Serine racemase: A glial enzyme synthesizing d -serine to regulate glutamate- N -methyl- d -aspartate neurotransmission

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Abstract
Although d amino acids are prominent in bacteria, they generally are thought not to occur in mammals. Recently, high levels of d -serine have been found in mammalian brain where it activates glutamate/ N -methyl- d -aspartate receptors by interacting with the “glycine site” of the receptor. Because amino acid racemases are thought to be restricted to bacteria and insects, the origin of d -serine in mammals has been puzzling. We now report cloning and expression of serine racemase, an enzyme catalyzing the formation of d -serine from l -serine. Serine racemase is a protein representing an additional family of pyridoxal-5′ phosphate-dependent enzymes in eukaryotes. The enzyme is enriched in rat brain where it occurs in glial cells that possess high levels of d -serine in vivo . Occurrence of serine racemase in the brain demonstrates the conservation of d -amino acid metabolism in mammals with implications for the regulation of N -methyl- d -aspartate neurotransmission through glia-neuronal interactions.