Purified human plasma fibronectin (catalog number F2006; Sigma-Aldrich, St. Louis, MO) was treated with or without H2O2 (catalog number H1009; Sigma-Aldrich, St. Louis, MO) in the presence or absence of HRP and MPO. Fluorescence intensity of o,o′-dityrosine formation was measured in a kinetic assay with the BioTek Synergy Mx system (BioTek Instruments, Winooski, VT) using Gen5 2.05 software at the excitation wavelength of 320 nm and emission of 405 nm. Mouse lung fibroblasts were isolated from male C57BL/6 as previously described (39). Human plasma fibronectin and MPO (5 U/ml) was added to TGF-β1 (4 ng/ml)–stimulated fibroblasts. Total mouse lung lysate was collected in boiling 1% SDS lysis buffer and subjected to Western blot analysis under reducing conditions.