Identification of n-hydroxy acid metabolites in electron impact ionization mass spectrometry

The catabolism of 4-hydroxy acid (drugs of abuse and lipid peroxidation products) generates a series of hydroxy acids with different carbon chain lengths and hydroxyl group at different locations. The identification of these hydroxy acid metabolites is important to uncover the catabolic pathways of drugs of abuse and lipid peroxidation products.

We characterized the fragmentation of trimethylsilyl (TMS) derivatives of hydroxy acids by electron impact ionization (EI) mass spectrometry (MS) with the aid of an isotope-labeled compound. The metabolites (hydroxy acids) of 4-hydroxy acid in isolated rat livers were identified by their characterized fragmentation patterns in gas chromatography (GC)/EI-MS.

TMS migration to both ester and ether groups was found in the fragmentation of 2-hydroxy acid- and 3-hydroxy acid-TMS derivatives, but only migration to the ester group was observed in the fragmentation of n-hydroxy acid-TMS (n ≥4) derivatives. TMS migration to the ester group generates the following fragments from different hydroxy acids: (i) the characteristic fragment at m/z 190 from 2-hydroxy acid; (ii) the fragment at m/z 204 from both 3-hydroxy acid and 4-hydroxy acid; and (iii) a characteristic fragment at m/z 218 from 4-hydroxy acid containing more than four carbons in the carbon skeleton. TMS migration to the ether group in 2-hydroxy acid and 3-hydroxy acid yields variant fragments depending on the carbon skeleton length. The identified metabolites of 4-hydroxy acid confirmed the catabolic pathways of 4-hydroxy acid in the isolated rat livers.

With the characterized fragmentation patterns of each hydroxy acid in EI-MS, we successfully identified the various hydroxy acid metabolites of 4-hydroxyoctanoic acid (and other 4-hydroxy acids from C(5) to C(11)) in the rat livers.