Enzymatically-derived PGF2α is an optically pure compound whereas PGF2α derived from the free radical-catalyzed peroxidation of arachidonate is a racemic mixture. Ent-PGF2α is the opposite enantiomer of PGF2α. This compound can only be generated via the isoprostane pathway of free radical-catalyzed lipid peroxidation and has been implicated as a marker of oxidative stress. Levels of ent-PGF2α are elevated in human urine from heavy cigarette smokers and patients with hypercholesterolemia.