Identification Of Unusual Oxysterols And Bile acids With 7-Oxo Or 3β,5α,6β-Trihydroxy Functions In Human Plasma By Charge-Tagging Mass Spectrometry With Multistage Fragmentation

7-Oxocholesterol (7-OC), 5,6-epoxycholesterol (5,6-EC) and its hydrolysis product cholestane-3β,5α,6β-triol (3β,5α,6β-triol) are normally minor oxysterols in human samples, however, in disease their levels may be greatly elevated. This is the case in plasma from patients suffering from some lysosomal storage disorders e.g. Niemann Pick disease type C, or the inborn errors of sterol metabolism e.g. Smith-Lemli-Opitz syndrome and cerebrotendinous xanthomatosis. A complication in the analysis of 7-OC and 5,6-EC is that they can also be formed ex vivo from cholesterol during sample handling in air causing confusion with molecules formed in vivo. When formed endogenously 7-OC, 5,6-EC and 3β,5α,6β-triol can be converted to bile acids. Here, we describe methodology based on chemical derivatisation and liquid chromatography mass spectrometry with multistage fragmentation (MSn) to identify the necessary intermediates in the conversion of 7-OC to 3β-hydroxy-7-oxochol-5-enoic acid and 5,6-EC and 3β,5α,6β-triol to 3β,5α,6β-trihydroxycholanoic acid. Identification of intermediate metabolites is facilitated by their unusual MSn fragmentation patterns. Semi-quantitative measurements are possible, but absolute values await the synthesis of isotope-labelled standards.