Cofactors of serine racemase that physiologically stimulate the synthesis of the N -methyl- d -aspartate (NMDA) receptor coagonist d -serine

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Abstract
High levels of d-serine occur in the brain, challenging the notion that d-amino acids would not be present or play a role in mammals. d-serine levels in the brain are even higher than many l-amino acids, such as asparagine, valine, isoleucine, and tryptophan, among others. d-serine is synthesized by a serine racemase (SR) enzyme, which directly converts l- to d-serine. We now report that SR is a bifunctional enzyme, producing both d-serine and pyruvate in cultured cells and in vitro. Transfection of SR into HEK 293 cells elicits synthesis of d-serine and augmented release of pyruvate to culture media. We identified substances present in HEK 293 and astrocyte cell extracts that strongly stimulate d-serine production by SR and elicit production of pyruvate. Experiments with recombinant enzyme reveal that Mg(2+) and ATP present in the cell extracts are physiological cofactors and increase 5- to 10-fold the rates of racemization and production of pyruvate. As much as three molecules of pyruvate are synthesized for each molecule of d-serine produced by SR. This finding constitutes a previously undescribed mechanism underlying d-amino acid synthesis in mammals, different from classical amino acid racemases present in bacteria. Our data link the production of d-serine to the energy metabolism, with implications for the metabolic and transmitter crosstalk between glia and neurons.

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