Factor VIII, along with calcium and phospholipid, acts as a cofactor for factor IXa when it converts factor X to the activated form, factor Xa. Interacts with vWF. vWF binding is essential for the stabilization of F8 in circulation (By similarity). Found in most tissues. The binding of vWF and activation depend on the sulfation of Tyr-1669. Proteolytically cleaved by cathepsin CTSG to produce a partially activated form. Belongs to the multicopper oxidase family.