Iso[7]LGD2-protein adducts are abundant in vivo and free radical-induced oxidation of an arachidonyl phospholipid generates this D series isolevuglandin in vitro

Abstract

Isolevuglandins (isoLGs) are a family of gamma-ketoaldehydes, aka isoketals or neuroketals, that are generated by free radical-induced oxidation of polyunsaturated fatty acid-containing lipids. Because of their high reactivity toward epsilon-amino groups of lysyl residues, isoLGs are found as protein adducts in vivo. Plasma levels of isoLG-derived protein modifications are orders of magnitude higher than levels of the corresponding isoprostane. This suggests that while isoprostanes are rapidly cleared from the circulation, isoLG-protein adducts accumulate over the lifetime of the protein, which can be weeks, and this may provide a dosimeter for oxidant stress. We now confirm the postulated formation of the first D series isoLG, iso[7]LGD(2), by free radical-induced oxidation of 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphocholine in vitro. We also show that iso[7]LGD(2)-protein adduct levels in blood are the highest known for an isoLG-derived epitope. They average 30-fold higher than isoLGE(2)-protein and 3-fold higher than iso[4]LGE(2)-protein levels. Similarly, iso[7]LGD(2)-derived epitope levels in oxidized low density lipoprotein are 20 times higher than isoLGE(2)-protein and five times higher than iso[4]LGE(2)-protein levels. Previous studies showed that plasma levels of protein-bound E series isoLGs, i.e., isoLGE(2) and iso[4]LGE(2), are elevated in individuals with atherosclerosis as compared with age-matched controls. Plasma iso[7]LGD(2)-protein immunoreactivity in individuals with atherosclerosis averages 8.5 +/- 3.1 nmol/mL, significantly higher (P = 0.01) than the 3.5 +/- 0.1 nmol/mL in healthy controls. Plasma levels of iso[7]LGD(2)-protein adducts are strongly correlated with iso[4]LGE(2)- (r = 0.933) and isoLGE(2)-protein adducts (r = 0.877). This supports the hypothesis that isoLGs are generated in vivo by parallel competing radical-induced pathways.