Cross‐linking of serine racemase dimer by reactive oxygen species and reactive nitrogen species

Abstract

Serine racemase (SR) is the only identified enzyme in mammals responsible for isomerization of L-serine to D-serine, a co-agonist at NMDA receptors in the forebrain. Our previous data reported that an apparent SR dimer resistant to SDS and β-mercaptoethanol was elevated in microglial cells after proinflammatory activation. Because the activation of microglia is typically associated with an oxidative burst, oxidative cross-linking between SR subunits was speculated. In this study, an siRNA technique was employed to confirm the identity of this SR dimer band. The oxidative species potentially responsible for the cross-linking was investigated with recombinant SR protein. The data indicate that nitric oxide, peroxynitrite, and hydroxyl radical were the likely candidates, while superoxide and hydrogen peroxide per se failed to contribute. Furthermore, the mechanism of formation of SR dimer by peroxynitrite oxidation was studied by mass spectrometry. A disulfide bond between Cys6 and Cys113 was identified in both SIN-1 treated SR monomer and dimer. Activity assays indicated that SIN-1 treatment decreased SR activity, confirming our previous conclusion that noncovalent dimer is the most active form of SR. These findings suggest a compensatory feedback whereby the consequences of neuroinflammation might dampen D-serine production to limit excitotoxic stimulation of NMDA receptors.

Serine racemase as a prime target for age‐related memory deficits

“…These observations strongly suggest that SR could be targeted by the age‐related accumulation of reactive oxygen species, leading to cognitive defects through the deregulation of the d ‐serine‐related pathway. This possibility is consolidated by the in vitro observations that SR activity is inhibited by nitric oxide‐mediated S‐nitrosylation (Mustafa et al ., ) and sullfhydryl oxidation (Wolosker et al ., ), whereas SR conformational states are affected by peroxynitrite oxidation (Wang & Barger, ). To confirm the hypothesis of reactive oxygen species targeting SR in the course of ageing, rodents were recently treated with the reducing agent N ‐acetyl‐cysteine from middle age to senescence to prevent oxidative damage in the ageing brain (Haxaire et al ., ).…”

Improved NMDA Receptor Activation by the Secreted Amyloid-Protein Precursor-α in Healthy Aging: A Role for D-Serine?

“…Accordingly, transcriptional SR expression is promoted by the APP fragment that is associated with D-serine release [ 57 ], though these effects have, as of yet, only been characterized in microglia cultures. Alternatively, increased oxidation [ 89 ] and/or changes in dimer active conformation [ 90 ] by changes in redox status are thought to alter SR activity, especially in the aging brain in which potent oxidative stress occurs (see [ 4 ]). Considering that sAPPα activates signaling pathways that protect synapses against excitotoxicity linked to increased oxidation and impaired energy metabolism [ 91 , 92 , 93 , 94 ], this antioxidative property may help sAPPα to optimize SR activity and D-serine production.…”

d-serine in physiological and pathological brain aging

“…They are also prevented in aged animals receiving a long-term treatment with the reducing compound N-acetyl-cysteine in which the extent of OS is minimized [57]. An increased oxidation of sulfydryl groups of SR [58] and/or changes in its dimer active conformation [59] may be viewed as possible mechanisms underlying the age-J o u r n a l P r e -p r o o f related SR deregulation by OS. Interestingly, aged LOU/C rats show not only intact synaptic plasticity and NMDAr activation but also preserved memory abilities [55,56,60].…”