Hooked to vitamin B12 since 1955: A historical perspective

Abstract In our pioneering work in 1956, two binders of vitamin B 12 (B 12 ) alias cobalamin (Cbl) were identified in gastric juice, S with slow electrophoretic mobility, a 70 kD protein with intrinsic factor (IF) activity and another rapid (R), not IF active but probable digestion product. Numerous sources contained a protein immunologically identical to R (haptocorrin, Hc). Another IF-active component (I) was found. Isoelectric focusing showed that S, I and R were assemblies of “isoproteins” with different pI's due to varying glycosidation. Isolation of S, I and R in microquantities was achieved in 1962 using a series of ion exchange chromatographies and gel filtration. Ponderable products were obtained in 1965–1966. The B 12 -IF complex was a dimer, contained 13% carbohydrate and showed a different absorption spectrum than B 12 . Using the Schilling test, B 12 absorption was shown to require Ca ++ , bound in vitro to the ileal receptor and IF, but most of Ca ++ could be removed with sialidase. The receptor–substrate complex contained Ca ++ and carbohydrate. The purified receptor was shown to contain two main subunits. The Imerslund–Grasbeck syndrome was discovered 1958–1960; it is caused by mutations in either of two genes, cubilin or amnionless, which form the multiligand receptor cubam. Testicular biopsies during and after B 12 -treated deficiency showed remarkable improvement after therapy. Studies of the turnover of radioactive B 12 revealed biliary and fecal excretion, enterohepatic circulation and allowed calculation of biological half-life and daily need. The B 12 coenzymes largely behaved like B 12 . To study whether radiocobalt in B 12 was representative of the rest of the B 12 molecule, 32 P and 57 Co labeled hydroxocobalamins were biosynthesized and shown to behave identically when given simultaneously to rats. The complex metabolism of B 12 explains the pathogenesis of B 12 deficiencies. Some of its mechanisms are not restricted to B 12 , e.g. the endocytosis of B 12 -IF also applies to other macromolecules.