Fast LC–MS/MS analysis of free oxysterols derived from reactive oxygen species in human plasma and carotid plaque

A rapid liquid chromatography–tandem mass spectrometric (LC–MS/MS) method was developed and validated for the quantification of reactive oxygen species (ROS) derived free oxysterols and cholesterol in human plasma and atherosclerotic plaque. In vitro autoxidation of cholesterol during sample pretreatment was avoided by applying only one protein precipitation and re-concentration step using 80 μl plasma. For preparation of 10 mg atherosclerotic plaques an additional liquid–liquid extraction was included. Free 7-keto-, 7-α/ß-hydroxy-, 5,6-α-epoxy-, 5,6-β-epoxycholesterol, cholestane-3ß,5α,6ß-triol and cholesterol were separated within 7 min on a monolithic column. An API 4000 tandem mass spectrometer was applied in positive ionization mode using atmospheric pressure chemical ionization. The detection limit was 0.1 ng/ml and the linearity ranged from 0.5 to 0.75 to 2000 ng/ml for the oxysterols and from 50 to 1000 μg/ml for cholesterol. Recovery was between 80.9 and 107.9%. Between-run imprecision ranged from 7.9 to 11.7%. Analysis of plasma samples from additional 50 middle-aged volunteers revealed a large inter-individual variability (e.g. 7-ketocholesterol 2.63-30.47 ng/ml). Oxysterol concentrations normalized to cholesterol were about 43 times higher in carotid plaque compared to plasma (n = 5). This rapid LC–MS/MS method enables reliable quantification focused on especially ROS-derived oxysterols in human plasma and atherosclerotic plaque samples under high-throughput conditions.