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

摘要

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 recom-binant 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.