Specific analysis in plasma and urine of 2,3-dinor-5,6-dihydro- isoprostane F(2α)-III, a metabolite of isoprostane F(2α)-III and an oxidation product of γ-linolenic acid

Abstract

F2-isoprostanes (iPs) are free radical-catalyzed isomers of prostaglandin F(2α). Circulating and urinary iPs have been used as indices of lipid peroxidation in vivo. Utilizing an 18O-labeled homologous internal standard, we developed a gas chromatography/mass spectrometry assay for the 2,3-dinor-5,6-dihydro (dinor-dihydro) metabolite of iPF(2α)-III. Although urinary excretion of iPF(2α)-III reflects systemic lipid peroxidation, the 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 urine 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 lipid peroxidation. Also, iPF(2α)-III may be formed in blood samples in a cyclooxygenase-dependent manner by platelets ex vivo. By contrast, the metabolite 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(2α)-III is elevated and increases further during reperfusion following orthoptic liver transplantation. In addition to its formation as an iPF2 metabolite, analysis of γ-linolenic acid autooxidation products and the compound present in freeze-thawed plasma suggests that γ- linolenic acid may also be an important source of dinor-dihydro-iPF(2α)-III.