Studies on the Transcobalamins

S ummary . Vitamin B 12 absorbed from the gut is transported on all the vitamin B 12 binding proteins (transcobalamins) and not primarily on Transcobalamin II. Absorbed vitamin B 12 can still be detected on the vitamin B 12 binding proteins after 10 days suggesting that the very rapid plasma clearance of the TCII‐vitamin B 12 complex previously reported was probably due to damage to TCII prior to re‐injection. Vitamin B 12 binder III is part of Transcobalamin I complex showing immunological identity with other binders in tissue fluids (R binders). Heparin given in vivo or in vitro prevented separation of Binder III from TCI and this effect was neutralized by protamine.