Production of transcobalamin II by various murine and human cells in culture.

The ability of various murine and human cell types to secrete in vitro transcobalamin II (TCII), the vitamin B12 transport protein, was investigated. All cell types tested were found to secrete into the culture medium biologically active TCII molecules, capable of facilitating B12 uptake. The largest amounts of TCII were produced by primary cultures of murine fibroblasts and macrophages. Large quantities of TCII were also secreted by myeloma, erythroid leukemia, and macrophage-like tumor cell lines. Murine thymus cells of T lymphocyte tumors secreted only small quantities of TCII. Mouse monocytes and fibroblasts secreted considerably larger quantities of TCII than did their human counterparts. The data indicate that many cell types have the potential to produce biologically active TCII in vitro. Whether this in vitro potential also reflects in vivo biosynthetic activity is discussed.