Identification of metabolites from type III F2-isoprostane diastereoisomers by mass spectrometry.

F(2)-isoprostanes (F(2)-iPs) are prostaglandin (PG)-like products of non-enzymatic free radical-catalyzed peroxidation of arachidonic acid that are now widely used as indices of lipid peroxidation in vivo. Knowledge of the metabolic fate of F(2)-iPs in vivo is still scant, despite its importance for defining their overall formation and biological effects in vivo. Type III F(2)-iPs, which are diastereoisomers of cyclooxygenase-derived PGF(2alpha), may be metabolized through the pathways of PG metabolism. We therefore studied the in vitro metabolism of eight synthetic Type III F(2)-iP diastereoisomers in comparison with PGF(2alpha). We used gas chromatography-mass spectrometry and high performance liquid chromatography-electrospray-tandem mass spectrometry for structural identification of metabolites formed after incubation of the various compounds with isolated rat hepatocytes. PGF(2alpha) was metabolized to several known products, resulting from a combination of beta-oxidation, reduction of Delta(5) and/or Delta(13) double bonds, and 15-OH oxidation, plus other novel products deriving from conjugation with taurine of PGF(2alpha) and its metabolites. Of the eight F(2)-iP diastereoisomers, some were processed similarly to PGF(2alpha), whereas others showed peculiar metabolic profiles according to specific stereochemical configurations. These data represent the first evidence of biodegradation of selected Type III F(2)-iP isomers other than 8-epi-PGF(2alpha), through known and novel pathways of PGF(2alpha) metabolism. The analytical characterization of these products may serve as a basis for identifying the most significant products formed in vivo.