A urine sample was obtained from participants at the post-intervention interview. The sample was aliquoted and stored at a -80C freezer without preservative. For assessment of oxidative stress, urine was analyzed for levels of F2-IsoPs, a stable marker of lipid peroxidation, following previously published protocols [29, 30, 36, 64]. Briefly, 1 ng of [2H4]-8-isoPGF2α an internal standard, was added to 1ml of thawed urine, followed by F2-IsoP extraction using C18 and Sep-pak cartridges. Pentaflurobenzyl esters were generated, purified, converted to trimethylsilyl ether derivatives and then subjected to analysis by tandem gas chromatography/negative ion chemical ionization-mass spectrometry. Urine creatinine was measured to adjust for variation of urine concentration between samples, with measures expressed as ng F2-isoprostanes/mg creatinine (ng/mg Cr).
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