Plasma R Binder Deficiency and Neurologic Disease

THE extracellular transport of vitamin B12 (cobalamin) is complicated and incompletely understood. Transport in plasma is associated with two carriers: transcobalamin II and R binder protein (R denotes "rapid" and refers to electrophoretic mobility).1 Transcobalamin II is required for efficient cellular uptake of cobalamin; in the rare cases of newborns with congenital deficiency of the protein, intractable megaloblastic anemia develops and is responsive only to pharmacologic doses of the vitamin.2 R binder proteins are a class of immunologically identical but electrophoretically distinct glycoproteins with molecular weights of 56,000 to 62,000. They are found in many secretions and differ only . . .