Catalyzes the oxidation of sulfhydryl groups in peptide and protein thiols to disulfides with the reduction of oxygen to hydrogen peroxide. May contribute to disulfide bond formation in a variety of secreted proteins. Also seems to play a role in regulating the sensitization of neuroblastoma cells for interferon-gamma-induced apoptosis. O2 + 2 R'C(R)SH = H2O2 + R'C(R)S-S(R)CR' Binds 1 FAD per subunit. Seems to be predominantly targeted to the nuclear and outer plasma membrane. Expressed in pancreas, brain, placenta, kidney, heart and fetal tissues. Weakly expressed in lung, liver and skeletal muscles. Belongs to the quiescin-sulfhydryl oxidase (QSOX) family.