Serine racemase modulates intracellular D-serine levels through an alpha,beta-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 alpha,beta-elimination of water. The enzyme also catalyzes water alpha,beta-elimination with L-serine and L-threonine. alpha,beta-Elimination with these substrates is observed both in vitro and in vivo. To investigate further the role of alpha,beta-elimination in regulating cellular D-serine, we generated a serine racemase mutant displaying selective impairment of alpha,beta-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 alpha,beta-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 alpha,beta-elimination and racemization reactions. alpha,beta-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 alpha,beta-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 alpha,beta-elimination, likely due to physical separation from serine racemase and its elimination activity.
Foltyn
Veronika N
VN
Department of Biochemistry, Technion-Israel Institute of Technology, The B. Rappaport Faculty of Medicine, Haifa 31096, Israel.
Bendikov
Inna
I
De Miranda
Joari
J
Panizzutti
Rogerio
R
Dumin
Elena
E
Shleper
Maria
M
Li
Pu
P
Toney
Michael D
MD
Kartvelishvily
Elena
E
Wolosker
Herman
H
D000255:Adenosine Triphosphate / Q000378:metabolism; D000595:Amino Acid Sequence; D002384:Catalysis; D002460:Cell Line; D006801:Humans; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D019998:Racemases and Epimerases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D012694:Serine / Q000378:metabolism*; D014867:Water / Q000378:metabolism; D000099300:Serine Racemase
D000255:Adenosine Triphosphate; D019998:Racemases and Epimerases; D011994:Recombinant Proteins; D012694:Serine; D014867:Water; D000099300:Serine Racemase