Purification of serine racemase: Biosynthesis of the neuromodulator <scp>d</scp> -serine

<jats:p> High levels of <jats:sc>d</jats:sc> -serine occur in mammalian brain, where it appears to be an endogenous ligand of the glycine site of <jats:italic>N</jats:italic> -methyl- <jats:sc>d</jats:sc> -aspartate receptors. In glial cultures of rat cerebral cortex, <jats:sc>d</jats:sc> -serine is enriched in type II astrocytes and is released upon stimulation with agonists of non- <jats:italic>N</jats:italic> -methyl- <jats:sc>d</jats:sc> -aspartate glutamate receptors. The high levels of <jats:sc>d</jats:sc> -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 <jats:sc>l</jats:sc> -serine to <jats:sc>d</jats:sc> -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 <jats:sc>l</jats:sc> -serine, failing to racemize any other amino acid tested. Properties such as pH optimum, <jats:italic>K</jats:italic> <jats:sub>m</jats:sub> values, and the requirement for pyridoxal phosphate resemble those of bacterial racemases, suggesting that the biosynthetic pathway for <jats:sc>d</jats:sc> -amino acids is conserved from bacteria to mammalian brain. </jats:p>