Sialic acid moiety of apolipoprotein E and its impact on the formation of lipoprotein particles in human cerebrospinal fluid.

Apolipoprotein (apo) E in the cerebrospinal fluid (CSF) is abundant with sialic acid (SA), and sialylation of certain proteins is known to modulate biological function. The aim of the present study was to quantify the SA content in CSF apoE and carry out the more detailed characterization of the CSF apoE-containing lipoproteins.The method for the determination of the SA in CSF apoE was based on the conversion of SA into p-aminobenzoic acid ethyl ester-derivatized N-acetylmannosamine, followed by HPLC analysis.The levels of CSF SA and serum SA were 25.9+/-1.5 and 2209+/-196 micromol/l, respectively; however, when the SA values were corrected by the total protein concentrations, CSF SA values were approximately 3.5-fold of those in the serum. The SA levels in the CSF apoE-containing lipoprotein fractions were 5.3+/-1.3% of total CSF SA, and were correlated with the CSF apoE concentrations. However, the ratios of SA to apoE were inversely proportional to the CSF lipid concentrations. The lipoprotein particle sizes were larger when the ratios of SA to CSF apoE were greater.The SA moiety of the CSF apoE molecules may affect the formation of the apoE-containing lipoprotein particles and the regulation of lipid delivery in CNS.