Determination of cholesterol oxidation products in human plasma by isotope dilution-mass spectrometry

A method based on isotope dilution-mass spectrometry was developed for the determination of nine cholesterol oxidation products in human plasma. The cholesterol oxidation products determined were cholest-5-ene-3 beta,7 alpha-diol, cholest-5-ene-3 beta,7 beta-diol (7 alpha- and 7 beta-hydroxycholesterol, respectively), 3 beta-hydroxycholest-5-en-7-one(7-oxocholesterol),5,6 alpha-epoxy-5 alpha- cholestan-3 beta-ol (cholesterol-5 alpha,6 alpha-epoxide),5,6 beta-epoxy-5 beta-cholestan-3 beta-ol (cholesterol-5 beta,6 beta-epoxide), (cholesterol-5 beta,6 beta-epoxide), cholestane-3 beta,5 alpha,6 beta-triol, cholest-5-ene-3 beta,24-diol (24-hydroxycholesterol), cholest-5-ene-3 beta,25-diol (25-hydroxycholesterol), and cholest-5-ene-3 beta,27-diol (27-hydroxycholesterol). A corresponding deuterium-labeled internal standard, containing 3 to 6 deuterium atoms, was synthesized for each cholesterol oxidation product except 5 beta,6 beta-epoxycholesterol which was determined using the internal standard for 5 alpha,6 alpha-epoxycholesterol. Plasma from 31 healthy volunteers was analyzed by the new method and 27-, 24-, and 7 alpha-hydroxycholesterol were the most abundant cholesterol oxidation products (mean values 154, 64, and 43 ng/ml, respectively). The other oxysterols determined were present in concentrations lower than 30 ng/ml. Males had higher 27-hydroxycholesterol concentrations in plasma than females. The 5,6-oxygenated products were present mainly unesterified while the other oxidation products were mostly in esterified form.