Selenium speciation in paired serum and cerebrospinal fluid samples

Se speciation was performed in 24 individual paired serum and cerebrospinal fluid (CSF) samples from neurologically healthy persons. Strong anion exchange (SAX) separation, coupled to inductively coupled plasma-dynamic reaction cell-mass spectrometry (ICP-DRC-MS), was employed. Species identification was done by standard matched retention time, standard addition and by size exclusion chromatography followed from SAX (2-D SEC-SAX-ICP-DRC-MS) and by SAX followed from CE-ICP-DRC-MS (2-D SAX-CE-ICP-DRC-MS). Limit of detection (LoD, 3 × standard deviation (SD) of noise) was in the range of 0.026-0.031 μg/L for all investigated species and thus was set uniformly to 0.032 μg/L. Quality control for total Se determination was performed by analysing control materials "human serum" and "urine", where determined values met target values. Several Se species were found in both sample types having following median values (sequence: serum/CSF, each in μg Se/L): total Se, 58.39/0.86; selenoprotein P (SePP), 5.19/0.47; Se-methionine (SeM), 0.23/<LoD; glutathione peroxidase (GPx), 4.2/0.036; thioredoxinreductase (TrxR), 1.64/0.035; Se IV, 12.25/0.046; Se-human serum albumin (Se-HSA), 18.03/0.068. Other Se species, such as Se-cystine (SeC), Se VI and up to four non-identified compounds were monitored (if ever) only in very few samples usually close to LoD. Therefore, their median values were <LoD. Linear relationships based on median values provide information about Se-species passage across neural barriers (NB): SePP_-serum is significantly correlated to total Se_-serum when the latter was > 65 μg/L; however, SePP_-CSF appeared independent of SePP_-serum. For Se-HSA_-serum versus (vs.) Se-HSA_-CSF, a weak linear relationship was found (r ² = 0.1722). On the contrary, for anti-oxidative Se-enzymes, higher r ² values were calculated: GPx_-serum vs. GPx_-CSF, r ² = 0.3837; TrxR _-serum vs. TrxR_-CSF, r ² = 0.6293. Q _-Se-species values (= ratios of CSF_-Se-species/serum _-Se-species) were compared with the Q _-Alb value (HSA _-CSF/HSA_-serum = clinical index of NB integrity) for deeper information about NB passage of Se species. The Q _-Se-HSA value (3.8 × 10^-3) was in accordance to the molecular mass dependent restriction at NB (Q _-Alb at 5.25 × 10^-3). Increased Q values were seen for TrxR (21.3 × 10^-3) and GPx (8.3 × 10^-3) which are not (completely) explained by molecular size. For these two anti-oxidative Se-enzymes (GPx, TrxR), we hypothesize that there might be either a facilitated diffusion across NB or they might be additionally synthesized in the brain.