The Nature of the Vitamin B12 Binding Protein in Human Serum*

It has been known for some years that the vitamin B12 circulating in the plasma is bound to protein. When this plasma binding capacity is exceeded, by the oral or intravenous administration of large doses of the vitamin, the excess unbound vitamin is rapidly excreted in the urine (Mollin and Ross, 1953). The exact nature of this binding protein has yet to be identified. In normal subjects and in patients with myeloproliferative disorders it migrates electrophoretically with the speed of an α1‐globulin, and is present in that fraction of the serum proteins which is not precipitated by 0.6 M‐perchloric acid, the seromucoid fraction (Weinstein, Weissman and Watkin, 1959). This serum fraction contains a number of antigenically distinct proteins (Burtin, 1964), two of which have been identified respectively as Orosomucoid and the α1‐acid–glycoprotein of Schultze (Hardwicke and St. Cyr, 1961). In this paper the binding of vitamin B12 to the constituent fractions of a seromucoid, obtained by (NH4)2 SO4 fractionation of serum, has been investigated by chromatography on DEAE‐cellulose. Binding into this seromucoid fraction of the serum proteins is confirmed, in normals and in myeloproliferative disorders, but this binding appears to be onto two separable serum proteins which are neither the Orosomucoid nor any other recognized α1‐glycoprotein.