May be involved in cell growth. 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 22 minutes and play a role in control of the ultradian cellular biological clock. Inhibited by the antitumor sulfonylurea LY181984, the vabilloid capsaicin, and retinoids. Extracellular and plasma membrane-associated. Found in the sera of cancer patients with a wide variety of cancers including breast, prostate, lung and ovarian cancers, leukemias, and lymphomas. Not found in the serum of healthy volunteers or patients with disorders other than cancer. Probably shed into serum by cancer cells. Found on the cell borders of renal, kidney and ovarian carcinomas but not on the borders of surrounding non-cancerous stromal cells. Glycosylated. Has several properties associated with prions including resistance to proteases, resistance to cyanogen bromide digestion, and the ability to form amyloid filaments resembling those of spongiform encephalopathies. Belongs to the ENOX family.