title: Serine racemase and D-serine transport in human placenta and evidence for a transplacental gradient for D-serine in humans.
abstract: OBJECTIVE
  To investigate the possible role of human placenta in providing D-serine to the developing fetus.
  
  
  METHODS
  Expression of serine racemase in placenta was determined by reverse transcriptase polymerase chain reaction and northern analysis and confirmed by subsequent cloning. The transport of D-serine by human ATB(0) was characterized by expressing the cloned cDNA transiently in mammalian cells using the vaccinia virus expression system. D-serine levels in maternal and fetal blood were measured by fluorescence high-performance liquid chromatography (HPLC) after derivatization of the amino acids with o-phthaldialdehyde and N-tertiary-butyloxycarbonyl-L-cysteine.
  
  
  RESULTS
  mRNA for serine racemase was detected in placenta. ATB(0) was capable of d-serine transport, and the transport process is obligatorily dependent on sodium (Na+) with a Na(+):substrate stoichiometry of 1:1 and saturable with a Michaelis-Menten constant of 310 +/- 30 microM. Furthermore, studies have shown that ATB(0) is not expressed in the maternal-facing brush border membrane of human placental syncytiotrophoblast. The circulating concentration of D-serine in maternal serum is 5.8 +/- 0.5 microM, and the corresponding value in the fetal serum is 14.6 +/- 1.2 microM, indicating a two- to three-fold higher concentration of D-serine in the fetus than in the mother.
  
  
  CONCLUSIONS
  We speculate that D-serine is synthesized in human placenta by the racemization of L-serine and that ATB(0), expressed on the basal membrane of the syncytiotrophoblast, mediates the efflux of D-serine into the fetal circulation in exchange for other amino acids in fetal blood.
authors.affiliation: Department of Biochemistry and Molecular Biology, Medical College of Georgia, 1120 Laney Walker Boulevard, Augusta, GA 30912-2100, USA.
authors.forename: Zhong
authors.initials: Z
authors.lastname: Chen
authors.forename: Wei
authors.initials: W
authors.lastname: Huang
authors.forename: Sonne R
authors.initials: SR
authors.lastname: Srinivas
authors.forename: Chandra R
authors.initials: CR
authors.lastname: Jones
authors.forename: Vadivel
authors.initials: V
authors.lastname: Ganapathy
authors.forename: Puttur D
authors.initials: PD
authors.lastname: Prasad
mesh_terms: D001692:Biological Transport; D003470:Culture Media; D005260:Female; D006801:Humans; D007700:Kinetics; D010920:Placenta / Q000201:enzymology*; D011247:Pregnancy; D019998:Racemases and Epimerases / Q000378:metabolism*; D012694:Serine / Q000378:metabolism*; D013237:Stereoisomerism; D000099300:Serine Racemase
chemical_list: D003470:Culture Media; D019998:Racemases and Epimerases; D012694:Serine; D000099300:Serine Racemase