Immunological characterization and clinical implication of cobalamin binding protein in human gastric cancer

Cobalamin (vitamin B12) binding protein was purified from gastric cancer extracts and from serum-free culture medium of cancer cell line KATOH-III. The molecular weight, determined by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, was 70,000 and the pI was 2.8 to 3.2. From biochemical and immunological properties, this cobalamin binding protein was considered to be an isoprotein of cobalamin R binder. Monoclonal antibodies were produced against saliva R and cobalamin binding protein in culture medium to study their antigenic determinants. Monoclonal antibody 55-D reacted to an epitope of peptide in both binders, whereas WK-1 and H-12 reacted to determinants of a carbohydrate moiety, including sialic acid, in cancer cell-derived binder. In addition, we carried out an enzyme-linked immunoassay and examined plasma levels of immunoreactive R binder in patients with gastric cancer (n = 72), benign gastrointestinal disease (n = 30), and healthy individuals (n = 40). Even in patients without liver metastasis, the level of immunoreactive R binder detected by monoclonal antibody H-12 was elevated in some patients and decreased after excision of the tumor. R binder was also elevated in cancer tissue extract. Immunoreactive binder was histochemically detected in the cytosol of cancer cells and metaplastic cells of the gastric mucosa. The present findings suggest that cobalamin R binder is de novo synthesized in gastric cancer cells and that its plasma level increases in some patients. This binding protein may be a useful diagnostic and therapeutic parameter.