Glycine-related amino acids stereoselectively affect N-methyl-D-aspartate receptor-mediated contractions of guinea pig ileum: comparison with the inhibition of strychnine-insensitive [3H]glycine binding to rat cortical membranes

Direct calcium binding results in activation of brain serine racemase

Serine racemase (SR) is a brain enzyme present in glial cells, where it isomerizes L-serine into D-serine that, in turn, diffuses and coactivates the N-methyl-D-aspartate receptor through the binding to the so-called "glycine site." We have developed a method for the slow expression of SR in a eukaryotic vector that permits the correct insertion of the prosthetic group into the active site, rendering functional SR with a K(m) toward L-serine of 4.8 mm. Divalent cations such as calcium or manganese were necessary for complete enzyme activity, whereas the presence of chelators such as EDTA completely inhibited the enzyme. Moreover, direct binding of calcium to SR was evidenced using (45)Ca(2+). Gel filtration of the recombinant SR revealed the protein to be in a dimer-tetramer equilibrium. The addition of EDTA to a calcium-saturated serine racemase evokes a profound conformational change, as monitored by both fluorescence and circular dichroism techniques. Fluorescence titration allowed us to calculate a binding constant for calcium of 6.2 microm. Reagents that react with sulfhydryl groups, such as cystamine, were potent inhibitors of SR, in a clear reflection that one or more cysteine residues are important for enzyme activity. Additionally, 16 serine analogues were tested as a putative SR substrate or inhibitors. Significant inhibition was only observed for L-Ser-O-sulfate, L-cycloserine, and L-cysteine. Finally, activation of brain SR as a result of the changes in calcium concentration was studied in primary astrocytes. Treatment of astrocytes with the calcium ionophore, as well as with compounds that augment the intracellular calcium levels such as glutamate or kainate led to an increase in the amount of d-serine present in the extracellular medium. These results suggest that there might be a glutamatergic-mediated regulation of SR activity by intracellular calcium concentration.

Free D-aspartate and D-serine in the mammalian brain and periphery

It has long been assumed that L-forms of amino acids exclusively constitute free amino acid pools in mammals. However, a variety of studies in the last decade has demonstrated that free D-aspartate and D-serine occur in mammals and may have important physiological function in mammals. Free D-serine is confined predominantly to the forebrain structure, and the distribution and development of D-serine correspond well with those of the N-methyl-D-aspartate (NMDA)-type excitatory amino acid receptor. As D-serine acts as a potent and selective agonist for the strychnine-insensitive glycine site of the NMDA receptor, it is proposed that D-serine is a potential candidate for an NMDA receptor-related glycine site agonist in mammalian brain. In contrast, widespread and transient emergence of a high concentration of free D-aspartate is observed in the brain and periphery. Since the periods of maximal emergence of D-aspartate in the brain and periphery occur during critical periods of morphological and functional maturation of the organs, D-aspartate could participate in the regulation of these regulation of these developmental processes of the organs. This review deals with the recent advances in the studies of presence of free D-aspartate and D-serine and their metabolic systems in mammals. Since D-aspartate and D-serine have been shown to potentiate NMDA receptor-mediated transmission through the glutamate binding site and the strychnine-insensitive glycine binding site, respectively, and have been utilized extensively as potent and selective tools to study the excitatory amino acid system in the brain, we shall discuss also the NMDA receptor and uptake system of D-amino acids.

Excitatory amino acid receptors in the mammalian periphery

Aspartate and glutamate occur ubiquitously in free and chemically bound forms and have been considered primarily as substances of metabolic relevance. This focus has changed with the more recent discovery of their specific role as excitatory synaptic transmitters in the mammalian CNS. Enthusiasm for this concept has overshadowed the possibility that glutamate and aspartate may also have specific, receptor-mediated functions in the periphery. In this review, Sándor Erdö summarizes the current knowledge of excitatory amino acid (EAA) receptors outside the CNS, through which EAAs may modulate various functions in peripheral organs and tissues.