Mechanisms for the Formation of Isoprostane Endoperoxides from Arachidonic Acid

Abstract

The isoprostanes are a class of autoxidn. products generated from arachidonic acid (or its esters) by a free radical initiated process. The potent biol. activity of these compds. has been attracting intense research interest since they were detected in humans as well as animal models in the early 1990s. The measurement of these compds. has been regarded as one of the most useful non-invasive biomarkers for oxidative stress status. Two mechanisms for the formation of these compds. have been proposed. In the first mechanism, a peroxyl radical undergoes successive 5-exo cyclizations analogous to the enzymic mechanism proposed for prostaglandin biosynthesis. The second mechanism starts with a 4-exo cyclization of a peroxyl radical leading to an intermediate dioxetane, a mechanism that has also been proposed for prostaglandin biosynthesis as well as for the formation of 4-hydroxy nonenal (HNE). Autoxidn. of cholesteryl-15-HpETE under free radical conditions provides Type IV isoprostanes. The "dioxetane" mechanism for isoprostane generation from 15-HpETE requires that optically pure products are formed from an optically pure reactant, whereas an alternate mechanism for the process involving β-fragmentation of the 15-peroxyl would give racemic isoprostane products. We have carried out a test of the mechanism based upon these stereochem. requirements. The results of anal. of the product mixt. derived from autoxidn. of optically pure Ch-15-HpETE by atm. pressure chem. ionization-mass spectrometry coupled with chiral high performance liq. chromatog. indicate that the major isoprostane diastereomers are formed as a racemic mixt. These exptl. results are consistent with a mechanism for isoprostane formation involving β-fragmentation of the 15-peroxyl radical followed by re-addn. of oxygen to form the 11-HPETE peroxyl, and they exclude a mechanism proceeding through the formation of a dioxetane intermediate. [on SciFinder(R)]