Specific analysis in plasma and urine of 2,3-dinor-5,6-dihydro-isoprostaneF-2 alpha-III, a metabolite of isoprostane F-2 alpha-III and an oxidation product of gamma-linolenic acid

F-2-isoprostanes (iPs) are free radical-catalyzed isomers of prostaglandin F-2 alpha. Circulating and urinary iPs have been used as indices of lipid p eroxidation in vivo. Utilizing an O-18-labeled homologous internal standard , we developed a gas chromatography/mass spectrometry assay for the 2,3-din or-5,6 dihydro (dinor-dihydro) metabolite of iPF(2 alpha)-III. Although uri nary excretion of iPF(2 alpha)-III reflects systemic lipid peroxidation, th e metabolite is more abundant (median of 877 (range of 351-1831) versus 174 (range of 56-321) pg/mg of creatinine; p < 0.01) than the parent iP in uri ne and can be measured in plasma. Metabolite analysis may be preferable in plasma due to the abundance of arachidonic acid as a source of ex vivo lipi d peroxidation. Also, iPF(2 alpha)-III may be formed in blood samples in a cyclooxygenase-dependent manner by platelets ex vivo. By contrast, the meta bolite is not formed by aggregated platelets (0.71 +/- 0.08 versus 0.65 +/- 0.09 pg/ml). Although the metabolite/parent ratio is altered in cirrhosis, urinary dinor-dihydro-iPF(2 alpha)-III is elevated and increases further d uring reperfusion following orthoptic liver transplantation. In addition to its formation as an iPF(2) metabolite, analysis of gamma-linolenic acid au tooxidation products and the compound present in freeze-thawed plasma sugge sts that gamma-linolenic acid may also be an important source of dinor-dihy dro-iPF(2 alpha)-III.