Precolumn extraction and reversed-phase high-pressure liquid chromatography of prostaglandins and leukotrienes

Prostaglandins, leukotrienes, and other metabolites of arachidonic acid can be conveniently and efficiently extracted from biological media using a precolumn containing octadecylsilyl silica connected to a 6-port switching valve that is in line with an analytical HPLC column. This procedure makes it possible to extract complex mixtures of eicosanoids and to analyze them by reversed-phase HPLC in a single step. The requirement to evaporate solvents from extracts prior to HPLC is therefore eliminated, saving time and reducing the possibilities for loss and contamination. The effects on recoveries of various media for loading the sample onto the precolumn were investigated, and it was concluded that 15% methanol at neutral pH gives the best overall results. It is therefore not necessary to acidity the sample prior to extraction, which simplifies the procedure and improves the recoveries of acid-labile eicosanoids. Following extraction, eicosanoids can be introduced onto the HPLC column by changing the position of the 6-port switching valve. We have investigated several approaches to the analysis of complex mixtures of these products by reversed-phase HPLC. The best results were obtained using a ternary gradient with a non-end-capped column of octadecylsilyl silica. Metabolites of arachidonic acid other than peptido-leukotrienes were first eluted by increasing the concentrations of acetonitrile and methanol in the mobile phase, which contained a constant concentration of trifluoroacetic acid (0.001%). Peptido-leukotrienes were then eluted with a second gradient, in which the concentrations of acetonitrile and methanol were kept constant, but the concentration of trifluoroacetic acid was increased to 0.0091%. Leukotrienes C4, D4, and E4 appear as sharp peaks at the end of the chromatogram and are completely separated from other types of arachidonic acid metabolites.