FAD-dependent sulfhydryl oxidase that catalyzes disulfide bond formation (By similarity). Required for the import and folding of small cysteine-containing proteins in the mitochondrial intermembrane space. Forms a redox cycle with MIA40 that involves a disulfide relay system. Important for maintaining the cysteine residues in MIA40 in an oxidized state (By similarity). O2 + 2 R'C(R)SH = H2O2 + R'C(R)S-S(R)CR' FAD. Homodimer. Contains three disulfide bonds; one catalytic disulfide (Cys-119 to Cys-122), one structural disulfide (Cys-148 to Cys-165), and one shuttle disulfide (Cys-177 to Cys-182).