Serine Racemase Modulates Intracellular D-Serine Levels through an α,β-Elimination Activity

Mammalian brain contains high levels of d-serine, an endogenous co-agonist of N-methyl d-aspartate type of glutamate receptors. d-Serine is synthesized by serine racemase, a brain enriched enzyme converting l- to d-serine. Degradation of d-serine is achieved by d-amino acid oxidase, but this enzyme is not present in forebrain areas that are highly enriched in d-serine. We now report that serine racemase catalyzes the degradation of cellular d-serine itself, through the α,β-elimination of water. The enzyme also catalyzes water α,β-elimination with l-serine and l-threonine. α,β-Elimination with these substrates is observed both in vitro and in vivo. To investigate further the role of α,β-elimination in regulating cellular d-serine, we generated a serine racemase mutant displaying selective impairment of α,β-elimination activity (Q155D). Levels of d-serine synthesized by the Q155D mutant are several-fold higher than the wild-type both in vitro and in vivo. This suggests that the α,β-elimination reaction limits the achievable d-serine concentration in vivo. Additional mutants in vicinal residues (H152S, P153S, and N154F) similarly altered the partition between the α,β-elimination and racemization reactions. α,β-Elimination also competes with the reverse serine racemase reaction in vivo. Although the formation of l- from d-serine is readily detected in Q155D mutant-expressing cells incubated with physiological d-serine concentrations, reversal with wild-type serine racemase-expressing cells required much higher d-serine concentration. We propose that α,β-elimination provides a novel mechanism for regulating intracellular d-serine levels, especially in brain areas that do not possess d-amino acid oxidase activity. Extracellular d-serine is more stable toward α,β-elimination, likely due to physical separation from serine racemase and its elimination activity.