Determination of HNE/protein adduct concentrations in normal (N) and osteoarthritic (OA) osteoblast. HNE/protein adduct levels were measured by enzyme-linked immunosorbent assay in cellular extracts from untreated (a) or treated (b) osteoblasts with increasing concentrations of H2O2 for 24 hours at the indicated concentrations. HNE/protein adduct levels were expressed in picograms of HNE/protein adducts per milligrams of total proteins. Data are mean ± standard error of the mean (n = 3). Statistics: Student unpaired t test; *p < 0.05, **P < 0.01, ***P < 0.001. HNE, 4-hydroxynonenal.