|M.Sc Student||Bendikov Inna|
|Subject||Mechanism of Synthesis and Degradation of Neuromodulator|
|Department||Department of Medicine||Supervisor||Professor Herman Wolosker|
Mammalian brain contains high levels of D-serine, an endogenous coagonist of N-methyl D-aspartate type of glutamate receptors. The presence of D-serine in the brain challenges the idea that only L-amino acids will be present or play a role in mammals. We now sought to understand the mechanism of biosynthesis and degradation of brain D-serine, which will shed light on several aspects of D-serine disposition and neurobiology. D-Serine is synthesized by serine racemase, a brain enriched enzyme converting L-serine 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 a,b-elimination of water. The enzyme also catalyzes water elimination with L-serine and L-threonine and elimination of these substrates is observed both in vitro and in vivo. In order to further investigate the role of elimination in regulating cellular D-serine, we generated a serine racemase mutant displaying selective impairment of elimination activity (Q155D). Levels of D-serine synthesized by the Q155D mutant are several folds higher than the wild type both in vitro and in vivo. This suggests that 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 a,b-elimination and racemization reactions. Elimination also competes with the reverse serine racemase reaction in vivo. While the formation of L-serine 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 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 elimination, likely due to physical separation from serine racemase and its elimination activity.