Synaptic vesicles transport l-glutamate, but not d-serine. A, Photometric trace showing the absence of d-serine-induced acidification of synaptic vesicles whereas l-glutamate can acidify these organelles (10 mM each, n = 3). Scale bar: 2 a.u. per 1 min. B, ATP-dependent uptake of d-[3H]serine and l-[3H]glutamate by isolated SVs. The values represent FCCP-sensitive uptake. N.D. not detectable. Values represent mean ± SEM of 2 independent experiments performed in triplicate. C, Western blot analysis of cortex homogenate (Input) and control or Sb2 immunoisolates (IP) for vesicular glutamate transporters vGlut1 and vGlut2 and for serine racemase (SR).