, unlabeled D-aspartate, and D,L-threo-hydroxyaspartate, a potent inhibitor of synaptosomal uptake, each competed for D-[3H]aspartate binding with IC50s of 7.0 +/- 4.3 microM, 5.4 +/- 1.5 microM, and 2.5 +/- 1.0 microM, respectively. N-methyl-D-aspartate (NMDA), quisqualate, and kainate had no affinity for this site. The regional distribution of D-aspartate binding sites was unique and did not conform to the distribution of neuronal uptake sites described by others. Striatal D-aspartate binding was unaffected by unilateral decortication or striatal quinolinic acid lesions.

A 30% decrease in osmolarity stimulated 3H-taurine, 3H-GABA and glutamate (followed as 3H-D-aspartate) efflux from rat hippocampal slices. 3H-taurine efflux was activated rapidly but inactivated slowly. It was decreased markedly by 100 microM 5-nitro-(3-phenylpropylamino)benzoic acid (NPPB) and 600 microM niflumic acid and inhibited strongly by tyrphostins AG18, AG879 and AG112 (25-100 microM), suggesting a tyrosine kinase-mediated mechanism. Hyposmolarity activated the mitogen-activated protein kinases (MAPK) extracellular-signal-related kinase-1/2 (ERK1/ERK2) and p38, but blockade of this reaction did not affect 3H-taurine efflux. Hyposmosis also activated phosphatidylinositide 3-kinase (PI3K) and its prevention by wortmannin (100 nM) essentially abolished 3H-taurine efflux. 3H-taurine efflux was insensitive to the protein kinase C (PKC) blocker chelerythrine (2.5 microM) or to cytochalasin E (3 microM). The release of 3H-GABA and 3H-D-aspartate occurred by a different mechanism, characterized by rapid activation and inactivation, insensitivity to NPPB, niflumic acid, tyrphostins or wortmannin. 3H-GABA and 3H-D-aspartate efflux was not due to external [NaCl] decrease, cytosolic Ca2+ increase or depolarization, or to reverse operation of the carrier.

In proteins and peptides, d-aspartic acid (d-Asp) and d-β-Asp residues can be spontaneously formed via racemization of the succinimide intermediate formed from l-Asp and l-asparagine (l-Asn) residues. These biologically uncommon amino acid residues are known to have relevance to aging and pathologies. Although nonenzymatic, the succinimide racemization will not occur without a catalyst at room or biological temperature. In the present study, we computationally investigated the mechanism of succinimide racemization catalyzed by dihydrogen phosphate ion, H₂PO₄;-;, by B3LYP/6-31+G(d,p) density functional theory calculations, using a model compound in which an aminosuccinyl (Asu) residue is capped with acetyl (Ace) and NCH₃ (Nme) groups on the N- and C-termini, respectively (Ace-Asu-Nme). It was shown that an H₂PO₄;-; ion can catalyze the enolization of the H;α;-C;α;-C=O portion of the Asu residue by acting as a proton-transfer mediator. The resulting complex between the enol form and H₂PO₄;-; corresponds to a very flat intermediate region on the potential energy surface lying between the initial reactant complex and its mirror-image geometry. The calculated activation barrier (18.8 kcal·mol;-1; after corrections for the zero-point energy and the Gibbs energy of hydration) for the enolization was consistent with the experimental activation energies of Asp racemization.