Enzymatically-derived PGF2.alpha. is an optically pure compound whereas PGF2.alpha. derived from the free radical-catalyzed peroxidation of arachidonate is a racemic mixture. Ent-PGF2.alpha. is the opposite enantiomer of PGF2.alpha.. 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-PGF2.alpha. are elevated in human urine from heavy cigarette smokers and patients with hypercholesterolemia.{14667}