A Vitamin B12 Binder with Transcobalamin I Characteristics Synthesized and Released by Human Granulocytes in Vitro

Summary. In‐vitro synthesis and release of a vitamin‐B12 binding protein by peripheral leucocytes derived from healthy subjects and patients with chronic myeloid leukaemia (CML), acute promyelocytic leukaemia (APL) and acute myeloblastic leukaemia (AML) were investigated. Myeloid cells, incubated in a nutrient medium containing [3H]leucine, incorporated labelled amino‐acid into the vitamin‐B12 binding protein with an increase of the unsaturated vitamin‐B12 binding capacity (UBBC) of the cells. The increase in the UBBC of the medium was due to the presence of the newly produced leucocyte binder which was released into the medium by the cells during incubation. On chromatography the newly synthesized binder was similar to transcobalamin I. The incorporation of [3H]leucine into the leucocyte binder by mature granulo‐cytes and undifferentiated myeloblasts was very limited and reached a maximum after 60 min of incubation. Cells from CML and APL patients showed a higher rate of incorporation of the labelled amino‐acid which reached a plateau after 40 hr. The increase in the UBBC of the leucocyte binder was paralleled by the incorporation of [3H]leucine. The percentage of immature myeloid cells in the incubation mixture (promyelocytes to stab forms) was correlated with both the incorporation of [3H]leucine and with the increase in UBBC.