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.