Purification of serine racemase: Biosynthesis of the neuromodulator d -serine

Abstract

High levels of D-serine occur in mammalian brain, where it appears to be an endogenous ligand of the glycine site of N-methyl-D-aspartate receptors. In glial cultures of rat cerebral cortex, D-serine is enriched in type II astrocytes and is released upon stimulation with agonists of non-N-methyl-D-aspartate glutamate receptors. The high levels of D-serine in discrete areas of rat brain imply the existence of a biosynthetic pathway. We have purified from rat brain a soluble enzyme that catalyzes the direct racemization of L-serine to D-serine. Purified serine racemase has a molecular mass of 37 kDa and requires pyridoxal 5-phosphate for its activity. The enzyme is highly selective toward L-serine, failing to racemize any other amino acid tested. Properties such as pH optimum, K m values, and the requirement for pyridoxal phosphate resemble those of bacterial racemases, suggesting that the biosynthetic pathway for D-amino acids is conserved from bacteria to mammalian brain.D-Amino acids are prominent in bacteria whereas in animal tissues L-amino acids occur exclusively, though there have been occasional reports of D-amino acids, usually in invertebrates (1, 2). Recently, D-serine (3-6) and D-aspartate (7,8) were reported in mammalian tissues, especially in the nervous system. Using highly selective antibodies, we localized D-aspartate to neuroendocrine tissues (9), whereas the immunohistochemical localizations of D-serine closely resemble N-methyl-D-aspartate (NMDA) receptors for the neurotransmitter glutamate, as the distribution of D-serine measured chemically (10, 11). Glutamate cannot activate the NMDA receptor in the absence of glycine, indicating a ''glycine site'' for the receptor (12, 13). D-Serine is up to three times more potent than glycine at this site (14), suggesting that D-serine is the endogenous ligand for this site. D-Serine is localized exclusively to type II astrocytes, a form of glia concentrated in gray matter in the same areas of the brain as NMDA receptors (10). Stimulation of the kainate subtype of glutamate receptors releases D-serine from type II astrocytes, implying that synaptic release of glutamate triggers release of D-serine from the astrocytes to activate NMDA receptors physiologically (10). Although in most parts of the brain the distribution of D-serine resembles NMDA receptors far better than glycine, in some areas glycine and NMDA receptors are colocalized, suggesting that D-serine is the predominant ligand for the receptor in most brain areas but that glycine serves this purpose in some sites (11).Understanding the neurobiology of D-serine requires delineation of its biosynthesis. D-Serine might be formed by direct racemization from L-serine. Dunlop and Neidle (15) reported the transformation of radiolabeled L-serine to D-serine in intact rats, but this might have involved multiple steps rather than any direct enzymatic racemization. In the present study we describe an enzyme activity that directly racemizes L-serine to D-serine and that is localized to the brain. We ...

Population genomics in a disease targeted primary cell model

“…The SRR protein was purified a decade ago (Wolosker et al 1999) and shown to convert L-serine to D-serine, the endogenous ligand of the glycine site of NMDA receptors in the brain, and since then the focus has been on its association to neurological disorders (Yamada et al 2005;Morita et al 2007). However, certain subunits of NMDA receptors are known to be expressed in osteoblasts and chondrocytes (Chenu et al 1998;Laketic-Ljubojevic et al 1999;Hinoi et al 2003), and recently it was shown that SRR negatively regulates chondrocyte differentiation Where a proxy SNP is not presented, the GWAS SNP corresponded to the strongest association.…”

Serine racemase: Activation by glutamate neurotransmission via glutamate receptor interacting protein and mediation of neuronal migration

“…4 E and F). In SR(K56G), the catalytic lysine is mutated to glycine, resulting in complete inactivation of the enzyme (19). Granule cell migration is markedly greater in preparations overlying SR-containing HEK-293 cells and over those with SR(K56G) or empty vector.…”

Glutamate receptor activation triggers a calcium-dependent and SNARE protein-dependent release of the gliotransmitter D-serine

“…6B, third line). Analysis of the D-serine content for each fraction revealed a peak of the amino acid in intermediate fractions [9][10][11][12] (Fig. 6C), where immunoreactivity for VAMP2 is higher.…”