Catalyzes the oxidation of sulfhydryl groups in peptide and protein thiols to disulfides with the reduction of oxygen to hydrogen peroxide (PubMed:17331072, PubMed:18393449, PubMed:23704371, PubMed:30367560, PubMed:23867277). Plays a role in disulfide bond formation in a variety of extracellular proteins (PubMed:17331072, PubMed:30367560, PubMed:22801504, PubMed:23867277). In fibroblasts, required for normal incorporation of laminin into the extracellular matrix, and thereby for normal cell-cell adhesion and cell migration (PubMed:23704371, PubMed:30367560, PubMed:23867277). O2 + 2 R'C(R)SH = H2O2 + R'C(R)S-S(R)CR' Binds 1 FAD per subunit. Monomer. A small proportion is secreted, probably via a proteolytic cleavage that removes the membrane anchor. Found in the extracellular medium of quiescent cells but is not found in proliferating cells. Expressed in heart, placenta, lung, liver, skeletal muscle, pancreas and very weakly in brain and kidney. Induced in quiescent cells just as fibroblasts begin to leave the proliferative cycle and enter quiescence. N-glycosylated (PubMed:17331072, PubMed:29757379). O-glycosylated on Thr and Ser residues (PubMed:29757379). 'Quiescin Q6' means that it was the sixth clone to be found at a higher level of expression in quiescent fibroblasts. Belongs to the quiescin-sulfhydryl oxidase (QSOX) family. Extended N-terminus.