Identification of the vitamin B12-binding protein in the serum of normals and of patients with chronic myelocytic leukemia.

By ROBERT S. MENDELSOIIN, DONALD M. WATKIN, ANN P. HORBETr AND J OHN L. FAHEY R ECENT STUDIES have demonstrated that virtually all the vitamin B12 in normal serum is bound to serum proteins.1 In chronic myelocytic leukemia tile serum B12 concentration may be increased from 2 to 30 times normal levels.24 At these increased levels, all or almost all of the B12 is protein-bound. However, normal serum can bind completely only an amount of B12 equal to two or three times the normal 35 Thus the increased serum B12 concentration of chronic myelocytic leukemia can be attril)uted either to an increase in the normal B12-binding protein or to the presence in the serum of components normally absent. These studies were undertaken to characterize the B12-binding protein of normal serum and of serum in chronic myelocytic leukemia. The present studies were conducted entirely with serum containing B12 bound in vivo to the serum proteins. In vitro B12 addition was not en1 )loyed in order to avoid ally artifacts attributable to overloading the normal B12-binding system or to contaminating materials. The recent introduction by Peterson and Sober of chromatographic procedures for protein fractionation has made possible new approaches to the separation and characterization of serum proteins.4’7 Normal and pathologic sera have been studied by Fahey, McCoy and Goulian,8 employing a modifled anion-exchange cellulose chromatographic procedure. This chromatographic procedure and electrophoretic procedures, employed individually and in sequence, have been used in this study to characterize the B12-binding protein in the serum of normal subjects and of patients with chronic myelocytic leukemia. Microbiologic assay was used to identify the B12. The results suggest that an alpha-i globulin binds B12 in normal serum and that an increasedi concentration of this proteimi in tile serum of patients with chronic myelocytic leukemia accounts for the elevated serum B12 levels in this disease.