Probably acts as a terminal oxidase of plasma electron transport from cytosolic NAD(P)H via hydroquinones to acceptors at the cell surface. Hydroquinone oxidase activity alternates with a protein disulfide-thiol interchange/oxidoreductase activity which may control physical membrane displacements associated with vesicle budding or cell enlargement. The activities oscillate with a period length of 24 minutes and play a role in control of the ultradian cellular biological clock. Not inhibited by the antitumor sulfonylurea LY181984, the vabilloid capsaicin, and retinoids. Extracellular and plasma membrane-associated. Expressed in lymphocyte cells, breast and breast cancer (at protein level). Found in the sera of cancer patients with a wide variety of cancers including breast, prostate, lung and ovarian cancers, leukemias, and lymphomas. Found also in the serum of healthy volunteers or patients with disorders other than cancer. Probably shed into serum by cancer cells. Belongs to the ENOX family.