Monocytic Vitamin B₁₂-binding Protein

Granulocytes are rich in vitamin B₁₂-binding protein resembling serum alpha-globulin binder. Lymphocytes and erythrocytes contain lesser amounts of the protein, but monocytes have not heretofore been studied. Characterization of our patient’s monocytic B₁₂ binder by gel filtration, radioautography, immunologic studies, and mediation of reticulocyte uptake of vitamin B₁₂ showed it was identical to granulocytic binder. Monocytic B₁₂-binding was low, but, due to the monocytosis in the subject, monocytes accounted for more total binding than did her granulocytes. The serum had markedly elevated B₁₂-binding capacity. However, the elevation was of beta-globulin primarily. The reason for this remains unclear. In vitro incubation of cellular extract with serum did not alter the characteristics of the cellular B₁₂-binding protein.